Low-frequency low-noise amplifier based on transformation input and feedback

Abstract

The invention relates to a low-frequency low-noise amplifier based on transformation input and feedback, which comprises a transformer coupling signal input circuit, a JFET discrete device amplification circuit, a low-noise amplification circuit, a transformer feedback circuit and a power supply, and is characterized in that the transformer coupling signal input circuit carries out transformation amplification on a signal; the JFET discrete device amplification circuit can provide low voltage noise and low current noise and can provide input capacitance as low as 20pF; the low-noise amplification circuit comprises three operational amplifiers; the transformer feedback circuit can significantly improve the long-term gain stability and the low-frequency passing characteristic of transformer amplification through voltage series negative feedback. And the power supply supplies power to the JFET discrete device amplification circuit. According to the circuit, the voltage noise density smaller than 100 pV / Hz can be achieved, and the long-term stability of the gain of the amplifier can be improved based on the structure fed back by the transformer; the high-pass characteristic of the pre-amplifier is improved; and differential amplification can be realized.

Description

Technical Field

[0001] The present invention relates to the technical field of signal amplification circuits, and in particular to a low-frequency low-noise amplifier based on variable-voltage input and feedback. Background Art

[0002] In weak signal measurement scenarios such as bioelectric signal detection, industrial precision measurement, and environmental monitoring, some low-level, low-source-impedance sensors have important applications. Typical examples include DC SQUIDs (Superconducting Quantum Interference Devices). However, these sensors must be connected to a low-noise, low-impedance preamplifier to exhibit their intrinsic high-resolution characteristics.

[0003] The prior art document with the publication number CN101316095B provides a gain-controllable broadband low-noise amplifier. A state configurator therein is used to generate input and output configuration signals and gain and band-switch control signals to control an input impedance matching circuit, a gain-controllable broadband low-noise amplifier, and an output impedance matching circuit respectively, so as to set appropriate input impedance, amplifier gain, operating bandwidth, and output impedance. The RF input signal can be transmitted to the gain-controllable amplifier with the highest efficiency through the input impedance matching circuit, generating less energy reflection. Then, due to the self-adaptability of the amplifier gain, the amplifier adjusts its own gain according to the amplitude of the input RF signal to make the output voltage stable, ensuring the normal operation of the subsequent circuit. Due to the existence of the output impedance matching circuit, the system output signal can be transmitted to the subsequent circuit with the highest efficiency, ensuring the working efficiency of the circuit. The invention can be applied to wireless transceivers with different impedance antennas and different operating frequency protocols and can perform adaptive gain adjustment.

[0004] Although the above prior art solution can achieve relevant beneficial effects through the structure of the prior art, there are still the following defects: 1. The input signal in the prior art has a large energy loss during transmission, and the signal integrity is damaged, thereby affecting the performance of the entire amplifier. 2. The overall noise control is poor: the existing amplifier may not be able to effectively reduce the overall noise, and the equivalent self-noise level at the input end is relatively high, affecting the signal quality and amplification accuracy. 3. The gain fluctuation is large: the existing transformer amplification technology may lack an effective gain stabilization mechanism and cannot suppress the gain fluctuation caused by factors such as temperature change and component aging, resulting in unstable gain of the amplifier during long-term operation and affecting the amplification effect. The existing amplifier may not be able to achieve differential mode operation and is difficult to effectively suppress common-mode interference. When working in a complex electromagnetic environment, the signal is easily interfered with and the stability is poor. Currently, there is a lack of a precise low-frequency low-noise amplifier on the market.

[0005] In view of this, the present invention proposes a low-frequency low-noise amplifier based on variable-voltage input and feedback. Summary of the Invention

[0006] 1. Technical problems to be solved

[0007] The object of the present invention is to overcome the deficiencies of the prior art and provide a low-frequency low-noise amplifier based on variable-voltage input and feedback to meet the usage requirements for amplifying low-frequency low-noise signals in fields such as superconducting quantum interference devices, bioelectrical signal detection, industrial precision measurement, and environmental monitoring.

[0008] 2. Technical solutions

[0009] To achieve the above object, the present invention adopts the following technical solutions:

[0010] The present invention provides a low-frequency low-noise amplifier based on variable-voltage input and feedback, including a transformer-coupled signal input circuit, a JFET discrete device amplification circuit, a low-noise amplification circuit, a transformer feedback circuit, and a power supply.

[0011] The transformer-coupled signal input circuit includes a pair of impedance-matched transformers. To achieve a low-noise effect, the turns ratio is between 20 and 60, and the equivalent noise can reach 65 pV / √Hz.

[0012] The JFET discrete device amplification circuit is composed of a pair of low-noise JFET discrete devices. The JFET pair can provide low voltage noise and low current noise, and can also provide an input capacitance as low as pF.

[0013] The low-noise amplification circuit includes three operational amplifiers. The first operational amplifier is connected to the JFET discrete device amplification circuit, and the output of the first operational amplifier is connected to the second operational amplifier for further amplification. The output of the second operational amplifier is AC-coupled to the third operational amplifier; there are a capacitor C1, a resistor R5, a resistor R6, and a capacitor C2 connected between the low-noise operational amplifier A2 and the low-noise operational amplifier A3; the resistor R5, the resistor R6, and the capacitor C2 ensure the stability of the preamplifier at high frequencies.

[0014] The transformer feedback circuit can significantly improve the long-term gain stability and low-frequency pass characteristics of the transformer amplification through voltage series negative feedback. The transformer feedback circuit is connected to a resistor R A and a resistor R B， The resistor R A and a resistor R B together with the feedback transformer determine the gain of the preamplifier. The transformer feedback circuit enables the entire transformer to handle low-frequency signals as low as 1 Hz (currently 5 Hz in China).

[0015] The power supply provides power for the JFET discrete device amplification circuit.

[0016] 3. Beneficial Effects: One or more technical solutions provided in the embodiments of the present application have at least the following technical effects or advantages:

[0017] 1. In the present invention, a pair of impedance-matched transformers are used to achieve efficient coupling between the signal source and the subsequent amplification circuit. Impedance matching can minimize energy loss during the transmission of the input signal, ensuring signal integrity and laying the foundation for the high-performance performance of the entire amplifier.

[0018] 2. The overall noise can be effectively reduced. When selecting an appropriate turns ratio (such as between 20 - 60) and combining with the noise level of a low-noise operational amplifier, the equivalent self-noise at the input end of the preamplifier can reach 65 pV / √Hz.

[0019] 3. It can provide low voltage noise and low current noise, effectively avoiding the introduction of noise in the primary stage of signal amplification and ensuring signal purity. The JFET pair provides an input capacitance as low as the pF level. The smaller input capacitance can reduce signal distortion, enabling the input signal to be processed more accurately by the amplification circuit and contributing to improving the overall performance of the amplifier.

[0020] 4. Through the voltage series negative feedback mechanism, the long-term gain stability of the transformer amplification is significantly improved, effectively suppressing gain fluctuations caused by factors such as temperature changes and component aging, enabling the amplifier to always maintain stable amplification performance. It greatly improves the low-frequency passing characteristics, allowing low-frequency signals to pass through the amplification circuit more effectively and reducing signal attenuation and distortion in the low-frequency band. At the same time, the feedback transformer can perform special processing and compensation on low-frequency signals, further optimizing the amplification effect of low-frequency signals.

[0021] 5. Enhanced anti-interference ability: The preamplifier is enabled to operate in differential mode, which can effectively suppress common-mode interference, improve the anti-interference ability of the signal, and enable the amplifier to operate stably in a complex electromagnetic environment. The feedback transformer provides an equivalent feedback resistance as low as possible, which can reduce the introduction of high-frequency noise and enable the amplifier to maintain good noise performance in the high-frequency band. The input transformer and the feedback transformer have similar characteristics, generating unit gain to ensure the consistency of the signal during input and feedback processes and enhancing the overall performance of the amplifier.

[0022] 6. Three low-noise operational amplifiers are used for multi-stage amplification. Combining with the low-noise characteristics of the JFET pair, the signal is further amplified while ensuring a low-noise level. The stability of the preamplifier at high frequencies is ensured, ensuring the stable operation of the entire amplification circuit. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 : Overall system block diagram of the preamplifier based on a transformer;

[0024] Figure 2:Detailed description diagram of the transformer-based preamplifier;

[0025] Figure 3 :System block diagram of transformer input and transformer feedback;

[0026] Figure 4 :Noise source diagram of transformer input and transformer feedback;

[0027] Figure 5 :Schematic diagram of the transformer-based preamplifier based on transformer feedback. Specific implementation manners

[0028] The specific implementation manners of the present invention will be further described in detail below with reference to the accompanying drawings.

[0029] Refer to Figures 1 - 5 ,The present invention provides a low-frequency and low-noise amplifier based on transformer input and feedback, including a transformer-coupled signal input circuit, a JFET discrete device amplification circuit, a low-noise amplification circuit, a transformer feedback circuit, and a power supply;

[0030] The transformer-coupled signal input circuit includes a pair of impedance-matched transformers. These two transformers play a crucial role in the entire amplifier system. They can achieve efficient coupling between the signal source and the subsequent amplification circuit. Through impedance matching, the input signal can minimize energy loss during transmission, ensure the integrity of the signal, and transmit the signal to the subsequent circuit module in the best state, laying the foundation for the high-performance performance of the entire amplifier. In order to achieve the low-noise effect, it is necessary to determine the turns ratio of the transformer. The derivation process is as follows:

[0031] As Figure 1 and Figure 2 shown, in the preamplifier, e S is the signal source, R S is the source resistance of the signal source, N F is the turns ratio of the input transformer, R O is the feedback equivalent resistance, is the resistance of the primary winding of the signal input transformer, the self-inductance of the primary winding of the signal input transformer, is the self-inductance of the secondary winding of the signal input transformer, is the resistance of the secondary winding of the signal input transformer, V O is the output signal, where V O , i A and e S are three independent variables. The transfer function of the preamplifier is shown in formula (1). When the voltage noise and current noise of amplifier A are zero, the transfer function of the preamplifier is a first-order high-pass filter. The gain of the preamplifier V O / e S Only depends on the feedback coefficient H O .

[0032]

[0033] Figure 2 A detailed description of the circuit is given, and the transfer function and noise characteristics of the circuit can be calculated. From formula (1), the equivalent input noise of the preamplifier can be expressed as formula (2). Where and are the thermal noises of the source resistance R S , the resistance of the secondary winding of the transformer the resistance of the primary winding of the transformer and the feedback resistance R O , and are the voltage noise and current noise of the amplifier A.

[0034]

[0035] When the input terminal is short-circuited, formula (2) can be simplified to formula (3):

[0036]

[0037] The input current noise of the preamplifier is:[[]]

[0038]

[0039] The input voltage noise and current noise are and equations of variables. There is a certain correlation among the four variables, but this correlation can be ignored under the optimal source impedance. At extremely low winding resistances, formulas (3) and (4) can be simplified to formulas (5) and (6)

[0040]

[0041] In the case where there is no energy loss in the transformer, that is the optimal matching source impedance of the preamplifier is only 1 / N of the optimal matching source impedance of the amplifier A F 2 . To reduce the overall noise, the feedback equivalent resistance R O should be as small as possible. Under the condition of selecting a suitable magnetic core, the energy loss of the transformer can be ignored. For a given winding cross-sectional area S, the resistivity ρ of the wire, the average winding length l, and when the cross-sectional areas of the primary and secondary windings are equal, the equivalent resistance of the transformer can be expressed as formula (7), where K is the filling coefficient of the transformer is the number of turns of the primary winding of the transformer.

[0042]

[0043] The high-pass cut-off frequency of the preamplifier is as shown in Equation (8):

[0044]

[0045] In order to make the cut-off frequency of the low frequency lower, a transformer with a high self-inductance of the primary winding should be selected.

[0046] However, high self-inductance means high resistance, which increases thermal noise. Therefore, the noise problem needs to be considered comprehensively, that is, the thermal noise caused by the transformer resistance should be comparable to the equivalent voltage noise level at which the amplifier A is coupled to the input terminal In practice, other noise terms in Equation (6) can be ignored. From Equations (1) to (8), Equation (9) can be obtained:

[0047]

[0048] When N F is selected between 20 and 60, combined with the current noise level of low-noise operational amplifiers, the equivalent input self-noise of the preamplifier can reach 65 pV / √Hz;

[0049] Preferably, the turns ratio of the preamplifier N F is 30, the number of turns of the primary coil is 10, the copper wire diameter is 1 mm, the resistance is 34 mΩ, and the inductance is 575 μH; the number of turns of the secondary coil is 300, the copper wire diameter is 0.12 mm, the resistance is 47 Ω, and the inductance is 515 mH.

[0050] The JFET discrete device amplifier circuit is composed of a pair of low-noise JFET discrete devices. The JFET pair can provide low voltage noise and low current noise, and can provide an input capacitance as low as pF; the JFET pair can provide extremely low voltage noise and low current noise. During the signal amplification process, the introduction of noise will seriously affect the signal quality, and the low-noise characteristics of the JFET pair can effectively avoid this problem, ensuring that the signal can maintain a high purity at the primary stage of amplification. At the same time, the JFET pair can also provide an input capacitance as low as the pF level. The smaller input capacitance can reduce signal distortion, enabling the input signal to be processed more accurately by the amplifier circuit, which helps to improve the overall performance of the amplifier. The JFET discrete device amplifier circuit can provide an input capacitance as low as 20 pF (the current lowest in China is 100 pF).

[0051] such as Figure 3As shown, the transformer feedback circuit can significantly improve the long-term gain stability and low-frequency pass characteristics of transformer amplification through voltage series negative feedback. The transformer feedback circuit can not only make the preamplifier operate in differential mode, and due to the introduced feedback transformer N R << 1, the equivalent resistance of the resistor r in the feedback loop can be reduced to the mΩ level. At the same time, the feedback transformer in the feedback loop can also improve the low-frequency pass characteristics of the circuit.

[0052] Through the voltage series negative feedback mechanism, the transformer feedback circuit can significantly improve the long-term gain stability and low-frequency pass characteristics of transformer amplification. During long-term operation, the gain stability of the amplifier is crucial. Voltage series negative feedback can effectively suppress gain fluctuations caused by factors such as temperature changes and component aging, enabling the amplifier to always maintain stable amplification performance. In terms of low-frequency signal processing, it can greatly improve the low-frequency pass characteristics, allowing low-frequency signals to pass through the amplification circuit more effectively and reducing signal attenuation and distortion in the low-frequency band.

[0053] The transformer feedback circuit can not only make the preamplifier operate in differential mode. The differential mode can effectively suppress common-mode interference and improve the anti-interference ability of the signal, enabling the amplifier to operate stably in a complex electromagnetic environment. And due to the introduced feedback transformer turns ratio << 1, the equivalent resistance of the resistor r in the feedback loop can be reduced to the mΩ level. The lower equivalent resistance can reduce the power loss of the signal in the feedback loop and improve the feedback efficiency. At the same time, the feedback transformer in the feedback loop can also improve the low-frequency pass characteristics of the circuit. It can perform special processing and compensation on low-frequency signals to further optimize the amplification effect of low-frequency signals.

[0054] To obtain better noise performance at high frequencies, the feedback transformer should provide the lowest possible equivalent feedback resistance. The low equivalent feedback resistance can reduce the introduction of high-frequency noise, enabling the amplifier to maintain good noise performance in the high-frequency band. In addition, the input transformer and the feedback transformer should have similar characteristics, which enables the generation of unit gain, ensuring the consistency of the signal during input and feedback processes and further improving the overall performance of the amplifier.

[0055] Preferably, the turns ratio of the feedback transformer is 1 / 20, the number of turns of the primary coil is 160, the copper wire diameter: 0.25 mm, the resistance: 3.8 Ω, the inductance: 180 mH, the number of turns of the secondary coil is 8, the copper wire diameter: 1.6 mm, the resistance: 8 mΩ, the inductance: 465 μH.

[0056] Refer to Figure 4 , the input-output transfer function can be expressed by formula (10):

[0057]

[0058] As can be seen from Equation (10), the transformer preamplifier based on transformer feedback is a band-pass filter, and since within the required range of a few Hz to 100 kHz, its band-pass characteristics can be completely ignored. Secondly, when , at a source impedance lower than the optimum, the preamplifier gain will have a peak.

[0059] Since transformer amplification is adopted, the equivalent noise analysis is discussed from two aspects: high frequency and low frequency. The noise at high frequency and the noise at low frequency are shown in Equations (11) and (12).

[0060]

[0061] Equation (11) shows that in order to obtain better noise performance at high frequency, the feedback transformer should provide the lowest possible equivalent feedback resistance, and the input transformer and the feedback transformer should have similar characteristics, which makes N R N F produce unit gain. The feedback loop introduces N R 2 e nr 2 noise to the preamplifier.

[0062] The power supply provides power for the JFET discrete device amplifier circuit. A stable power supply is crucial for the normal operation of the JFET discrete device amplifier circuit. It can ensure that the JFET discrete device operates at an appropriate working voltage, guaranteeing the stable performance of its low noise, high gain, etc. If the power supply is unstable, it will directly affect the working state of the JFET discrete device, and then lead to a decline in the performance of the entire amplifier, such as increased noise, unstable gain, etc.

[0063] Such as Figure 5As shown, the low-noise amplifier circuit includes three operational amplifiers (low-noise operational amplifier A1, low-noise operational amplifier A2, and low-noise operational amplifier A3). After the signal is amplified by a transformer, it is amplified using a pair of low-noise JFET discrete devices. The JFET pair can provide low voltage noise and low current noise, and can also provide an input capacitance as low as pF. The output of the JFET is connected to the low-noise operational amplifier A1. The gain of the JFET and the low-noise operational amplifier A1 is determined by the resistor R1 and the resistor R2. The output of the low-noise operational amplifier A1 is connected to the low-noise operational amplifier A2 for further amplification. The output of the low-noise operational amplifier A2 is AC-coupled to the low-noise operational amplifier A3 buffer to make the DC current in the feedback loop zero. A capacitor C1, a resistor R5, a resistor R6, and a capacitor C2 are connected between the low-noise operational amplifier A2 and the low-noise operational amplifier A3; a capacitor C1 is connected to one side of the low-noise operational amplifier A2, a resistor R6 is connected to one side of the capacitor C1, and the resistor R6 is connected to the low-noise operational amplifier A3; a resistor R6 and a capacitor C2 are connected between the resistor R6 and the low-noise operational amplifier A3; the resistor R5, the resistor R6, and the capacitor C2 ensure the stability of the preamplifier at high frequencies. The transformer feedback circuit is connected to a resistor R A and a resistor R B , the resistor R B is a variable resistor; the resistor R A and the resistor R B together with the feedback transformer determine the gain of the preamplifier.

[0064] The present invention realizes the efficient coupling between a signal source and a subsequent amplification circuit through a pair of impedance-matched transformers. Impedance matching can minimize energy loss during the transmission of input signals, ensure signal integrity, and lay a foundation for the high-performance performance of the entire amplifier. It can effectively reduce the overall noise. When selecting an appropriate turns ratio and combining with the noise level of a low-noise operational amplifier, the equivalent self-noise at the input terminal of the preamplifier can reach 65 pV / √Hz. It can provide low voltage noise and low current noise, effectively avoiding the introduction of noise in the primary stage of signal amplification and ensuring the purity of the signal. The JFET pair provides an input capacitance as low as the pF level. The smaller input capacitance can reduce signal distortion, enabling the input signal to be processed more accurately by the amplification circuit and contributing to improving the overall performance of the amplifier. Through the voltage series negative feedback mechanism, the long-term gain stability of the transformer amplification is significantly improved, effectively suppressing gain fluctuations caused by factors such as temperature changes and component aging, and enabling the amplifier to always maintain stable amplification performance. It greatly improves the low-frequency passing characteristics, allowing low-frequency signals to pass through the amplification circuit more effectively and reducing signal attenuation and distortion in the low-frequency band. At the same time, the feedback transformer can perform special processing and compensation on low-frequency signals, further optimizing the amplification effect of low-frequency signals. The feedback transformer provides an equivalent feedback resistance as low as possible, which can reduce the introduction of high-frequency noise and enable the amplifier to maintain good noise performance in the high-frequency band. The input transformer and the feedback transformer have similar characteristics, generating unit gain, ensuring the consistency of the signal during input and feedback, and enhancing the overall performance of the amplifier. Three low-noise operational amplifiers are used for multi-stage amplification. Combining with the low-noise characteristics of the JFET pair, the signal is further amplified and the low-noise level is ensured. Ensure the stability of the preamplifier at high frequencies and ensure the stable operation of the entire amplification circuit.

[0065] The above is the implementation mode of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, various modifications and improvements can be made, which should also be regarded as the protection scope of the present invention.

Claims

1. A low-frequency low-noise amplifier based on voltage-variable input and feedback, comprising: Transformer coupling signal input circuit, JFET discrete device amplifier circuit, low noise amplifier circuit, transformer feedback circuit and power supply: It is characterized by: The transformer-coupled signal input circuit performs voltage transformation and amplification on the signal; The JFET discrete device amplifier circuit provides low voltage noise and low current noise, and provides an input capacitance as low as pF; The low-noise amplifier circuit comprises three operational amplifiers, the first operational amplifier is connected to the JFET discrete device amplifier circuit, the output of the first operational amplifier is connected to the second operational amplifier for further amplification, and the output of the second operational amplifier is connected to the third operational amplifier through AC coupling; The transformer feedback circuit improves the long-term gain stability and low-frequency pass characteristics of transformer amplification through voltage series negative feedback; The power supply provides power for the JFET discrete device amplifier circuit.

2. A low-frequency low-noise amplifier based on voltage transformation input and feedback according to claim 1, characterized in that: The transformer-coupled signal input circuit includes a pair of impedance-matched transformers, the transformer turns ratio is between 20 and 60, and the equivalent noise reaches 65 pV / √Hz.

3. A low-frequency low-noise amplifier based on voltage transformation input and feedback according to claim 1, characterized in that: The JFET discrete device amplifier circuit is composed of a pair of low-noise JFET discrete devices. The JFET pair of tubes provides low voltage noise and low current noise, and provides an input capacitance as low as pF.

4. A low-frequency low-noise amplifier based on voltage transformation input and feedback according to claim 1, characterized in that: The low-noise amplifier circuit includes three operational amplifiers, the first operational amplifier is connected to the JFET discrete device amplifier circuit, the output of the first operational amplifier is connected to the second operational amplifier for further amplification, and the output of the second operational amplifier is connected to the third operational amplifier through AC coupling; a capacitor C1, a resistor R5, a resistor R6 and a capacitor C2 are connected between the second operational amplifier and the third operational amplifier; the resistor R5, the resistor R6 and the capacitor C2 ensure the stability of the preamplifier at high frequencies.

5. A low-frequency low-noise amplifier based on voltage transformation input and feedback according to claim 1, characterized in that: The transformer feedback circuit significantly improves the long-term gain stability and low-frequency pass characteristics of transformer amplification through voltage series negative feedback. A resistor R is connected to one side of the transformer feedback circuit. A and resistor R B , resistor R B is a variable resistor; resistance R A and resistor R B Together with the feedback transformer it determines the gain of the preamplifier.

6. A low-frequency low-noise amplifier based on voltage transformation input and feedback according to claim 2, characterized in that: The impedance matching transformer has a turns ratio of 30, a primary coil turns of 10, a copper wire diameter of 1 mm, a resistance of 34 mΩ, and an inductance of 575 uH; a secondary coil turns of 300, a copper wire diameter of 0.12 mm, a resistance of 47 Ω, and an inductance of 515 mH.

7. A low-frequency low-noise amplifier based on voltage transformation input and feedback according to claim 5, characterized in that: The turns ratio of the feedback transformer is 1 / 20, the primary coil has 160 turns, the copper wire diameter is 0.25mm, the resistance is 3.8Ω, the inductance is 180mH, the secondary coil has 8 turns, the copper wire diameter is 1.6mm, the resistance is 8mΩ, and the inductance is 465uH.

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