Amplifier circuit and sensor system for broadband and low-noise amplification of capacitive current source

Abstract

The invention relates to an amplifier circuit (1) for broadband and low-noise amplification of a capacitive current source, preferably a thermoelectric sensor, comprising: a signal input (3) which can be connected to the capacitive current source in a node K1; the invention relates to an amplifier (1) for amplifying an input voltage, comprising an input stage (A1), which is connected to a node K1 at an input (7) of the input stage (A1) and has a node K2 at an output (9) of the input stage (A1), the input stage (A1) being designed to amplify the input voltage by at least three times, the input stage (A1) being designed to provide a high-ohm input resistance at the input of the input stage (A1), the input stage (A1) is designed to provide a stable and load-independent voltage at the output of the input stage (A1); the invention relates to a power amplifier comprising a first amplifier (A1) and a second amplifier (A3), an amplifier cascade (11) having at least a first amplifier and a second amplifier (A2, A3), each having an input (13, 17) and an output (15, 19), the output (15) of the first amplifier (A2) being connected in a node K3 to the input (17) of the second amplifier (A3), the input (13) of the first amplifier (A2) being connected to a node K2, the output (15, 19) being connected in a node K3 to the input (17) of the second amplifier (A3). An output (19) of the second amplifier (A3) is connected to a node K4, the amplifier cascade (11) being arranged to produce a high signal gain with a low phase shift over a wide frequency range; a feedback network (F1), which is connected to the input (7) of the input stage (A1) in a node K1 and to the output (19) of the second amplifier (A3) in a node K4, said feedback network (F1) being designed to provide a high-ohm feedback resistance with a parasitic capacitance of less than 0.5 pF, the feedback network (F1) is arranged to provide a negative feedback to a structure comprising the input stage (A1) and the amplifier cascade (11), and a signal output (21) connected to a node K5, which is connected to or corresponds to a node K4. The invention also relates to a sensor system.

Description

Technical Field

[0001] The invention relates to an amplifier circuit for broadband and low-noise amplification of a capacitive current source, preferably a pyroelectric sensor, and to a sensor system. The subject matter of the invention is defined in the appended claims. Background Art

[0002] Highly sensitive detection of electromagnetic radiation in the infrared and terahertz bands is important for various applications. For example, thermal sensors are not only used in motion and fire alarms, but also in gas analysis or in spectrometers for chemical analysis. Due to their simple structure and the resulting low cost, capacitive current sources, such as thermoelectric sensors, are widely used in high-demand measurements at room temperature. The core element of a thermoelectric sensor is a crystal of thermoelectric material. In these materials, the charge centers of positive and negative ions do not coincide, thus forming an electric polarization that can be oriented along the crystal axis. Even very small changes in crystal temperature, such as temperature changes caused by irradiation with thermal radiation, can lead to changes in this polarization. On the one hand, an increase in temperature directly leads to a decrease in spontaneous polarization, and on the other hand, an increase in temperature also indirectly promotes a change in dipole orientation through the expansion of the material. As a result, surface charges are formed on the interface of the crystal perpendicular to its polar axis in proportion to the temperature change. The charge can flow out through the electrodes arranged on these surfaces and can therefore be measured as a current.

[0003] Since the currents generated by pyroelectric crystals are generally in the order of magnitude of a few picoamperes, low-noise measuring electronics with high-gain amplifier circuits are required for the analysis. This places high demands on the amplifier circuits used for this purpose if the largest possible frequency bandwidth is to be achieved at the same time. This has a significant impact on the overall performance of the sensor system given the size of the sensor element.

[0004] Circuits for measuring such small currents are known in the prior art.

[0005] A disadvantage of the amplifier circuits known in the prior art is the relatively narrow bandwidth in the case of high amplification of the signals of capacitive current sources, such as pyroelectric sensors. In addition, the amplified signal often has strong noise. In addition, the amplifier circuits known in the prior art generally show a strong dependence on their input capacitance. Summary of the invention

[0006] Therefore, the object of the present invention is to provide a transimpedance amplifier (TIA) or an amplifier circuit which can amplify the signal of a capacitive current source, in particular a pyroelectric sensor, with a high bandwidth and low noise, and thereby overcome the disadvantages of the prior art.

[0007] This object is achieved in a first aspect of the present invention by an amplifier circuit according to the invention according to claim 1. Furthermore, this object is achieved in a second aspect of the present invention by a sensor system according to the invention according to claim 20. Preferred embodiments according to the invention result from the dependent claims and the following embodiments.

[0008] In a first aspect of the present invention, this object is achieved by an amplifier circuit for amplifying capacitively a current source, preferably a thermoelectric sensor, in a broadband and low-noise manner, according to the features of claim 1. According to claim 1, the amplifier circuit according to the invention for amplifying capacitively a current source, preferably a thermoelectric sensor, in a broadband and low-noise manner comprises: a signal input terminal, which can be connected to the capacitive current source at a node K 1 ; an input stage, wherein the input stage is connected at an input terminal of the input stage to the node K 1 and has at an output terminal of the input stage a node K 2 , wherein the input stage is arranged to amplify an input voltage by at least a factor of 3, wherein the input stage is arranged to provide a high-ohmic input resistance at the input terminal of the input stage, wherein the input stage is arranged to provide a stable and load-independent voltage at the output terminal of the input stage; an amplifier cascade, wherein the amplifier cascade has at least a first amplifier and a second amplifier, the first amplifier and the second amplifier each having an input terminal and an output terminal, wherein the output terminal of the first amplifier is connected at a node K 3 to the input terminal of the second amplifier, wherein the input terminal of the first amplifier is connected to the node K 2 , wherein the output terminal of the second amplifier is connected to the node K 4 , wherein the amplifier cascade is arranged to produce a high signal amplification with a low phase shift over a wide frequency range; a feedback network, wherein the feedback network is connected at a node K 1 to the input terminal of the input stage and at a node K 4 to the output terminal of the second amplifier, wherein the feedback network is arranged to provide a high-ohmic feedback resistance with a parasitic capacitance of less than 0.5 pF, wherein the feedback network is arranged to provide negative feedback to a structure comprising the input stage and the amplifier cascade; and a signal output terminal, the signal output terminal being connected to the node K 5 , wherein the node K 5 is connected to the node K 4 or corresponds to the node K 4 .

[0009] The amplifier circuit according to the invention is a circuit of a transimpedance amplifier (TIA). Hereinafter, the terms "amplifier circuit" and "transimpedance amplifier" are used synonymously.

[0010] In the context of the present invention, a capacitive current source is a current source with an output impedance that can be well approximated by a capacitor. This output impedance corresponds to the source impedance at the input of a downstream TIA. Examples of capacitive current sources can be photodiodes, CCD pixels, tunnel current sensors, pressure and tactile sensors, (Geiger-Müller) counter tubes, photomultipliers (such as microchannel plates), acceleration sensors or preferably thermoelectric sensors. In the context of the present invention, a thermoelectric sensor is a component in which a temperature difference causes a change in the voltage of the component due to its thermoelectric properties.

[0011] A thermoelectric sensor can be described by an equivalent circuit diagram consisting of a current source I py , a crystal capacitor C py and a loss resistance R py in parallel. The resistivity of the thermoelectric material can be very high and is generally on the order of a few 10 10 Ωcm. For a typical crystal size with a diameter of a few millimeters, the capacitance of the sensor element can vary between 100 pF and 1 nF depending on the crystal thickness.

[0012] A thermoelectric sensor within the scope of the present invention has a crystal with a thermoelectric material or comprises or consists of a thermoelectric material. The capacitance C py of the thermoelectric sensor is mainly determined by the thickness and area of the thermoelectric crystal. Therefore, in order to amplify different sensor elements broadband, a transimpedance amplifier is required, which is particularly insensitive to changes in the source impedance.

[0013] The amplifier circuit according to the present invention (i.e., the transimpedance amplifier) converts an input current signal into an output voltage proportional thereto and can thus be regarded as a current-controlled voltage source. This property is particularly advantageous when measuring and amplifying small current signals. This property is particularly suitable for measuring the small currents of thermoelectric sensor elements. The current generated at the interface of the thermoelectric material (also called thermoelectric medium) is thus converted into a well-measurable voltage. This voltage can then be read out, for example, by an analog-to-digital converter. Preferably, the capacitive current source has a current on the order of that of a thermoelectric sensor.

[0014] In the context of the present invention, the signal input describes the hardware interface of the amplifier circuit to which the current signal of the capacitive current source can be applied. For this purpose, the signal input can be connected to the capacitive current source. In order to connect to the capacitive current source, the signal input can have, for example, one or more of the following connection means: terminal blocks, crimp connectors, plug connectors, screw connectors, pads, solder joints, high-frequency connectors.

[0015] It is also conceivable that the signal input of the amplifier circuit according to the invention is directly connected to a capacitive current source. In this case, the amplifier circuit according to the invention can form a unit with the capacitive current source. For example, the amplifier circuit according to the invention can be integrated into the capacitive current source.

[0016] Connection to node K x (where x is a natural number) describes within the scope of the present invention a direct or indirect electrical connection, preferably a direct electrical connection, to node K x . A node describes, in the sense of the present invention, a connection point of at least two conductor paths of the amplifier circuit. A direct connection to node K x is a conductive connection to node K x that does not include additional components and detours via other nodes. An indirect connection to node K x is a conductive connection to node K x that includes at least one electrical and / or electronic component or one or more groups of components. A group of components is, in the sense of the present invention, a group of components that includes at least two or more electrical and / or electronic components. Additionally or alternatively, an indirect connection can also include a detour via one or more other nodes.

[0017] The input stage is, in the sense of the present invention, a component or a group of components arranged downstream of the signal input and upstream of a subsequent stage (such as an amplifier cascade). The input stage is constructed and arranged such that the input signal (i.e., the current signal of the capacitive sensor applied at the signal input or node K 1 ) is amplified. The gain is at least 3 times, preferably at least 5 times, more preferably at least 10 times. Additionally or alternatively, the gain can be at most up to 25 times, preferably at most 20 times. Particularly preferably, the gain is in the range from 5 to 10 times. In this range, generally good amplification exists with optimized low noise. The signal output from the input stage at the output of the input stage can be inverting or non-inverting. Preferably, the amplification is performed as a linear regulation, preferably by using a linear amplifier. Alternatively, the amplification can also be performed using the advantageous, non-linear characteristic curves of the corresponding components for amplification.

[0018] The input stage is arranged to provide a high-ohmic input resistance at the input of the input stage and at the same time a stable and load-independent voltage at the output of the input stage. Preferably, the input stage is very low-noise here. A low-noise input stage is, in the sense of the present invention, an input stage having a spectral noise density of less than . The low-noise input stage results from the minimization of the associated voltage noise and current noise U n and I n at the input of the input stage.

[0019] To achieve this, the input stage is preferably constructed discretely, i.e., consisting of at least two semiconductor components which are arranged for controlling voltage and / or current. For example, the input stage can be constructed by transistors. Here, the transistors can be bipolar transistors or field effect transistors or a combination consisting of at least one bipolar transistor and at least one field effect transistor.

[0020] Preferably, the discrete structure of the input stage includes components or groups of components that provide amplification of the input signal and components or groups of components that reduce or adjust the impedance at the output of the input stage. Thus, the input stage provides a voltage at its output, the value of which does not depend on the load caused by the downstream circuit.

[0021] Preferably, the discrete structure includes at least one low-pass (Tiefpass), which is also referred to as a low-pass filter (Tiefpass-Filter), for noise suppression of the supply voltage of the input stage.

[0022] An amplifier cascade in the sense of the present invention is a series circuit or link of at least two amplifiers that amplify the input signal, i.e., at least one first amplifier having a first amplification factor G Q3 and a second amplifier having a second amplification factor G Q4 The amplifier cascade is located downstream of the input stage.

[0023] The amplifier cascade can include two, three, four or more amplifiers. Preferably, the amplifier cascade includes two amplifiers.

[0024] The high signal gain achieved by the amplifier cascade describes, within the scope of the present invention, an inherent gain of at least 10 4 factors for the input signal. In the sense of the present invention, the low phase shift of the entire amplifier cascade including the input stage describes a phase shift of a maximum of -160°. In the sense of the present invention, an additional frequency range describes a frequency bandwidth of at least 500 Hz, preferably at least 1 kHz, further preferably at least 10 kHz, particularly preferably at least 100 kHz. In the case of a frequency bandwidth of 10 kHz, a signal high gain of 10 8 factors is advantageous here. Particularly advantageous is a signal gain of 10 9 factors at a frequency bandwidth of 10 kHz, or a signal gain of 10 8 factors at a frequency bandwidth of 100 kHz.

[0025] The amplification can be carried out in open-loop gain or closed-loop gain. Preferably, the amplification is carried out in open-loop gain. Preferably, the first amplifier is a linear regulator which controls the function of the amplifier circuit through a feedback network. Preferably, the second amplifier has a smaller gain GQ4 This gain can be fixedly defined or adjustable. In this case, the second amplifier can amplify the output signal of the first amplifier with a larger bandwidth and a reduced phase shift. Additionally, for a given negative feedback, a larger frequency range with a smaller input impedance can be provided.

[0026] The first and second amplifiers and, if necessary, other downstream amplifiers can be implemented as inverting, non-inverting, or a combination of inverting and non-inverting amplifiers. Preferably, the first and second amplifiers are non-inverting.

[0027] Preferably, the first amplification factor G Q3 is much greater than the second amplification factor G Q4 : G Q3 >> G Q4 . For example, the ratio of G Q3 / G Q4 can be at least 10 3 , preferably 10 4 , more preferably 10 5 .

[0028] The feedback network is described in the context of the present invention as a group of components that at least includes a high-ohmic feedback resistor R for providing negative feedback to a structure composed of an input stage and an amplifier cascade fb . This resistor defines a transimpedance amplifier as the ratio of the output voltage to its input current. In the context of the present invention, the high-ohmic feedback resistor has an ohmic resistance value of 10 GΩ at a transimpedance gain of 10 GV / A.

[0029] To amplify the very low current of a capacitive current source (preferably a thermoelectric sensor), a very large signal gain is required. Here, the amplification factor should preferably exceed the factor 10 6 . Particularly advantageously, at a frequency bandwidth of 10 kHz, the signal gain is the factor 10 9 , or at a frequency bandwidth of 100 kHz, the signal gain is the factor 10 8 . In other words, the amplifier circuit according to the present invention can have a TIA gain-bandwidth product greater than 10 TV / AHz.

[0030] For the noise characteristics of the amplifier circuit according to the present invention, only the actual transimpedance part of the amplifier circuit according to the present invention is important. The downstream voltage gain can no longer improve the noise, even if it involves a voltage filter circuit. Using the downstream voltage gain or voltage filter circuit can only change the noise bandwidth, but cannot change the noise density that is important for the quality of the amplified signal.

[0031] In order to obtain an optimized signal-to-noise ratio of the amplified signal, in the case of a pre-given high gain of the amplifier circuit according to the invention, it is important that the input current noise of the amplifier circuit is as small as possible. Here, the input current including the input current noise is converted into a voltage by the amplifier circuit according to the invention. Here, the input current noise is mainly affected by the selected feedback, i.e., the Johnson-Nyquist noise of the feedback resistor. Preferably, at room temperature, at a gain of 500 MV / A, the input current noise is about More preferably Further preferably, at room temperature, at a gain of 5 GV / A, the input current noise is about Particularly preferably about

[0032] The amplifier circuit according to the invention is preferably a linear amplifier circuit. This is achieved by negative feedback of the input stage and an amplifier stage with a linear feedback network in cascade. For this reason, the feedback network is connected to nodes K 1 and K 4 connected.

[0033] Preferably, the feedback network has a compensation circuit which is arranged to compensate for a potentially present unwanted parasitic capacitance of the high-ohmic feedback resistor. In other words, the compensation circuit is arranged to minimize, preferably eliminate, the frequency dependence of the feedback network. For example, the compensation circuit can have at least one capacitor which advantageously interacts with the high-ohmic feedback resistor. Additionally or alternatively, the compensation circuit can include a band-pass filter, for example a high-pass and / or a low-pass filter.

[0034] Preferably, the compensation circuit has electronic or electrical components which can be variably adjusted in terms of their ohmic resistance or in terms of their capacitance. This enables the compensation circuit to be precisely coordinated with the parasitic capacitance of the ohmic resistance such that the parasitic capacitance is balanced and the frequency dependence of the feedback network (almost) disappears.

[0035] The signal output describes, within the scope of the present invention, the following hardware interface of the amplifier circuit on which an amplified output signal can be output broadband and with low noise. For further processing and use of the output signal, the signal output can be connected to an external device. For example, the signal output can be connected to an analog-to-digital converter and subsequent measurement and analysis devices (such as a measurement computer). For connection to an external device, the signal output can for example have one or more of the following connection means: terminal blocks, crimp connectors, plug connectors, screw connectors, pads, solder joints, high-frequency connectors.

[0036] The amplifier circuit according to the present invention has the surprising advantage that it achieves extremely low noise and at the same time very high gain in the case of a high frequency bandwidth. Thereby, the amplifier circuit according to the present invention provides an optimized signal-to-noise ratio. For example, a gain G of, for example, G≈10 9 V / A and a bandwidth greater than 10 kHz, in particular up to 100 kHz, can be achieved by the amplifier circuit according to the present invention. At such a gain, this is significantly faster compared to what is common for amplifier circuits hitherto available on the market. The amplifier circuit is robust against an increase in the input capacitance at the signal input. This allows the use of thinner and thus more sensitive thermoelectric sensor elements even at high gains.

[0037] In a preferred embodiment according to a first aspect of the present invention, the input stage includes a junction field effect transistor and a bipolar transistor, wherein the drain of the junction field effect transistor is connected to the base of the bipolar transistor, wherein the junction field effect transistor is wired as a source circuit (Source-Schaltung), wherein the bipolar transistor is wired as an emitter follower, wherein the first and second amplifiers of the amplifier cascade are operational amplifiers, wherein the voltage gain of the first operational amplifier is greater than 10 4 , wherein the voltage gain of the second operational amplifier is at most 10 3 , wherein the feedback network has a high-ohm feedback resistor with a parallel capacitor, and wherein the feedback network includes a low-pass filter connected in series.

[0038] Junction field effect transistors are known in the prior art. A junction field effect transistor generally has three terminals: source, gate, and drain. Bipolar transistors are known in the prior art. A bipolar transistor generally has three terminals: collector, base, and emitter.

[0039] Preferably, the gate terminal of the junction field effect transistor is connected to node K 1 Preferably, the emitter terminal of the bipolar transistor is connected to node K 2

[0040] The source circuit of the junction field effect transistor and the emitter follower circuit of the bipolar transistor are well known in the art.

[0041] The source circuit is configured to invert and amplify the input signal at the junction field effect transistor by at least a factor of 3, preferably a factor of 5 to 20. However, it is also conceivable to amplify the input signal non-inversely.

[0042] The emitter follower is configured to reduce the output impedance of the input stage without additional voltage amplification. This allows at node K 2 ​transmits voltage stably, and the voltage value of this node is independent of the load caused by subsequent circuits. The emitter follower utilizes the fixed voltage drop between the base and emitter of a bipolar transistor to compensate for the offset potential at the drain output terminal of a field-effect transistor. Through this offset correction, the two operational amplifiers in the cascaded amplifier after the input stage must regulate less strongly in the reverse direction in order to regulate their input signals to zero. In an actual operational amplifier, the common-mode input voltage should be close to zero. Without compensation, offsetting the signal level by a field-effect transistor will cause the subsequent high-gain amplifier cascade to saturate or limit the dynamic range of the gain.

[0043] Preferably, the drain terminal of the junction field-effect transistor and / or the collector terminal and emitter terminal of the bipolar transistor each have a resistor or a series circuit composed of at least two resistors. Through a suitable resistor or a series circuit of resistors on the drain terminal of the junction field-effect transistor and / or on the collector terminal and emitter terminal of the bipolar transistor, an optimized operating point of the input stage can be set. The optimized operating point of the input stage is given at the maximum gain with a high bandwidth and low noise of the output signal.

[0044] Preferably, at least the series circuit of resistors has a low-pass filter in parallel at the drain terminal of the junction field-effect transistor and / or at the collector terminal and emitter terminal of the bipolar transistor. The low-pass filter can advantageously suppress the noise of the power supply voltage. Further preferably, each series circuit of resistors has a low-pass filter in parallel at the drain terminal of the junction field-effect transistor and at the collector terminal and emitter terminal of the bipolar transistor.

[0045] Preferably, the voltage gain G of the first operational amplifier Q3 is greater than 10 6 , and more preferably greater than 10 7 .

[0046] Preferably, the voltage gain G of the second operational amplifier Q4 is at most 1000, more preferably at most 400, and even more preferably at most 250.

[0047] A capacitor C in parallel with a high-ohm feedback resistor 5 and a low-pass filter in series therewith (such as a low-pass filter composed of an ohmic resistor R 8 and a capacitor C 6 constitute a feasible implementation of the above compensation circuit. The compensation circuit is provided for reducing the inevitable parasitic capacitance of the high-ohm feedback resistor to zero or close to zero. In this case, close to zero describes that the parasitic capacitance is less than 0.5 pF, preferably less than 0.1 pF.

[0048] Preferably, the low-pass filter is connected to node K 4 Preferably, the parallel circuit consisting of the high-ohm feedback resistor R 7 and the capacitor C 5 is connected to at least node K 1 .

[0049] The parasitic capacitance of the high-ohm feedback resistor R 7 generates a pole in the control loop, and the pole can be compensated by the low-pass filter composed of R 8 and C 6 . This can be shown by decomposing the compensation network into its two components: the transfer function of, for example, an inverting transimpedance amplifier with the feedback Z 7 constituted by R 5 and C f alone is:

[0050]

[0051] Herein, is the voltage at node K f . The low-pass filter directly upstream constituted by R 8 and C 6 alone has the following transfer function:

[0052]

[0053] wherein the potential is the potential at node K 4 . By substitution, the total transfer function of the compensation circuit is obtained:

[0054]

[0055] From which it can be directly seen that when R 8 C 6 = R 7 C 5 , the transfer function loses its frequency dependence. Therefore, by appropriately adjusting the low-pass filter, the parasitic capacitance of the high-ohm feedback resistor R 8 can be compensated.

[0056] Preferably, the high-ohm feedback resistor R 8 is a combination of a fixed resistor and a potentiometer.

[0057] The amplifier circuit according to the present invention has the following advantages: It is constructed from readily available, cost-effective, and reliable electronic components.

[0058] Furthermore, a specific arrangement structure of different component groups, in particular the nesting of the input stage, amplifier cascades, and feedback network, enables a performance with high gain that is broadband and low-noise for signals of capacitive current sources, preferably thermoelectric sensors, which has not been achieved hitherto in the prior art.

[0059] Junction field effect transistors have very low input current noise. Thereby, the noise of the signal at the signal output of the amplifier circuit according to the invention is kept at a low level, and the signal-to-noise ratio is improved.

[0060] Therefore, this embodiment particularly provides a very cost-effective amplifier circuit, which has the advantages of broadband and low-noise high gain mentioned above.

[0061] In a preferred embodiment according to the first aspect of the invention, the amplifier circuit further has an output stage, wherein the output stage is connected at the input of the output stage to node K 4 and is connected at the output of the output stage to the signal output at node K 5 wherein the output stage is arranged to amplify the voltage on the input of the output stage up to 20 times, and wherein the output stage is arranged to filter DC interference on the input of the output stage and to adjust the signal level of the signal at the output of the output stage at K 5 .

[0062] In the sense of the present invention, the output stage is a component group located downstream of the amplifier cascade, which is arranged to amplify the voltage on the input of the output stage, filter DC interference on the input of the output stage, and adjust the signal level of the voltage at the output of the output stage.

[0063] Preferably, the gain of the voltage at the input of the output stage is at most 15 times, more preferably at most 10 times. The gain can be an inverting or non-inverting gain. The gain can be achieved, for example, by means of one or more of the following amplification components: field effect transistors, unipolar transistors, bipolar transistors, operational amplifiers.

[0064] Capacitive current sources, preferably thermoelectric sensors, generally measure temperature changes. Therefore, the (amplified) voltage signal derived from the current signal of the capacitive current source is also a time-varying signal. Therefore, the DC voltage component in the voltage signal, for example, the DC voltage component from non-optimal offset correction in the input stage, does not provide information for measurement and is therefore undesirable. Therefore, filtering DC interference at the input of the output stage significantly improves the signal quality.

[0065] Filtering of the DC interference can be carried out, for example, using a frequency-dependent circuit (such as a band-pass filter) or using at least one frequency-dependent component (such as a capacitor). The frequency-dependent circuit or the at least one frequency-dependent component is preferably connected downstream of the input of the output stage and preferably upstream of the component that performs amplification.

[0066] Adjustment of the signal level can be carried out, for example, using solutions known in the prior art. For example, an amplifier circuit based on an operational amplifier can be employed.

[0067] The output stage and, if necessary, subsequent amplification following it have the following advantages: The signal level can be optimally adapted to the dynamic range of generally subsequent analysis electronics (such as an analog-to-digital converter). In addition, by further increasing the signal level, possible interference signals become less significant compared to the signal.

[0068] Therefore, an optimized signal-to-noise ratio is obtained in the subsequent measuring electronics.

[0069] In a preferred embodiment according to the first aspect of the present invention, the output stage includes an inverting band-pass amplifier, wherein the input of the band-pass amplifier is AC-coupled.

[0070] Preferably, the input of the amplifier of the output stage is connected to node K through a serially connected capacitor. 4 Preferably, the output of the amplifier of the output stage is connected to node K 5 and the signal output. Alternatively, the band-pass amplifier can also be implemented non-inverting.

[0071] The output stage can be composed of easily obtainable, cost-effective, and reliable electronic components. Therefore, this embodiment particularly provides a very cost-effective amplifier circuit, which has the advantages of broadband, low noise, and high gain mentioned above.

[0072] In a preferred embodiment according to the first aspect of the present invention, the amplifier circuit further includes an amplitude limiting device, wherein the amplitude limiting device is connected to node K 4 or K 5 at the input of the amplitude limiting device, and is connected to node K 3 at the output of the amplitude limiting device, wherein the amplitude limiting device is arranged to limit the amplitude of the output signal at the signal output when a threshold at node K 4 or node K 5 is exceeded.

[0073] In the sense of the present invention, the amplitude limiting device is a group of components arranged to limit the amplitude of the output signal at the signal output when a threshold is exceeded.

[0074] The limitation of the amplitude is achieved by means of a non-linear transfer function. The non-linear limitation of the amplitude can be carried out, for example, with the aid of a Zener diode (Z-diode). Alternatively, other components with a non-linear transfer function can also be used.

[0075] Using this non-linear feedback increases the stability of the circuit. Precisely in the case of amplifying the signal of a thermoelectric sensor, it is possible that too much light impinges on the sensor, which can saturate the amplification stage. Thus, overshoot (übersteuern) at the signal output of the amplifier circuit according to the invention is prevented.

[0076] In particular, in the interaction with the output stage, the following cooperation relationship is produced: the non-linear negative feedback is implemented at node K 3 instead of node K 1 to reduce or prevent an increase in the noise of the output signal at the signal output at node K 5 due to the possible capacitance of at least one component in the amplitude limiting device. In order to be able to carry out a controlled negative feedback at node K3, the polarity of the gain along the feedback closed-loop must be overall inverting (negative). For this purpose, the output stage is preferably designed to be inverting, and the amplitude limiting device is arranged along the connection between nodes K 5 and K 3

[0077] Another reason for arranging the amplitude limiting device after the first amplifier in the amplifier cascade is that the amplifiers in the amplifier cascade are very sensitive to additional capacitance and reduced input impedance. In particular, non-linear components such as diodes will add strong noise.

[0078] Finally, the oscillation or overshoot of the amplifier cascade can be reduced or completely prevented by the interaction of the amplitude limiting device with the output stage.

[0079] In a preferred embodiment according to the first aspect of the invention, the amplitude limiting device comprises two Zener diodes connected in series in opposite polarities.

[0080] Zener diodes are known in the prior art. A Zener diode has a cathode and an anode. In the series circuit of the two Zener diodes connected in opposite polarities, the respective cathodes (or respective anodes) of the Zener diodes are preferably connected to node K 3 and to the signal output at node K 5 respectively. The two anodes (or two cathodes) of the two Zener diodes are connected to each other. Negative feedback starts when the voltage on K 5 exceeds a threshold value. The threshold value depends on K 5 ​Breakdown voltage of the connected Zener diode. The threshold can be set by correspondingly selecting the Zener diode.

[0081] Preferably, the anode of the Zener diode is grounded via a series circuit composed of an ohmic resistor and a capacitor.

[0082] Zener diodes are simple, reliable and cost - advantageous components. Therefore, they are particularly suitable for amplitude - limiting devices. In this regard, the same advantages as described for the amplitude - limiting device in the previous paragraphs also apply. To avoid unnecessary repetition, reference is made here to the description above and it will not be repeated again.

[0083] In a preferred embodiment according to the first aspect of the present invention, the amplifier circuit includes a frequency - response compensation device which is connected to node K 3 and node K 1 and is arranged to reduce the oscillation tendency of the amplifier cascade.

[0084] In the sense of the present invention, the frequency - response compensation device is a component or a group of components that reduce the unwanted oscillation tendency of the entire amplifier circuit. Within the scope of the present invention, the oscillation tendency of the amplifier circuit describes the tendency of the amplifier circuit towards unwanted oscillations.

[0085] For example, the frequency - response compensation device may have frequency - dependent components or a group of frequency - dependent components. The frequency - response compensation device may, for example, have at least one or more of the following frequency - dependent components or groups of frequency - dependent components: capacitors connected in series, capacitors connected in parallel, band - pass filters (such as high - pass filters and / or low - pass filters).

[0086] The frequency - dependent components or the group of frequency - dependent components are used to stabilize the amplifier cascade through internal frequency compensation. When the phase of the signal rotates 180° (the loop gain approaches - 1) through the feedback loop of the amplifier circuit according to the present invention, it may change from negative feedback to positive feedback, and the amplifier cascade tends to oscillate. This characteristic can be compensated by the frequency - dependent components or the group of frequency - dependent components. Here, the frequency - dependent components or the group of frequency - dependent components are preferably dimensioned such that the circuit stability of the amplifier cascade and the entire amplifier circuit according to the present invention is ensured, but the bandwidth is not overly restricted at the same time.

[0087] Thus, with the aid of the frequency response compensation device, the stability of the amplifier circuit according to the invention at high frequencies can be improved. Thereby, oscillations of the amplifier circuit are reduced or avoided and the performance of the amplifier circuit is improved. In particular, a flat transfer can be achieved by means of the frequency response compensation device, that is, a linear transfer function is achieved at high frequencies and high gains. The fluctuations of the transfer function are reduced, preferably to less than 5% of the maximum amplitude, particularly preferably to less than 3% of the maximum amplitude. This is particularly necessary in order to be able to quantify the signal amplitude in broadband measurements.

[0088] In a preferred embodiment according to the first aspect of the invention, the frequency response compensation device comprises negative feedback via a capacitor.

[0089] The capacitor of the negative feedback is preferably connected to nodes K 1 and K 3 and.

[0090] The capacitor of the negative feedback serves to stabilize the entire amplifier circuit by internal frequency compensation. When the phase of the signal rotates by 180° (the loop gain approaches -1) through the feedback loop of the amplifier circuit according to the invention, it is possible to change from negative feedback to positive feedback, and the amplifier circuit tends to oscillate. This characteristic can be compensated by the capacitor of the negative feedback. The capacitance value of the capacitor is preferably designed such that the circuit stability of the amplifier cascade and the entire amplifier circuit according to the invention is ensured, but the bandwidth is not restricted too strongly at the same time.

[0091] The capacitor is a simple, reliable and cost - advantageous component. Therefore, it is a simple and cost - advantageous implementation to realize the frequency response compensation device by using the capacitor of the negative feedback.

[0092] Furthermore, in this regard, the same advantages for the frequency response compensation device described in the above paragraphs also apply. To avoid unnecessary repetition, reference is made to the above description here and it will not be repeated again.

[0093] In a preferred embodiment according to the first aspect of the invention, the input stage has a component with negative capacitance.

[0094] In the sense of the present invention, a component with negative capacitance is a component in which a decrease in the applied voltage causes an increase in the charge of the component. For this purpose, the component can have a material with negative capacitance. For example, initial experimental evidence regarding such materials can be found in M. Hoffmann, S. Slesazeck, and T. Mikolajick, "Progress and future prospects of negative capacitance electronics: A materials perspective", APL Mater. 9, 020902 (2021), and A. K. Yadav, K. X. Nguyen, Z. Hong et al., "Spatially resolved steady-state negative capacitance", Nature 565, 468 (2019).

[0095] Examples of materials in which negative capacitance can be formed are ferroelectric materials, such as HfO 2 or lead zirconate titanate or heterostructures composed of these ferroelectric materials and dielectric materials, such as strontium titanate.

[0096] Preferably, the component with negative capacitance is combined with a signal input or an input stage such that the negative capacitance of the component equalizes the normal positive capacitance of a capacitive current source (preferably a thermoelectric sensor). Alternatively or additionally, the component with negative capacitance can also be included in a feedback network and there compensate for the parasitic capacitance of a high-ohm feedback resistor.

[0097] Using such a component significantly reduces noise and thus improves the signal-to-noise ratio when measuring the signal of a capacitive current source (preferably a thermoelectric sensor).

[0098] In a preferred embodiment according to the first aspect of the present invention, the amplifier circuit or at least a part of the amplifier circuit is mounted on a circuit board, wherein at least one electronic component of the amplifier circuit is soldered to the circuit board by means of pads on the circuit board, and wherein the area of the circuit board outside the pads and under the at least one electronic component is removed.

[0099] A circuit board in the sense of the present invention means any substrate suitable for permanently mechanically fixing and electrically connecting the amplifier circuit according to the present invention or a part of the amplifier circuit according to the present invention. Examples of circuit boards are flexible or rigid circuit boards. A circuit board can for example include fiber-reinforced plastics or cardboard as well as metal claddings and / or conductor paths.

[0100] In the sense of the present invention, a pad describes areas on a circuit board that are generally provided with a metal coating and are suitable for making soldering connections by means of solder, for example for connecting electronic components.

[0101] Within the scope of the present invention, removing an area of the circuit board describes a circuit board area in which the material of the circuit board has been partially or completely removed. In other words, in such a removed area, there is no circuit board material (completely removed) or there is little circuit board material (partially removed). The removal of the circuit board material can be carried out, for example, by a cutting process, such as milling, drilling or sawing. Alternatively, corresponding cutouts without circuit board material can also be provided during the manufacture of the circuit board material.

[0102] Each device, such as an ohmic resistor, has parasitic capacitances formed between the terminals of the component and between the component and the surroundings. Here, especially in the case of an ohmic resistor, the parasitic capacitance increases with the component size. In particular, the high-ohmic feedback resistors of the input stage and the transistors react sensitively to changes in environmental conditions. Even a minimal deviation in the material surrounding the circuit board, in the protective lacquer applied to the components and the circuit board, or in the flux residues caused by the soldering process can generate leakage currents on the order of the current in the negative feedback loop of the amplifier circuit according to the present invention, which can significantly and adversely affect the performance of the amplifier electronics according to the present invention. In particular, these effects cause undesirably large quality fluctuations within a production batch during the production process, which require subsequent processing or result in rejects and thus increase costs.

[0103] Preferably, the circuit board is removed centrally under the component, so that only the area of the pad remains. The component connected to the pad can bridge the gap generated in the circuit board by the removed area like a bridge in the state where the terminals of the component are fixedly soldered. In this case, no possible contamination can occur under the component.

[0104] Thereby, in production, the repeatability of the performance of the amplifier circuit according to the present invention is significantly improved. In addition, the costs for possible subsequent improvement of the amplifier circuit performance after production can be reduced or saved.

[0105] In a preferred embodiment according to the first aspect of the present invention, the at least one electronic component is one or more components of the feedback network or the entire feedback network.

[0106] The feedback network is particularly important for the function of the amplifier circuit according to the present invention and is at the same time very sensitive to parasitic capacitances. Therefore, reducing the parasitic capacitance by removing the circuit board material significantly improves the performance of the amplifier circuit according to the present invention.

[0107] In a preferred embodiment according to the first aspect of the present invention, the at least one electronic component is one or more components of an input stage, preferably a field effect transistor and / or a bipolar transistor.

[0108] The input stage is particularly important for the function of the amplifier circuit according to the present invention and is at the same time very sensitive to parasitic capacitance. Therefore, reducing the parasitic capacitance by removing the circuit board material significantly improves the performance of the amplifier circuit according to the present invention.

[0109] The amplifier circuit according to the present invention according to one of claims 1 to 19 can preferably be used to measure the current signal of a capacitive current source (preferably a thermoelectric sensor).

[0110] The amplifier circuit according to the present invention according to one of claims 1 to 19 can preferably be used to measure the current signal of a capacitive power source (preferably a thermoelectric sensor) in an infrared spectrometer, preferably an FTIR spectrometer (Fourier-Transform-Infrarot-Spektrometer).

[0111] Here, an FTIR spectrometer (Fourier transform infrared spectrometer) can include the following components: an infrared radiation source, an interferometer having at least one arm with variable length, a reference laser, a measurement unit with a sample interface (preferably an ATR crystal (Attenuated Total Reflection), which can come into contact with the sample), an infrared detector, a control system (the control system is arranged to change the length of the at least one arm of the interferometer), and a mirror device, which is located outside the interferometer and has at least two mirrors, each of which has a reflection surface and a substrate including the reflection surface, wherein the mirror device is at least arranged to deflect the light beam from the interferometer onto the sample interface and to deflect the light beam from the sample interface onto the infrared detector, wherein the substrate of at least one mirror or all mirrors of the mirror device is made of plastic material and / or 3D printed metal, or at least one mirror or all mirrors of the mirror device have plastic material and / or 3D printed metal.

[0112] By means of the ATR crystal, an evanescent wave can be coupled into the sample material or sample in contact with the ATR crystal. This effect is also known as the optical tunneling effect. The remaining light carries information about the interaction with the sample, which is led out of the ATR crystal again by means of total internal reflection and can then be guided, for example, by reflection to the infrared detector.

[0113] Combinations of all the above preferred embodiments are also preferred embodiments and describe a four-stage amplifier circuit, i.e., an amplifier circuit including four amplifier stages and having three feedback loops.

[0114] The first amplification stage is formed by the input stage. The second and third amplification stages are formed by the first and second amplifiers in an amplifier cascade. The fourth amplification stage is formed by the output stage.

[0115] The first feedback loop is formed by a feedback network. The second feedback loop is formed by an amplitude limiting means. Here, the third feedback loop is formed by an internal frequency compensation means.

[0116] Preferably, the input stage and the output stage are respectively inverting amplification stages. Preferably, the first and second amplifiers in the amplifier cascade are respectively or form non-inverting amplification stages. The cooperation of the inverting and non-inverting amplification stages allows for separate feedback loops, each of which contains only the first or the last inverting stage and otherwise only non-inverting stages. Thus, all feedback loops satisfy the condition for negative feedback (negative feedback coupling), i.e., the condition of an odd number of inverting amplification stages. The first feedback loop includes a linear component network for high gain with a capacitive compensation means. The second feedback loop produces a non-linear amplitude limitation. The third feedback loop is for compensating the frequency response of the entire circuit.

[0117] In a preferred embodiment according to the first aspect of the present invention, the amplifier circuit further includes a decoupler, wherein the decoupler is connected to node K 4 and the input terminal of the output stage, and wherein the decoupler is arranged to decouple the output terminal of the amplifier cascade from the output stage.

[0118] In a preferred embodiment according to the first aspect of the present invention, the decoupler includes an impedance converter, preferably a non-inverting impedance converter or a combination of two inverting impedance converters, or the decoupler is an impedance converter, preferably a non-inverting impedance converter or a combination of two inverting impedance converters. Particularly preferably, the decoupler is a non-inverting impedance converter or has a non-inverting impedance converter.

[0119] The impedance converter is a technically simple, cost-effective and reliable component or group of components by means of which the desired decoupling can be achieved. The advantage of a non-inverting impedance converter is that it generally has a very high-ohm input, which is particularly advantageous for such a circuit.

[0120] In a preferred embodiment according to the first aspect of the present invention, the impedance converter comprises at least one or more of the following components or is one of the following components: MOSFET, bipolar transistor, operational amplifier, operational amplifier with high input resistance and low inherent noise, operational amplifier with high input resistance and low inherent noise, wherein the output terminal of the operational amplifier is directly or indirectly coupled back to its inverting input terminal.

[0121] In a particularly preferred embodiment according to the first aspect of the present invention, the impedance converter comprises at least one operational amplifier with high input resistance and low inherent noise, wherein the output terminal of the operational amplifier is directly coupled back to the inverting input terminal.

[0122] Within the scope of the present invention, decoupling the output terminal of an amplifier cascade from the input terminal of an output stage by means of a decoupler particularly describes the decoupling of the frequency dependence of the output stage from the upstream amplifier cascade. This frequency decoupling can be complete or only partial. In the case of complete frequency decoupling, the ideal frequency dependence of the output stage is not distorted, and the frequency dependence of the amplifier cascade is also not affected by it. In the case of partial frequency decoupling, the output stage loads the upstream amplifier cascade, and the frequency dependence of the output stage is distorted by the frequency-dependent output impedance of the amplifier cascade. The greater the feedback resistance of the amplifier cascade (i.e., the weaker the feedback intensity), the stronger this distortion. Preferably, the decoupling is complete or almost complete.

[0123] MOSFETs, bipolar transistors, and operational amplifiers are advantageous, reliable, and technically simple components that are suitable for use as or in a decoupler. Operational amplifiers with high input resistance and low inherent noise are components preferably used in a decoupler because they can provide particularly strong decoupling up to complete or almost complete decoupling. At the same time, due to their low inherent noise, they contribute only little or no distortion to the signal amplified in the amplifier circuit according to the present invention. Operational amplifiers with high input resistance and low inherent noise are particularly preferred, wherein the output terminal of the operational amplifier is directly coupled back to the inverting input terminal because they have the same advantageous characteristics as operational amplifiers with high input resistance and low inherent noise and further improve the decoupling.

[0124] The decoupler is preferably connected to node K 4 and the input terminal of the output stage. In particular, the decoupler can be directly or indirectly connected to node K 4 and the input terminal of the output stage. In this case, a direct connection describes the connection of "K 4 —decoupler—input terminal of the output stage". An indirect connection can, for example, include other electronic components between K 4 and the decoupler and / or between the decoupler and the input terminal of the output stage.

[0125] The decoupler and the non-inverting impedance converter are arranged to provide frequency decoupling of the amplifier cascade from the input of the output stage. This achieves a significantly flatter transfer function between the amplifier cascade and the output stage. As a result, the frequency dependence at the output of the amplifier circuit according to the invention is reduced or improved.

[0126] At the same time, the decoupler improves the gain of the amplifier circuit according to the invention. In particular, the input signal of the amplifier circuit according to the invention can be amplified with a flat transfer function, for example, with a gain of 50 mV / A up to 360 kHz.

[0127] In a preferred embodiment according to the first aspect of the invention, the amplifier circuit further includes an intermediate load, wherein the intermediate load is connected at node K 3 to the output of the first amplifier and the input of the second amplifier of the amplifier cascade, and the intermediate load is arranged to minimize the natural oscillation of the amplifier circuit.

[0128] In the sense of the invention, the intermediate load being connected at node K 3 to the output of the first amplifier of the amplifier cascade means that the intermediate load can be connected directly or indirectly, for example, by means of one or more intermediate electronic components, to the output of the first amplifier at node K 3 . In the case of an indirect connection, the preferred electronic component can be an ohmic resistor.

[0129] In the sense of the invention, the intermediate load being connected at node K 3 to the input of the second amplifier of the amplifier cascade means that the intermediate load can be connected directly or indirectly, for example, by means of one or more intermediate electronic components, to the input of the second amplifier at node K 3 . In the case of an indirect connection, the preferred electronic component can be an ohmic resistor.

[0130] In a preferred embodiment according to the first aspect of the invention, the intermediate load is arranged to minimize natural oscillation by phase correction.

[0131] In a preferred embodiment according to the first aspect of the invention, the output of the first amplifier is connected to node K 3 via a first ohmic resistor (R 17 ), and node K 3 is grounded via a second ohmic resistor (R 15 ). The first ohmic resistor (R 17 ) and the second ohmic resistor (R 15 ) preferably form a voltage divider here, and at node K 3A partial voltage can be intercepted thereon. The voltage divider advantageously limits an over-strong load on, in particular, an upstream amplifier stage or an upstream operational amplifier.

[0132] In a preferred embodiment according to the first aspect of the present invention, the intermediate load has at least one or more of the following component combinations or consists of at least one or more of the following component combinations: an inductor and / or a capacitor connected in series to ground and at least one ohmic resistor, preferably the second ohmic resistor; an inductor and / or a capacitor connected in parallel to ground and at least one ohmic resistor. Particularly preferably, the intermediate load has or consists of an inductor or a capacitor connected in series to ground and at least the second ohmic resistor. Further preferably, the intermediate load has an inductor connected in series to ground and at least the second ohmic resistor, or consists of an inductor connected in series to ground and at least the second ohmic resistor R 15 formed.

[0133] Within the scope of the present invention, the intermediate load describes each component or each component combination suitable for minimizing the inherent oscillation of the amplifier circuit according to the present invention. Preferably, the intermediate load is arranged to introduce a phase correction that changes the frequency response such that the amplifier circuit according to the present invention is more robust with respect to the inherent oscillation. Within the scope of the present invention, the robustness with respect to the inherent oscillation means that the feedback circuit, in particular the feedback network and / or the frequency response compensation device, satisfies the Nyquist stability criterion. That is to say, the phase shift of the feedback is so small that the desired negative feedback does not unexpectedly become positive feedback.

[0134] The above-mentioned component combination is simple in structure and robust in use. In addition, although simple, it has a surprising effect of strongly improving the advantageous characteristics of the amplifier circuit according to the present invention.

[0135] Particularly preferably, the inductance value of the intermediate load in the component combination consisting of an inductor and at least one ohmic resistor (preferably the first and the second ohmic resistors) is in the range of 1 μH to 20 μH, preferably in the range of 1 μH to 5 μH. Particularly preferably, the values of the first ohmic resistor and the second ohmic resistor in the component combination of the intermediate load consisting of an inductor and an ohmic resistor are in the range of 10 ohms to 100 ohms, preferably in the range of 10 ohms to 20 ohms. Preferably, the first resistor and the second resistor are of the same size. These values surprisingly show particularly good results when minimizing the undesired inherent oscillation.

[0136] The arrangement of the intermediate load advantageously stabilizes the amplifier circuit according to the present invention when there are large variations in the input capacitance of the capacitive power supply. In particular, this enables the use of different capacitive current sources and / or thermoelectric sensors. Therefore, the circuit according to the present invention becomes more versatile and can be put into use without costly and technically complex adaptation of the circuit.

[0137] In a second aspect of the present invention, the object of the present invention is solved by a sensor system having the features of claim 20. The sensor system according to the invention according to claim 20 comprises a capacitive current source and an amplifier circuit according to any one of claims 1 to 19.

[0138] The embodiments made in connection with the first aspect of the present invention are also applicable to the sensor system in connection with the second aspect of the present invention. To avoid unnecessary redundancy, reference is made here to the above description and will not be repeated again.

[0139] The capacitive current source may preferably have a component with negative capacitance.

[0140] In the sense of the present invention, a component with negative capacitance is a component in which a decrease in the applied voltage causes an increase in the charge of the component. For this purpose, the component may have a material with negative capacitance. For example, initial experimental evidence regarding such materials can be found in M. Hoffmann, S. Slesazeck, and T. Mikolajick, "Progress and future prospects of negative capacitance electronics: A materials perspective", APL Mater. 9, 020902 (2021), and A. K. Yadav, K. X. Nguyen, Z. Hong, et al., "Spatially resolved steady-state negative capacitance", Nature 565, 468 (2019).

[0141] Examples of materials with negative capacitance are ferroelectric materials such as HfO 2 or lead zirconate titanate, and heterostructures composed of these ferroelectric materials and dielectric materials such as strontium titanate.

[0142] Preferably, the component with negative capacitance is connected to the capacitive current source, for example, at the output of the capacitive current source. Alternatively, the component with negative capacitance may be part of the capacitive current source (preferably a thermoelectric sensor). In both cases, the negative capacitance of the component can balance the normal positive capacitance of the capacitive current source (preferably a thermoelectric sensor).

[0143] The use of such a component significantly reduces the noise and thus improves the signal-to-noise ratio when measuring the signal of the capacitive current source (preferably a thermoelectric sensor).

[0144] In a preferred embodiment according to the second aspect of the present invention, the capacitive current source is a thermoelectric sensor.

[0145] The thermoelectric sensor can for example comprise lithium tantalate (LiTaO 3 ) or triglycine sulfate (TGS).

[0146] Due to the special characteristics of the amplifier circuit according to the invention in the sensor system according to the invention, the signal of the thermoelectric sensor can be amplified particularly broadband and with low noise.

[0147] In a preferred embodiment according to the second aspect of the present invention, the thermoelectric sensor is implemented in a plate-like manner and has a maximum thickness of 40 μm, preferably a maximum of 10 μm.

[0148] The thermoelectric sensor generally comprises a crystal made of a thermoelectric material.

[0149] Within the scope of the present invention, the thickness of the thermoelectric sensor describes the thickness of the thermoelectric material, i.e. the thickness or average thickness of the thermoelectric crystal.

[0150] The temperature increase of the thermoelectric sensor or crystal is proportional to the absorption A of radiation in the crystal th and inversely proportional to its heat capacity c th (photothermal conversion). By means of a suitable, broadband absorbent coating of the crystal, as large a temperature change as possible can be achieved. In addition, a smaller heat capacity can be achieved by a smaller crystal volume or, given a given sensor area, by a small crystal thickness. The heating process of the sensor element is impeded by heat dissipation (for example by heat conduction through the crystal support). As poor a heat conduction as possible causes as high a temperature difference ΔT as possible between the irradiated and non-irradiated crystal and thus a correspondingly high current signal. As the modulation frequency ω of the incident light increases, ΔT becomes smaller and holds. So, a small heat capacity (i.e. a thinner crystal) is favorable for the fastest possible thermal response of the sensor. Thus, if the increased capacitance of the sensor element due to the small thickness does not exceed the reasonable limit of the input capacitance of the subsequent amplifier circuit, a thermoelectric sensor with a small thickness enables the fastest possible thermal response and favorable properties of the thermoelectric sensor. Within the scope of the present invention, the thickness or average thickness of the crystal is preferably between 2 μm and 40 μm.

[0151] Another factor is thermoelectric conversion: a temperature change in a thermoelectric crystal generates a surface charge Q that is proportional to the area A of the crystal and to the thermoelectric coefficient p, which describes the specific strength of the thermoelectric effect in the material: Q = pAΔT. Thus, a time-varying temperature generates a time-varying charge, i.e., a current I. It follows that a thermoelectric sensor can only respond to a change in radiation. In a steady state, no current flows anymore. At high frequencies, the thermoelectric current is constant and independent of the frequency of the optical excitation. For the current, i.e., at high frequencies, a low heat capacity (thin crystal) is also desirable for obtaining the largest possible current signal.

[0152] In addition, the smaller thickness of the thermoelectric sensor causes an increase in the bandwidth of the amplifier circuit according to the invention up to the electronic bandwidth. In this case, both the optothermal conversion and the thermoelectric conversion are optimized due to the reduced heat capacity of the thermoelectric sensor.

[0153] The sensor system according to the invention according to any one of claims 20 to 22 can preferably be used in an FTIR spectrometer (Fourier transform infrared spectrometer) and / or used together with an FTIR spectrometer. In particular, the sensor system according to the invention can be part of or used in the above-mentioned FTIR spectrometer.

[0154] It should be clarified here that one or more of the above-described preferred embodiments can be combined with each other as long as they do not contradict each other, and these combinations are also preferred embodiments.

[0155] In particular, one or more of the different aspects of the present invention in the above-described preferred embodiments can also be combined with each other as long as they do not contradict each other, and these combinations are also preferred embodiments of the present invention. Description of the Drawings

[0156] The preferred embodiments of the present invention will be described in detail below with reference to the drawings. In the drawings:

[0157] Figure 1 shows a schematic diagram of a circuit of a first embodiment of an amplifier circuit according to the present invention,

[0158] Figure 2 shows a circuit diagram of an embodiment of an input stage,

[0159] Figure 3 shows a circuit diagram of an embodiment of an amplifier cascade with a feedback network,

[0160] Figure 4 shows a schematic diagram of a circuit of a second embodiment of an amplifier circuit according to the present invention,

[0161] Figure 5A circuit diagram showing one embodiment of the output stage

[0162] Figure 6 A schematic diagram of a circuit showing a third embodiment of the amplifier circuit according to the present invention

[0163] Figure 7 A circuit diagram showing one embodiment of the amplitude limiting device

[0164] Figure 8 A schematic diagram of a circuit showing a fourth embodiment of the amplifier circuit according to the present invention

[0165] Figure 9 A circuit diagram showing one embodiment of an amplifier cascade with a feedback network and frequency compensation

[0166] Figure 10 A circuit diagram showing a fifth embodiment of the amplifier circuit according to the present invention

[0167] Figure 11a Figures a and b show components on a circuit board together with removed areas

[0168] Figure 12 Shows one embodiment of a sensor system according to the present invention

[0169] Figure 13a 、 Figure 13b Shows the measurement results of the performance of the amplifier circuit according to the present invention

[0170] Figure 14a 、 Figure 14b 、 Figure 14c Shows the measurement results of the performance of the sensor system according to the present invention

[0171] Figure 15 Shows the measurement results of the transfer characteristics at different gains

[0172] Figure 16 A schematic diagram of a circuit showing a sixth embodiment of the amplifier circuit according to the present invention

[0173] Figure 17 A schematic diagram of a circuit showing a seventh embodiment of the amplifier circuit according to the present invention

[0174] Figure 18 A circuit diagram showing one embodiment of an intermediate load

[0175] Figure 19 A schematic diagram of a circuit showing an eighth embodiment of the amplifier circuit according to the present invention

[0176] Figure 20A circuit diagram showing a ninth embodiment of the amplifier circuit according to the present invention, and

[0177] Figure 21 shows a second embodiment of the sensor system according to the present invention. Detailed Description

[0178] Figure 1 A schematic diagram of a circuit showing a first embodiment of the amplifier circuit 1a according to the present invention. The amplifier circuit 1a is an amplifier circuit for amplifying a capacitive current source, preferably a thermoelectric sensor, with broadband and low noise. The amplifier circuit 1a includes: a signal input terminal 3, which can be connected to a capacitive current source at a node K 1 ; an input stage A1, wherein the input stage A1 is connected to the node K at the input terminal 7 of the input stage A1 1 and has a node K at the output terminal 9 of the input stage A1 2 , wherein the input stage A1 is arranged to amplify the input voltage by at least 3 times, wherein the input stage A1 is arranged to provide a high-ohmic input resistance at the input terminal 7 of the input stage A1, wherein the input stage A1 is arranged to provide a stable and load-independent voltage at the output terminal 9 of the input stage A1; an amplifier cascade, wherein the amplifier cascade 11 has at least one first amplifier A2 and a second amplifier A3, the first amplifier and the second amplifier respectively having input terminals 13 or 17 and output terminals 15 or 19, wherein the output terminal 15 of the first amplifier A2 is connected to the input terminal 17 of the second amplifier A3 at the node K 3 , wherein the input terminal 13 of the first amplifier A2 is connected to the node K 2 , wherein the output terminal 19 of the second amplifier A3 is connected to the node K 4 , wherein the amplifier cascade 11 is arranged to generate a high signal gain with low phase shift over a wide frequency range; a feedback network F1, wherein the feedback network F1 is connected to the input terminal 7 of the input stage A1 at the node K 1 and is connected to the output terminal 19 of the second amplifier A3 at the node K 4 , wherein the feedback network F1 is arranged to provide a high-ohmic feedback resistor with a parasitic capacitance of less than 0.5 pF, wherein the feedback network F1 is arranged to provide negative feedback to a structure including the input stage A1 and the amplifier cascade 11; and a signal output terminal 21, the signal output terminal is connected to the node K 5 , wherein the node K 5 is connected to the node K 4 or corresponds to the node K 4 .

[0179] Figure 2 Shows an example of the input stage A1. The input stage A1 includes a junction field effect transistor Q 1and the bipolar transistor Q 2 The junction field effect transistor Q 1 has three terminals: source S, gate G, and drain D. The bipolar transistor Q 2 has three terminals: collector C, base B, and emitter E. The junction field effect transistor Q 1 has its drain connected to the base B of the bipolar transistor Q 2 The junction field effect transistor Q 1 serves as the source circuit wiring. The bipolar transistor Q 2 serves as the emitter follower wiring.

[0180] Additionally or alternatively, the input stage A1 can have a component with negative capacitance.

[0181] Figure 3 An example of an amplifier cascade 11 with a feedback network F1 is shown. The first and second amplifiers A2, A3 of the amplifier cascade 11 are operational amplifiers Q 3 , Q 4 respectively. The first operational amplifier Q 3 can have a voltage gain greater than 10 4 . The second operational amplifier Q 4 can have a voltage gain of at most 10 3 .

[0182] The feedback network F1 includes a high-ohm feedback resistor R 5 with a parallel capacitor C 7 . Additionally, the feedback network F1 includes a low-pass filter connected in series. The low-pass filter has a resistor R 8 and a grounded capacitor C 6 . The low-pass filter is connected to the feedback resistor R 7 and the parallel capacitor C 5 at node K f .

[0183] Figure 4 A schematic diagram of a circuit showing a second embodiment of the amplifier circuit 1b according to the present invention is shown. Figure 4 The circuit shown in Figure 1 is an extension of the circuit in Figure 1 and thus has all the elements and their functions in Figure 4 An output stage A4 is also shown. The output stage A4 is connected to node K 4 at the input terminal 23 of the output stage A4, and is connected to the signal output terminal 21 at node K 5 at the output terminal 25 of the output stage A4.

[0184] The output stage A4 is arranged to amplify the voltage at the input of the output stage by up to 20 times. In addition, the output stage A4 is arranged to filter out DC interference on the input 23 of the output stage A4 and to adjust the signal level at the output 25 of the output stage A4 at K 5 at.

[0185] Figure 5 A circuit diagram showing an example of the output stage A4. The output stage A4 includes an inverting band-pass amplifier Q 5 . The inverting input 27 of the band-pass amplifier A4 is AC-coupled by means of a series-connected capacitor C 7 .

[0186] Figure 6 A schematic diagram of a circuit showing a third embodiment of the amplifier circuit 1c according to the invention. The circuit shown in Figure 6 is an extension of the circuits in Figure 1 and Figure 4 and thus has all the elements and their functions in Figure 1 and Figure 4 .

[0187] Figure 6 Additionally shown is the amplitude limiting means F2 included in the amplifier circuit 1c. The amplitude limiting means F2 is connected at the input 31 of the amplitude limiting means F2 to node K 4 or K 5 and at the output 29 of the amplitude limiting means F2 to node K 3 . The amplitude limiting means F2 is arranged to limit the amplitude of the output signal at the signal output 21 when a threshold value at node K 4 or node K 5 is exceeded.

[0188] Figure 7 A circuit diagram showing an example of the amplitude limiting means F2. The amplitude limiting means F2 has two oppositely poled Zener diodes Q 6 , Q 7 .

[0189] Figure 8 A schematic diagram of a circuit showing a fourth embodiment of the amplifier circuit 1d according to the invention. Figure 8 The circuit shown in Figure 1 , Figure 4 and Figure 6 is an extension of the circuits in Figure 1 , Figure 4 and Figure 6 and thus has all the elements and their functions in

[0190] Figure 8Also shown is a frequency response compensation device F3 included in the amplifier circuit 1d. The frequency response compensation device F3 is connected to node K 3 and node K 1 and is arranged to reduce the oscillation tendency of the amplifier cascade 11.

[0191] Figure 16 A schematic diagram of a circuit according to a sixth embodiment of the amplifier circuit 1f according to the present invention is shown. Figure 16 The amplifier circuit 1f shown in Figure 8 is an extension of the circuit of Figure 16 Shown is Figure 8 the amplifier circuit 1d shown in 4 together with an additional decoupler A5. The decoupler A5 is connected to node K

[0192] and the input terminal 23 of the output stage A4. The decoupler is arranged to decouple the output terminal of the amplifier cascade 11 from the output stage A4. 8 Preferably, the decoupler A5 is or has a non-inverting impedance converter Q

[0193] Figure 17 A schematic diagram of a circuit according to a seventh embodiment of the amplifier circuit 1g according to the present invention is shown. Figure 17 The amplifier circuit 1g shown in Figure 8 is an extension of the circuit of Figure 17 Shown is in Figure 8 the amplifier circuit 1d shown in 3 together with an additional intermediate load F4. The intermediate load F4 is connected in node K

[0194] Figure 18 to the output terminal 15 of the first amplifier A2 and the input terminal 17 of the second amplifier A3 of the amplifier cascade 11. The intermediate load F4 is arranged to minimize the inherent oscillation of the amplifier circuit 1g. Preferably, the intermediate load F4 is arranged to minimize the inherent oscillation of the amplifier circuit 1g by phase correction. 15 An example of implementing the intermediate load F4 is shown. The intermediate load F4 is shown exemplarily as an inductor L1 grounded in series and an ohmic resistor R

[0195] Figure 19 A schematic diagram of a circuit according to an eighth embodiment of the amplifier circuit 1h according to the present invention is shown. Figure 19 The amplifier circuit 1h shown in Figure 16 is an extension of the circuit of

[0196] Figure 9 Shown isFigure 3 An example of the amplifier cascade 11 shown, together with an additional example of the frequency response compensation means F3. The frequency response compensation means F3 includes a capacitor connected in series, such as capacitor C 4 . Capacitor C 4 is connected to node K 1 and K 3 . The frequency response compensation means F3 is arranged to reduce the tendency of the cascade amplifier 11 to oscillate.

[0197] Figure 10 A circuit diagram showing a fifth embodiment of the amplifier circuit 1e according to the present invention is shown. Here, the amplifier circuit will be formed by combining the components shown in Figure 2 , Figure 3 , Figure 5 , Figure 7 and Figure 9 into the overall amplifier circuit 1e according to the present invention. Additionally, Figure 10 a capacitive current source 41 is shown, which is implemented as a thermoelectric sensor D py , which capacitive current source can be connected to and is connected to the amplifier circuit 1e at node K 1 . The amplifier circuit 1e is particularly preferred.

[0198] Figure 20 A circuit diagram showing a ninth embodiment of the amplifier circuit 1k according to the present invention is shown. Here, the amplifier circuit 1k combines the amplifier circuit shown in Figure 10 with the intermediate load shown in Figure 18 and the decoupler A5 in the form of another operational amplifier Q Figure 16 shown in 8 . The output of the operational amplifier Q 8 is directly coupled back to its inverting input.

[0199] The amplifier circuit 1k is particularly preferred.

[0200] The output terminal of the first operational amplifier Q 3 is connected to node K 3 via the first resistor R 17 , where node K 3 is connected to the non-inverting input of the second operational amplifier Q 9 via the ohmic resistor R 4 .

[0201] The intermediate load F4 is exemplarily shown as an inductor L 1 connected in series to ground and a second ohmic resistor R 15 . However, alternative embodiments of the intermediate load F4 are also conceivable. The ohmic resistor R 17 and the ohmic resistor R15 A voltage divider is formed. The voltage divider advantageously limits the excessive loading of the preceding amplifier stage or the preceding operational amplifier.

[0202] Preferably, the resistor R 15 and R 17 are of the same magnitude. The aforementioned values surprisingly show particularly good results in minimizing unwanted natural oscillations.

[0203] The first to fifth embodiments of the amplifier circuits 1a, 1b, 1c, 1d, 1e can preferably be used to measure the current signal of a capacitive current source (preferably a thermoelectric sensor). Any embodiment of the amplifier circuits 1a, 1b, 1c, 1d, 1e, 1f, 1g, 1h, 1k (abbreviated as amplifier circuit 1) according to the present invention, especially also the sixth to ninth embodiments, can preferably be used to measure the current signal of a capacitive current source (preferably a thermoelectric sensor).

[0204] The amplifier circuits 1a, 1b, 1c, 1d, 1e, 1f, 1g, 1h, 1k according to the present invention can preferably be used to measure the current signal of a capacitive current source (preferably a thermoelectric sensor) in an infrared spectrometer, preferably in an FTIR spectrometer (Fourier-Transform-Infrarot-Spektrometer).

[0205] FIG. 11 shows a partial view of the circuit board 33, which can also be referred to as a circuit card. The amplifier circuits 1a, 1b, 1c, 1d, 1e, 1f, 1g, 1h, 1k (hereinafter only referred to as amplifier circuit 1 for better readability) or at least a part of the amplifier circuit 1 are mounted on the circuit board 33. FIG. 11 exemplarily shows the electronic components 35 of the amplifier circuit 1 on the circuit board 33. The electronic components 35 are soldered to the circuit board 33 by means of pads 37. The area 39 of the circuit board 33 outside the pads 37 and beneath the at least one electronic component 35 is removed.

[0206] The area 39 of the circuit board 33 can preferably be removed beneath one or more components of the feedback network F1 or beneath the entire feedback network F1.

[0207] Alternatively or additionally, preferably, the area 39 of the circuit board 33 beneath one or more components of the input stage A1 can be removed. Particularly preferably, the area 39 beneath the field-effect transistor Q 1 and / or beneath the bipolar transistor Q 2 is removed.

[0208] Figure 12Shows an embodiment of a sensor system 43 according to the present invention, the sensor system comprising a capacitive current source 41 and an amplifier circuit 1. The capacitive current source of the sensor system 43 is preferably a thermoelectric sensor D py .

[0209] Figure 21 Shows another embodiment of a sensor system 43 according to the present invention, comprising a capacitive current source 41 and an amplifier circuit 1k. The capacitive current source of the sensor system 43 is preferably a thermoelectric sensor D py .

[0210] Thermoelectric sensor D py can preferably be implemented in a plate-like manner and have a maximum thickness d of 40 μm, preferably a maximum of 10 μm.

[0211] The sensor system 43 according to the present invention can preferably be used in and / or with an FTIR spectrometer (Fourier Transform Infrared Spectrometer).

[0212] Figure 13a 、 Figure 13b Shows the performance measurement results of the amplifier circuit according to the present invention. Figure 14a 、 Figure 14b 、 Figure 14c Respectively show the performance measurement results of the sensor system according to the present invention, and Figure 15 shows the measurement results of the transfer characteristics at different gains.

[0213] Figure 13 shows the measurement data of the transfer for a four-stage amplifier circuit 1 according to the present invention and demonstrates the capabilities and advantages of the present invention. Compared to a general TIA where the gain is typically only 10 MV / A, the amplifier circuit 1 according to the present invention exhibits a flat transfer at a 3 dB cut-off frequency up to 90 kHz despite a gain that is 40 times higher, 400 MV / A. The amplifier circuit 1 thus achieves the same bandwidth as a TIA with a smaller gain of 10 MV / A known in the prior art. In other words, for a similar bandwidth, the amplifier circuit 1 achieves a significantly greater gain compared to the prior art. In addition, the proposed circuit is robust with respect to changes in the input capacitance, especially those caused by the capacitive current source 41, such as the thermoelectric sensor D py causing changes in the input capacitance. The transfer functions for 0 pF to 270 pF are almost identical. Even in the case of a relatively large input capacitance of 2.2 nF, there is no overshoot in the frequency response as occurs in a TIA known in the prior art

[0214] The noise voltage of the amplifier circuit according to the present invention is at Figure 13bis shown and at low frequencies is only slightly above the minimum possible Johnson - Nyquist noise of the 400 MΩ feedback resistor, i.e.,

[0215] at higher frequencies starting from 10 4 Hz, the noise depends on the input capacitance: the higher the input capacitance, the earlier and more strongly the noise rises. This is due on the one hand to the reduction of the input impedance of the capacitive current source 41, which causes a higher amplification of the input voltage noise. On the other hand, the dielectric losses of the sensor increasing with frequency effectively reduce its resistance R py . This makes a further contribution to the noise.

[0216] Preferably, the capacitance of the thermoelectric sensor D py can be compensated by a passive structural element having a negative capacitance (see above). According to Figure 13b , the combination of a component having a negative capacitance with the capacitive current source 41 (preferably a thermoelectric sensor D py ) having generally a positive capacitance ensures a further significant reduction of the noise and thus also a significant improvement of the signal - to - noise ratio.

[0217] The ability of the amplifier circuit 1 according to the invention is tested in an optical device when working in cooperation with the thermoelectric sensor D py , i.e., as a sensor system according to the invention (see Figures 14a to 14c ). For this purpose, the sensitivity at a gain of 5 GV / A is measured with a pulsed diode laser with a power of 145 μW in a frequency range up to 100 kHz, and compared with a single - stage TIA known in the prior art with the same gain using the thermoelectric sensor D py . In the case of the same thickness of 30 μm of the thermoelectric sensor D py , the bandwidth of 5 kHz of the four - stage amplifier circuit 1 according to the invention (see Figures 13a to 1 3c) is significantly greater than the bandwidth of 200 Hz of the single - stage TIA (see Figures 13a to 1 3c). In this case, the cut - off frequency of 5 kHz corresponds to the thermal time constant of the thermoelectric sensor D py with a thickness of 30 μm. The amplifier circuit 1 according to the invention has a very high electronic cut - off frequency of 8 kHz in this favorable case of high gain.

[0218] In the case of reducing the thickness of the thermoelectric sensor D py , significantly more signal can be obtained and the bandwidth is further increased up to the electronic bandwidth of the amplifier circuit 1 according to the invention. In this case, due to the reduced heat capacity of the thermoelectric sensor, both the photo - thermal conversion and the thermoelectric conversion are optimized. Figures 14a to 14c The corresponding curves inpy shows this effect. Compared with the thicker thermoelectric sensor D py , in the case of the same amplifier circuit according to the invention, the signal is more than doubled, and the bandwidth is increased up to an electronic bandwidth of 8 kHz.

[0219] As Figure 13b already shown in Figures 14a to 14c , in py , the noise fraction at low frequencies is mainly the Johnson - Nyquist noise of the feedback resistor. As expected, the noise of the sensor system with the four - stage amplifier 1 according to the invention increases with increasing frequency. The absolute value of the noise mainly depends on the capacitance C py of the thermoelectric sensor D py and its loss factor tan(δ). Both of these parameters are larger in the thinner 7 - μm thermoelectric sensor (C py ≈250 pF) than in the 30 - μm thick thermoelectric sensor (C py ≈120 pF). For the selected thickness of the thermoelectric sensor, the signal improvement of the thinner thermoelectric sensor overcompensates for this effect, so that it still has a significantly lower Noise Equivalent Power (NEP) or a better signal - to - noise ratio (see Figure 14c ).

[0220] In summary, the two sensor systems with the four - stage amplifier circuit 1 according to the invention perform significantly better over the entire frequency range, but especially at frequencies above 200 Hz, than the sensor systems known in the prior art. The bandwidth is larger at the same gain, and measurements above 1 kHz can be achieved without problems. Since the four - stage amplifier circuit 1 according to the invention is insensitive to larger input capacitances, thinner thermoelectric crystals can be used in thermoelectric sensors with larger capacitances and larger signals without reducing the bandwidth or causing excessive transmission.

[0221] If the gain is reduced (see Figure 15 ), the bandwidth of the 7 - μm thick thermoelectric sensor can be increased far beyond 8 kHz with a flat transfer function using the four - stage amplifier circuit 1 according to the invention. At a gain of 400 mV / A, the 70 - kHz cut - off frequency of the detector is no longer limited by the electronic bandwidth (90 kHz).

Claims

1. An amplifier circuit (1) for capacitively amplifying a current source, preferably a thermoelectric sensor, in a broadband and low-noise manner, comprising: - A signal input terminal (3) which can be connected to a capacitive current source at node K 1 in - an input stage (A1), The input stage (A1) is connected between the input terminal (7) of the input stage (A1) and node K 1 and has node K at the output terminal (9) of the input stage (A1). 2 , wherein the input stage (A1) is configured to amplify an input voltage by at least a factor of 3, the input stage (A1) is configured to provide a high-ohmic input resistance at the input of the input stage (A1), the input stage (A1) is configured to provide a stable and load-independent voltage at the output of the input stage (A1), - an amplifier cascade, wherein the amplifier cascade (11) has at least a first amplifier and a second amplifier (A2, A3), the first amplifier and the second amplifier each having an input (13, 17) and an output (15, 19), The output terminal (15) of the first amplifier (A2) is connected to the input terminal (17) of the second amplifier (A3) at node K 3 ​ The input terminal (13) of the first amplifier (A2) is connected to node K 2 connected The output terminal (19) of the second amplifier (A3) is connected to node K 4 connected the amplifier cascade (11) is configured to produce a high signal gain with a low phase shift over a wide frequency range, - a feedback network (F1), The feedback network (F1) is connected to the input terminal (7) of the input stage (A1) at node K 1 and is connected to the output terminal (19) of the second amplifier (A3) at node K 4 in which wherein the feedback network (F1) is configured to provide a high-ohmic feedback resistor with a parasitic capacitance of less than 0.5 pF, the feedback network (F1) is configured to provide negative feedback to a structure comprising the input stage (A1) and the amplifier cascade (11), and - Signal output terminal (21), the signal output terminal is connected to node K 5 is connected, node K 5 is connected to node K 4 or corresponds to node K 4 .

2. The amplifier circuit (1) according to claim 1, wherein, The input stage (A1) includes a junction field effect transistor (Q 1 ) and a bipolar transistor (Q 2 ). the drain of the junction field effect transistor (Q1) is connected to the base of the bipolar transistor, The junction field effect transistor (Q 1 ) serves as the source circuit connection wire, The bipolar transistor (Q 2 ) is wired as an emitter follower, - The first and second amplifiers (A2, A3) of the amplifier cascade (11) are operational amplifiers (Q 3 , Q 4 ). The voltage gain of the first operational amplifier (Q 3 ) is greater than 10 4 , The voltage gain of the second operational amplifier (Q 4 ) is at most 10 3 , the feedback network (F1) has a high-ohmic feedback resistor with a parallel capacitor, the feedback network (F1) includes a low-pass filter connected in series.

3. The amplifier circuit (1) according to claim 1 or 2, wherein, the amplifier circuit further has an output stage (A4), The output stage (A4) is connected to node K at the input of the output stage (A4) 4 and is connected to the signal output at the output of the output stage (A4) at node K 5 in the output stage (A4) is configured to amplify a voltage at the input of the output stage by up to a factor of 20, The output stage (A4) is arranged to filter DC interference at the input of the output stage and to adjust the signal level at the output of the output stage (A4) at K 5 therein.

4. The amplifier circuit (1) according to claim 3, wherein, the output stage (A4) includes an inverting band-pass amplifier, and the input of the band-pass amplifier is AC-coupled.

5. The amplifier circuit (1) according to any one of claims 1 to 4, wherein, the amplifier circuit further includes an amplitude limiting device (F2), The amplitude limiting device (F2) is connected between the input end of the amplitude limiting device (F2) and node K 4 or K 5 and between the output end of the amplitude limiting device (F2) and node K 3 and is connected The amplitude limiting device (F2) is arranged to limit the amplitude of the output signal at the signal output when a threshold value at node K 4 or node K 5 is exceeded.

6. The amplifier circuit (1) according to claim 5, wherein, The amplitude limiting device (F2) has two Zener diodes (Q 6 , Q 7 ) connected in series with opposite polarities.

7. The amplifier circuit (1) according to any one of claims 1 to 6, wherein, the amplifier circuit (1) has a frequency response compensation device (F3), The frequency response compensation device (F3) is connected to node K 3 and node K 1 is connected the frequency response compensation device (F3) is configured to reduce the tendency of the amplifier cascade (11) to oscillate.

8. The amplifier circuit (1) according to claim 7, wherein, the frequency response compensation device (F3) has negative feedback via a capacitor.

9. The amplifier circuit (1) according to any one of claims 1 to 8, wherein, the input stage (A1) includes a component having a negative capacitance.

10. The amplifier circuit (1) according to any one of claims 1 to 9, wherein, the amplifier circuit (1) or at least a part of the amplifier circuit (1) is mounted on a circuit board (33), At least one electronic component of the amplifier circuit (1) is soldered to the circuit board (33) by means of pads (37) on the circuit board (33). The area (39) of the circuit board (33) that is outside the pads and beneath the at least one electronic component is removed.

11. The amplifier circuit (1) according to claim 10, wherein, the at least one electronic component is one or more components of the feedback network (F1), or the entire feedback network (F1).

12. The amplifier circuit (1) according to claim 10 or 11, wherein, The at least one electronic component is one or more components of the input stage (A1), preferably the junction field effect transistor (Q 1 ) and / or the bipolar transistor (Q 2 ).

13. The amplifier circuit (1) according to any one of claims 3 to 12, wherein, the amplifier circuit (1) further includes a decoupler (A5), The decoupler (A5) is connected to node K 4 and to the input (23) of the output stage (A4). the decoupler (A5) is arranged to decouple the output of the amplifier cascade (11) from the output stage (A4).

14. The amplifier circuit (1) according to claim 13, wherein, the decoupler (A5) has an impedance converter, preferably a non-inverting impedance converter or a combination of two inverting impedance converters, or the decoupler (A5) is an impedance converter, preferably a non-inverting impedance converter or a combination of two inverting impedance converters.

15. The amplifier circuit (1) according to claim 14, wherein, The impedance converter includes at least one or more of the following components, or is one of the following components: MOSFET, bipolar transistor, operational amplifier (Q 8 ), an operational amplifier (Q 8 ) with a high input resistance and low inherent noise, an operational amplifier (Q 8 ) with a high input resistance and low inherent noise, wherein the output terminal of the operational amplifier (Q 8 ) is directly or indirectly coupled back to its inverting input terminal.

16. The amplifier circuit (1) according to any one of claims 1 to 15, wherein, the amplifier circuit (1) further includes an intermediate load (F4), The intermediate load (F4) is connected at node K 3 to the output terminal (15) of a first amplifier (A2) and to the input terminal (17) of a second amplifier (A3) cascaded with the amplifier (11). the intermediate load (F4) is arranged to minimize the natural oscillation of the amplifier circuit (1).

17. The amplifier circuit (1) according to claim 16, wherein, the intermediate load (F4) is arranged to minimize natural oscillation by phase correction.

18. The amplifier circuit (1) according to claim 16 or 17, wherein, The output terminal (15) of the first amplifier (A2) is connected to node K 3 via a first ohmic resistor (R 17 ). The node K 3 is grounded via a second ohmic resistor (R 15 ).

19. The amplifier circuit (1) according to claim 18, wherein, The intermediate load has at least one or more of the following component combinations or consists of them: an inductor (L 1 ) connected in series to ground and / or a capacitor and at least one ohmic resistor, preferably the second ohmic resistor (R 15 ); an inductor and / or a capacitor connected in parallel to ground and at least one ohmic resistor (R 15 ).

20. A sensor system (43), the sensor system includes a capacitive current source and the amplifier circuit (1) according to any one of claims 1 to 19.

21. The sensor system (43) according to claim 20, wherein, The capacitive current source is a thermoelectric sensor (D py ).

22. The sensor system (43) according to claim 20 or 21, wherein, Thermoelectric sensor (D py ) is implemented in a plate shape and has a maximum thickness of 40 μm, preferably at most 10 μm.

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