AMPLIFIER DEVICE FOR AMPLIFYING SMALL CURRENTS

Description

[0001] The invention relates to a device and a method for amplifying small currents.

[0002] In many measuring instruments, such as mass spectrometers or vacuum gauges, measurements are based on currents generated by positively or negatively charged particles striking an electrode. The dynamic range of the ion and electron currents that occur and need to be measured is very large. It often extends from a few hundred attoamperes to a few microamperes, thus spanning several decades.

[0003] Small currents are particularly important to measure in vacuum gauges, in the lower pressure range, and in leak detectors. Mass spectrometers are frequently used for this purpose. When designing an amplifier capable of measuring larger currents for very small currents below 10 fA, circuit design difficulties arise if switching between measuring small and larger currents is necessary. Current amplifiers using a resistor must employ a high-value resistor for very small currents because the current noise of the resistor decreases with increasing resistance. However, for currents larger than, for example, 10 pA, the high-value resistor cannot be used because the voltage drop across the resistor becomes too large. Using capacitors as reference components presents similar challenges to those encountered with resistors.In state-of-the-art amplifiers, semiconductor switches or relays are used to switch resistors or current paths, enabling switching from an operating state for measuring small currents to an operating state for measuring larger currents. In all cases, this requires additional components that are connected to the sensitive input node where minute currents flow and must be measured.

[0004] To measure such particle currents, measuring devices are used that generate a measurable measuring potential from the particle current, i.e., ion current or electron current. The measuring device typically includes a current amplifier, usually an operational amplifier. A measuring resistor, which is placed, for example, in the feedback path of the operational amplifier, generates a measurable electrical potential from the amplified current.

[0005] To cover the entire dynamic range of the particle currents encountered with the same measuring circuit, both conventional and the amplifier according to the invention are designed to switch between a sensitive mode for measuring small currents of less than 100 pA (e.g., 1 pA) and a mode for measuring larger currents of at least 1 pA (e.g., 100 pA). In conventional current amplifiers, this switching is achieved using a relay, for example, in sector-field mass spectrometers. For resistor-based current amplifiers covering a large current range, range switching is essential, since only high-resistance resistors are suitable for small currents, and only low-resistance components can handle the larger current at higher currents. At small currents, the resistance noise limits the operating range.

[0006] EP 0 615 669 B1 describes an amplifier in which the current signal is transmitted using diodes. JP 2010 085384 A (OZAWA FUJIO) 15 April 2010 (2010-04-15) discloses a current measuring device with switchable measuring ranges and a protective device at the input. WO 2009 / 000236 A2 (FORSCHUNGSZENTRUM JUELICH GMBH [DE] ET AL.) 31 December 2008 (2008-12-31) discloses a current measuring device with automatically switchable measuring ranges.

[0007] The invention is based on the objective of creating an amplifier device for amplifying small currents, with which the switching to amplifying larger currents is improved, and of providing a corresponding method.

[0008] The amplifier device according to the invention is defined by independent claim 1.

[0009] According to the invention, the amplifier device has a first current path for amplifying small currents. The first current path includes an input amplifier device with at least one amplifier having at least one protection element, e.g., a protection diode, and a feedback element in a feedback path connecting the output of the input amplifier device to the inverting input. For measuring larger currents, a second current path, which differs at least partially from the first current path, is provided and configured. At least one of the protection elements of the first amplifier is included in the second current path.

[0010] Thus, according to the invention, an amplifier device is provided at whose input no additional components are required for measuring larger currents beyond those required for measuring very small currents, namely the current input, the measuring resistor in the form of the feedback element, and the input amplifier device with at least the first amplifier. Conventionally used input amplifiers already have protective elements in the input section, e.g., protection diodes, which are required in the integrated circuit to protect the input circuits. According to the invention, these protective elements are used for range switching. This ensures that the most sensitive section of the circuit for measuring very small currents is as stable as if no other sections for measuring other current quantities were present.When switching from the sensitive range to the range for measuring higher currents, the input current flows through one or more of the protection elements, thus making the input range low-impedance even for significantly higher input currents. Since the integrated circuit of the first amplifier must contain protection elements, not every amplifier module can be used. Both amplifier modules with protection elements for the supply voltages and amplifier modules with protection elements between the input lines can be used.

[0011] The input amplifier device can have a number n amplifiers, where n ≥ 1. In the simplest case of n = 1, the input amplifier device includes only the first amplifier. In the case of n ≥ 2, the input amplifier device can consist of at least two amplifiers forming an amplifier combination.

[0012] The input amplifier device has an inverting input and an output. The output is connected to the inverting input of the input amplifier device via a feedback path. A feedback element is included in the feedback path.

[0013] The amplifiers can be operational amplifiers and / or the protective elements can be protection diodes.

[0014] The following section provides a more detailed explanation of exemplary embodiments of the invention with reference to the figures. The figures show: Fig. 1 shows an embodiment in a first operating state, Fig. 2 a detail from Fig. 1 Fig. 3 shows the embodiment in a second operating state and Fig. 4 shows a detail from Fig. 1 or Fig. 3 according to a second embodiment.

[0015] The amplifier device of the illustrated embodiment has a first operational amplifier 1 which has an inverting input 30, a non-inverting input 31, an output 34, a positive supply terminal 32 and a negative supply terminal 33.

[0016] The operational amplifier includes protective elements 18, 19, 20, 21, in this case diodes, which are arranged in a bridge rectifier circuit between the inverting input 30 and the non-inverting input 31, as well as the two supply terminals 32, 33, and connect them to each other. The diodes 18, 19, 20, 21 are arranged such that they conduct from the negative supply terminal 33 towards the positive supply terminal 32 and block current in the opposite direction.

[0017] The expression "in the manner of a bridge rectifier circuit" means that the bridge branch includes the two inputs 30 and 31 of operational amplifier 1. This is based on the idea that, in an ideal operational amplifier, there is no voltage drop between the two inputs 30 and 31. One end of the bridge branch is connected to the inverting input 30, and the other end to the non-inverting input 31. The differential voltage between the two inputs, which is zero in an ideal operational amplifier, is therefore part of the bridge branch.

[0018] The output 34 of the first operational amplifier 1 is connected to the non-inverting input 35 of a second operational amplifier 3, whose output is connected via a feedback path 41 to the inverting input 30 of the first operational amplifier 1. The feedback path 41 contains two resistors 2 and 4, between which the feedback path 41 is connected via a third switch 12 to an input 22, at which a voltage corresponding to the diode forward voltage of approximately 0.5 volts is present.

[0019] The output 16 of the second operational amplifier 3 is further connected to ground via control elements 5. The control elements 5 consist of a resistor 5a, a capacitor 5b, and another resistor 5c. The inverting input 36 of the second operational amplifier 3 is electrically connected between the capacitor 5b and the other resistor 5c for its feedback.

[0020] The output 34 of the first operational amplifier 1 is connected via a first resistor 6a to the positive supply terminal 32 of the first operational amplifier 1 and via a second resistor 6b to the negative supply terminal 33. The negative supply terminal 33 can be connected via a first switch 10 alternately to a supply voltage of -5 volts applied to the voltage source 14 or to ground.

[0021] The positive supply terminal 32 of the first operational amplifier 1 is electrically connected to the inverting input 38 of a third operational amplifier 7, whose output 17 is fed back via a resistor 8. The resistor 8 is bridged by a diode 9, which conducts from the output 17 of the third operational amplifier 7 to the positive supply terminal 32 of the first operational amplifier 1 and blocks conduction in the opposite direction.

[0022] The first amplifier 1 and the second amplifier 3 form an input amplifier device 50, wherein the two amplifiers 1, 3 form an amplifier combination. The feedback path 41 connects the output of the input amplifier combination 50 to its inverting input 30.

[0023] The non-inverting input 37 of the third operational amplifier 7 can be connected via a second electrical switch 11 either to a voltage source 15, in this case 5 volts, or to ground.

[0024] In the illustrated embodiment of the amplifier device according to the invention, the first operational amplifier 1 is used as the input amplifier, its protection elements 18, 19, 20, 21 being provided as input protection diodes for the supply voltage. When the amplifier device is operated in its most sensitive range for measuring small currents, e.g., less than 100 pA, the input current signal reaches the first operational amplifier 1 via input 13. The first operational amplifier 1 receives its negative supply voltage from the voltage source 14 across the first switch 10 in the Fig. 1 in the switch position shown. The operational amplifier 1 receives its positive supply voltage from the voltage source 15, in this case +5 volts, via the second switch 11 in the Fig. 1 The switch position shown is at the non-inverting input 37 of the third operational amplifier 7.

[0025] The output voltage at output 17 of the third operational amplifier 7 settles to a value increased by the forward voltage of diode 9. This makes the voltage reaching the first operational amplifier 1 equal to the voltage at the non-inverting input 37 of the third operational amplifier 7.

[0026] Amplifier 7 is not part of the input amplifier device 50. Amplifier 7 serves to accept larger currents at the input 13 of the input amplifier device 50. The input current flows via diode 18 through amplifier 1 directly to the input 38 of amplifier 7. Smaller currents at input 13, on the other hand, are first amplified by the input amplifier device 50.

[0027] The third electrical switch 12 is in this one. Fig. 1 The operating state shown is open for measuring small currents. The feedback resistors 4, 2 together form the feedback element, in this case in the form of an electrical resistor.

[0028] In this case, input 13 at the inverting input 30 of the first operational amplifier 1 represents a virtual zero point, meaning that no significant voltage is present at input 13.

[0029] An inverted voltage proportional to the input current of the first operational amplifier 1 is applied to output 16 of the second operational amplifier 3. The protection diodes 18, 19, 20, 21 of the input amplifier are now each operated in reverse bias and therefore carry no significant current.

[0030] The input amplifier device 50, consisting of the first operational amplifier 1 and the second operational amplifier 3, forms a control loop. The control elements 5 must be dimensioned according to the cutoff frequency of the elements 5a, 5b, 5c used, so that the loop remains stable, for example 120 kΩ across the electrical resistor 5a, 1 nF across the capacitor 5b and 10 kΩ across the electrical resistor 5c.

[0031] Fig. 1 shows the entire amplifier device, while Fig. 2 The first operational amplifier 1 and the protective elements 18, 19, 20, 21 arranged therein and their electrical interconnection with the terminals of the first operational amplifier 1 are shown in detail.

[0032] Fig. 3 Figure 1 shows the amplifier device in its operating state for measuring larger currents via the second current path. For this purpose, the amplifier device is operated in an extended current range in which the first electrical switch 10 is switched to ground potential. This results in a voltage of approximately 0 volts at the negative supply terminal 33 of the input amplifier. The input protection elements 20 and 21 remain blocked.

[0033] The second switch 11 is also connected to ground potential. Since the third operational amplifier 7, with its feedback resistor 8, regulates itself so that there is no significant differential voltage between the inverting input 38 and the non-inverting input 37 of the third operational amplifier 7, the positive supply of the first operational amplifier 1 at the positive supply terminal 32 will also be close to 0 volts. If an input current now enters the input terminal 13, it flows through the protection diode 18 to the positive supply terminal 32.

[0034] The third electrical switch 12 is closed in this operating state, so that the voltage applied to terminal 22, in this case 0.5 volts, is applied to the feedback resistor 2 according to the diode forward voltage of the protection diode 18, so that as little current as possible flows through the feedback resistor 2.

[0035] The second current path for larger currents at input 13 now leads via the protection diode 18 to the feedback resistor 8 of the third operational amplifier 7. An inverted voltage proportional to the input current is applied across the feedback resistor 8, which can be tapped at the output 17 of the third operational amplifier 7. The diode 9 is reverse-biased in this case. The second current path for measuring larger currents then leads from the inverting input 30 of the first operational amplifier 1 via a protection diode 18 (one of the protection diodes 18, 19, 20, 21) to the positive supply terminal 32 of the first operational amplifier 1, and from there via the feedback resistor 8 of the third operational amplifier 7 to its output terminal 17.

[0036] In the described circuit arrangement, the input voltage 13 is increased by the forward voltage of the protective element 18 when operating in the current path for higher currents. To avoid this characteristic, the ground points at switches 10 and 11 can be reduced by the forward voltage of the protective element 18. In this case, the feedback element 2 is de-energized when 220 volts are applied to the input.

[0037] A further improvement of the second current path occurs when diode 9 is moved to... Fig. 4 The current is split. If the second current path is activated, a negative voltage is present at output 17. The reverse current of the diode distorts the measurement. If the circuit is after Fig. 4 When the voltage is added, the negative voltage drops across diode 9b. At diode 9a, there is then approximately zero volts on both sides, which significantly reduces the reverse current.

[0038] The circuit for positive input currents has been described here. In principle, a circuit variant for negative currents, or even for both directions, is possible.

Claims

1. An amplifier device for receiving currents, comprising a first current path for measuring smaller currents of less than 100 pA, an input amplifier device (50) included in the first current path, comprising at least one first amplifier (1), an output, an inverting input (30), a non-inverting input (31), a first feedback path (41) connecting the output to the inverting input (30), and a feedback element (2) included in the first feedback path (41), wherein the first amplifier (1) has at least one protective element (18, 19, 20, 21), a second current path for measuring larger currents with a maximum current of at least 10 times the current to be received in the first current path, characterized in that, at least one of the protective elements (18, 19, 20, 21) of the input amplifier device (50) is part of the second current path so that when changing the measuring area from the measurement of small currents in the first current path to the measurement of large currents in the second current path, the input current flows through the at least one protective element, wherein a negative supply connection (33) of the first amplifier (1) is alternately connectible via first switch (10) to a negative supply voltage applied to a voltage source (14) or to ground for switching between the first and the second current path.

2. The amplifier device according to claim 1, characterized in that the supply voltage of the first amplifier (1) is applied to at least one of the protective elements (18, 19, 20, 21).

3. The amplifier device according to any one of the preceding claims, characterized in that at least one of the protective elements (18, 19, 20, 21) is arranged between two inputs (30, 31) of the first amplifier (1) and connected thereto.

4. The amplifier device according to any one of the preceding claims, characterized in that the feedback element (2) in the first feedback path (41) is an electrical resistor.

5. The amplifier device according to any one of the preceding claims, characterized in that the feedback component (2) in the first feedback path (41) is a capacitor.

6. The amplifier device according to any one of the preceding claims, characterized in that the input amplifier device (50) comprises at least one further amplifier (3) forming a control circuit with the first amplifier (1).

7. The amplifier device according to any one of the preceding claims, characterized in that at least one of the two supply connections (32, 33) of the first amplifier (1) is connected via an electrical resistor (6a, 6b) to the output of the first amplifier (1).

8. The amplifier device according to any one of the preceding claims, characterized in that the feedback path (41) connects the output of the last amplifier of the input amplifier device (50) consisting of the first amplifier (1) and the at least one further amplifier (3) to the inverting input (30) of the input amplifier device (50).

9. The amplifier device according to any one of the preceding claims, characterized in that the negative supply connection (33) of the first amplifier (1) is selectively connectible via a first switch (10) to a negative supply voltage (14) or to ground, while the positive supply connection (32) of the first amplifier (1) is connected in an electrically conductive manner to the inverting connection (38) of a third amplifier (7), the non-inverting input (37) of which is selectively connectible via a second switch (11) to a positive supply voltage (15) or to ground.

10. The amplifier device according to any one of the preceding claims, characterized in that the third amplifier (7) comprises a second feedback path (43) with an electrical resistor (8) and an electric diode (9) arranged in parallel with the resistor (8) and conducting in the direction from the output (17) of the third amplifier to the positive supply connection (32) of the first amplifier (1) and blocking in the opposite direction.

11. The amplifier device according to any one of the preceding claims, characterized in that the first feedback path (41) is selectively connectible via a third switch (12) to a voltage which avoids or allows the current flow in the feedback element (2) depending on the switch position.

12. The amplifier device according to any one of the preceding claims, characterized in that the ground references of the switches (10, 11) and the reference voltage (22) are changed by the forward voltage of the protective element (18).

13. The amplifier device according to any one of the preceding claims, characterized in that the second feedback path (43) comprises two diodes (9a, 9b) between which a resistor (39) leads with respect to the switch (11).

14. A gas detector comprising a mass spectrometric sensor or a total pressure sensor and an amplifier device according to any one of the preceding claims.

15. A method for measuring currents by using an amplifier device according to any one of claims 1-13, characterized in that switching from the first current path to the second current path takes place by connecting the negative supply connection (33) of the first amplifier (1) via the first switch (10) to ground.

16. The method according to any one of the preceding claims, characterized in that, for switching the current path, the non-inverting input of a third amplifier (7), whose inverting input is connected to the positive supply connection (32) of the first amplifier (1), is additionally connected to ground.