Charge amplifier circuit

Abstract

This invention provides a charge amplifier circuit that suppresses the dark current of the photodiode provided in the charge amplifier circuit while also suppressing an increase in the difficulty of circuit design and device selection. [Solution] The charge amplifier circuit 1 comprises a photodiode 11, an amplifier 12, a feedback capacitor 31, a state switch 21, an adjustment switch 22, and an adjustment capacitor 32. The state switch 21 is arranged in parallel with the feedback capacitor 31 and switches between a state in which charge is charged to the feedback capacitor 31 and a state in which the charge is reset. The adjustment switch 22 is arranged in parallel with the photodiode 11. The adjustment capacitor 32 holds a charge necessary to bring the photodiode 11 to a zero-bias state when the adjustment switch 22 is conducting, and can hold a charge at least when the adjustment switch 22 is not conducting.

Description

Technical Field

[0001] The present invention relates to a charge amplifier circuit.

Background Art

[0002] Patent Document 1 discloses a charge amplifier circuit. A charge amplifier circuit is a circuit that converts a minute charge (or a current which is a flow of charge) due to an output of a sensor or the like into a voltage. As a basic configuration, the charge amplifier circuit includes an amplifier, a capacitor, and a switch. The minute charge output from a sensor or the like is input to the amplifier. The capacitor is disposed in the feedback circuit of the amplifier and charges the charge output from the amplifier. Thereby, a voltage based on the charged charge is output. The switch is disposed in parallel with the capacitor and switches between a state in which the capacitor is charged with charge and a state in which the charge is reset.

[0003] Patent Document 2 discloses a photodetection circuit that converts a photocurrent output from a photodiode into a voltage signal by a transimpedance amplifier method. Specifically, Patent Document 2 discloses reducing the influence of dark current by operating the photodiode with zero bias. Patent Document 2 also discloses providing a dark current extraction circuit for reducing dark current when the above technique cannot be used, such as when receiving light having a long wavelength. The dark current extraction circuit has a configuration including two transistors such as MOSFETs. Further, Patent Document 2 also discloses a photodetection circuit that converts a photocurrent output from a photodiode into a voltage signal by a charge amplifier method. In this circuit, a circuit configuration for minimizing the reverse bias voltage is adopted, and the anode side of the photodiode is connected to the inverting input terminal of the operational amplifier.

Prior Art Documents

Patent Documents

[0004] [[ID=2�]]

Patent Document 1

[0005] The charge amplifier circuit in Patent Document 1 does not include a circuit to zero-bias the photodiode. The photodetector circuit in Patent Document 2 is primarily a transimpedance amplifier type. Furthermore, unlike the charge amplifier type circuit in Patent Document 2, a circuit is known in which the cathode side of the photodiode is connected to the inverting input terminal of the operational amplifier. In this type of circuit, in order to zero-bias the photodiode, the reference voltage of the operational amplifier must also be set to 0V, so in the case of a single-supply circuit, the difficulty of circuit design and device selection for the charge amplifier circuit increases.

[0006] This invention has been made in view of the above circumstances, and its main objective is to provide a charge amplifier circuit that suppresses the dark current of the photodiode provided in the charge amplifier circuit while suppressing an increase in the difficulty of circuit design and device selection. Means and effects for solving the problem

[0007] The problems that this invention aims to solve are as described above, and next, the means for solving these problems and their effects will be explained.

[0008] In view of the present invention, a charge amplifier circuit having the following configuration is provided: that is, it converts a charge based on a photocurrent into a voltage and outputs it. The charge amplifier circuit comprises a photodiode, an amplifier, a feedback capacitor, a state switch, an adjustment switch, and an adjustment capacitor. The photodiode generates a photocurrent when light is incident on it. The amplifier has an inverting input terminal connected to the cathode side of the photodiode, a non-inverting input terminal, and an output terminal. The feedback capacitor is arranged in the feedback circuit of the amplifier and charges the output of the photodiode to produce an output voltage. The state switch is arranged in parallel with the feedback capacitor and switches between a state in which a charge is charged to the feedback capacitor and a state in which the charge is reset. The adjustment switch is arranged in parallel with the photodiode. The adjustment capacitor holds a charge to bring the photodiode into a zero-bias state when the adjustment switch is conducting, and can hold the charge at least when the adjustment switch is not conducting.

[0009] This allows the photodiode to be brought to a zero-bias state by incorporating an adjustment capacitor and adjustment switch. As a result, it becomes unnecessary to set the voltage at the amplifier's non-inverting input terminal to 0V. Therefore, the increased difficulty in circuit design and device selection for charge amplifier circuits operating with a single power supply can be suppressed.

[0010] In the charge amplifier circuit described above, it is preferable to provide a capacitor that holds charge to cancel out the offset voltage of the amplifier.

[0011] This allows you to obtain the correct output voltage.

[0012] In the charge amplifier circuit described above, the following configuration is preferable: it includes a cancellation capacitor, which is a different capacitor from the adjustment capacitor. The cancellation capacitor holds a charge to cancel out the offset voltage of the amplifier.

[0013] This allows the two capacitors to perform their individual functions.

[0014] In the charge amplifier circuit described above, it is preferable that the adjustment capacitor holds a charge for bringing the photodiode to a zero-bias state and a charge for canceling out the offset voltage of the amplifier.

[0015] This allows a single capacitor to perform two functions.

[0016] In the charge amplifier circuit described above, it is preferable that a constant voltage source is connected to the non-inverting input terminal of the amplifier.

[0017] This prevents the amplifier's input dynamic range from becoming narrower. [Brief explanation of the drawing]

[0018] [Figure 1] A circuit diagram of a typical charge amplifier circuit. [Figure 2] Circuit diagram of the charge amplifier circuit of the first embodiment. [Figure 3] A graph showing the time variation of the switch state and output voltage, etc., in the first embodiment. [Figure 4] Circuit diagram of the charge amplifier circuit in the reset state of the second embodiment. [Figure 5] Circuit diagram of the charge amplifier circuit in the charged state according to the second embodiment. [Figure 6] A graph showing the time variation of the switch state and output voltage, etc., in the second embodiment. [Figure 7] Circuit diagram of the charge amplifier circuit of the third embodiment. [Figure 8] Graph showing the time variation of the switch state and output voltage, etc. of the third embodiment.

Mode for Carrying Out the Invention

[0019] Next, embodiments of the present invention will be described with reference to the drawings.

[0020] First, referring to FIG. 1, a general charge amplifier circuit 1 will be described. The charge amplifier circuit 1 is a circuit that converts the current output by a sensor or the like (specifically, the charge that moves as a current) into a voltage. In this specification, a photodiode 11 is assumed as the sensor.

[0021] The charge amplifier circuit 1 includes an amplifier 12. The amplifier 12 is an operational amplifier and includes an inverting input terminal, a non-inverting input terminal, and an output terminal. The operational amplifier functions as a differential amplifier that outputs the difference between the voltage at the inverting input terminal and the voltage at the non-inverting input terminal. However, the amplifier 12 is not limited to an operational amplifier. For example, the amplifier 12 is not limited to an operational amplifier (integrated circuit) and may be a discrete circuit.

[0022] The non-inverting input terminal of the amplifier 12 is grounded. The cathode side of the photodiode 11 is connected to the inverting input terminal of the amplifier 12, and the charge due to the photocurrent is input. A feedback circuit is connected to the output terminal of the amplifier 12, and a feedback capacitor 31 is arranged in the feedback circuit. The feedback capacitor 31 charges the output of the photodiode 11. As a result, the voltage of the feedback capacitor 31 rises, and the output voltage rises. Thereby, the charge amplifier circuit 1 can provide an output voltage to the outside.

[0023] Furthermore, the amount of charge that can be charged into the feedback capacitor 31 is limited, and it is necessary to return the voltage to the voltage when there is no input at the start of measurement. For this reason, a state switch 21 is provided to release the charge charged into the feedback capacitor 31. The state switch 21 is arranged in parallel with the feedback capacitor 31. The various switches described herein may be analog switches or switches using transistors such as MOSFETs.

[0024] When the state switch 21 is in a non-conductive state, the feedback capacitor 31 is charged. When the state switch 21 is in a conductive state, the positive and negative plates of the feedback capacitor 31 are electrically connected, and the charged charge is reset. In the following description, the state in which the feedback capacitor 31 is charged is referred to as the charge state, and the state in which the charged charge is reset (or the state in which the reset is completed) is referred to as the reset state. The charge amplifier circuit 1 continues to operate by repeatedly switching between the charge state and the reset state.

[0025] Next, we will explain the bias and dark current of the photodiode 11. In a typical charge amplifier circuit 1, the bias of the photodiode 11 and the voltage at the non-inverting input terminal of the amplifier 12 are the same value or close in value. As a result, when the voltage at the non-inverting input terminal is not 0V, the dark current generated in the photodiode 11 becomes a significant value that cannot be ignored. Since the magnitude of the dark current affects the magnitude of the noise, it is preferable to reduce the dark current in order to improve the accuracy of the output voltage. However, if the bias of the photodiode 11 is set to 0V, the voltage input to the non-inverting input terminal will also be 0V in a conventional circuit. As a result, the difficulty of circuit design and device selection for the charge amplifier circuit 1 increases.

[0026] In this regard, the charge amplifier circuit 1 of this embodiment has a function (hereinafter referred to as the zero-bias function) that maintains a state in which the bias of the photodiode 11 is 0V while setting the voltage input to the non-inverting input terminal of the amplifier 12 to an arbitrary value. Hereinafter, the state in which the bias of the photodiode 11 is substantially 0V will be referred to as the zero-bias state. The term "substantially 0V state" includes not only a state that is strictly equal to 0V, but also a state that is considered substantially 0V considering the technical field and common technical knowledge.

[0027] The charge amplifier circuit 1 of the first embodiment, including the zero-bias function, will be described in detail below with reference to Figures 2 and 3. Note that the explanation of components common to the general charge amplifier circuit 1 shown in Figure 1 may be simplified or omitted.

[0028] The charge amplifier circuit 1 of the first embodiment includes, in addition to the elements described above, an adjustment switch 22 and an adjustment capacitor 32.

[0029] The adjustment switch 22 is arranged in parallel with the photodiode 11. In the reset state, the adjustment switch 22 remains conductive, and in the charge state, the adjustment switch 22 remains non-conductive.

[0030] The adjustment capacitor 32 is positioned between the cathode of the photodiode 11 and the inverting input terminal of the amplifier 12, downstream of the connection point of the feedback circuit (on the amplifier 12 side).

[0031] In this embodiment, a constant voltage source 40 is connected to the non-inverting input terminal of the amplifier 12.

[0032] Here, as shown in Figure 2, the voltage on the cathode side of the photodiode 11 is referred to as Vpd. The anode side of the photodiode 11 is grounded. In other words, when the adjustment capacitor 32 is not present, Vpd is the voltage corresponding to the bias. In the reset state, since the state switch 21 and the adjustment switch 22 are conducting, the adjustment capacitor 32 is charged to a voltage corresponding to the bias of the photodiode 11. As a result, as shown in Figure 3, Vpd becomes 0V, and a zero-bias state is achieved.

[0033] Subsequently, in the charged state, the state switch 21 and the adjustment switch 22 become non-conductive. In the charged state, the photodiode 11 continuously outputs a photocurrent, gradually charging the feedback capacitor 31. As a result, the output voltage gradually increases, as shown in Figure 3. Meanwhile, since the adjustment switch 22 is non-conductive, the charge (i.e., the voltage based on the charge) charged in the adjustment capacitor 32 is maintained. As a result, the zero-bias state can be maintained even in the charged state, as shown in Figure 3.

[0034] In this embodiment, since a zero-bias state is maintained, noise caused by the dark current of the photodiode 11 can be reduced. Furthermore, in this embodiment, since the zero-bias state is achieved using the adjustment capacitor 32, it is not necessary to set the voltage input to the non-inverting input terminal to 0V (in reality, it is connected to the constant voltage source 40). As a result, the difficulty of circuit design and device selection for the charge amplifier circuit 1 can be suppressed.

[0035] Next, the charge amplifier circuit 1 of the second embodiment will be described with reference to Figures 4 to 6.

[0036] The charge amplifier circuit 1 of the second embodiment has an offset voltage rejection function in addition to the zero bias function. The offset voltage is a voltage that contributes to the output voltage due to the structure of the amplifier 12, etc., and there are individual differences in the amplifier 12. Since the offset voltage is superimposed on the output voltage, it leads to a decrease in accuracy. Furthermore, in the circuit diagrams from Figure 4 onward, Voff, which indicates the offset voltage, is indicated before the inverting input terminal.

[0037] The charge amplifier circuit 1 of the second embodiment includes, in addition to the elements of the charge amplifier circuit 1 of the first embodiment, a cancellation capacitor 33, a first switch 23, a second switch 24, and a third switch 25. Figures 4 to 6 show the switching states of these switches.

[0038] The cancellation capacitor 33 is a capacitor that realizes the offset voltage rejection function. The cancellation capacitor 33 is placed on a circuit with the non-inverting input terminal of the amplifier 12 as one end and the inverting input terminal as the other end. A second switch 24 is also placed on this circuit, but on the inverting input terminal side of the cancellation capacitor 33. The first switch 23 is placed on a circuit that connects the non-inverting input terminal of the amplifier 12 to the constant voltage source 40, and is also placed on a circuit that connects one plate of the cancellation capacitor 33 to the constant voltage source 40. The third switch 25 is placed on a circuit that connects the other plate of the cancellation capacitor 33 to the constant voltage source 40.

[0039] As shown in Figure 4, the charge amplifier circuit 1 of the second embodiment has a two-stage reset state. In the first stage, shown in the upper part of Figure 4, the state switch 21, adjustment switch 22, first switch 23, and second switch 24 are in a conductive state, and the third switch 25 is in a non-conductive state. As a result, one plate of the adjustment capacitor 32 is grounded, and the voltage on the other plate becomes the output voltage during reset, Vout (more specifically, Vref + Voff as described later), and a charge corresponding to this value is charged. The cancellation capacitor 33 is charged to become the offset voltage of the amplifier 12. Also, in the first stage, the constant voltage source 40 is directly connected to the non-inverting input terminal of the amplifier 12, so the input voltage is Vref. In the first stage, the offset voltage Voff is input to the inverting input terminal of the amplifier 12. Therefore, in the first stage, the output voltage is Vref + Voff.

[0040] Next, in the second stage of the reset state (lower part of Figure 4), the conduction and non-conduction states of the first switch 23, the second switch 24, and the third switch 25 are reversed. As a result, the voltage input to the non-inverting input terminal of the amplifier 12 drops by the voltage of the cancellation capacitor 33 (in other words, by the offset voltage), becoming Vref-Voff. Consequently, the output voltage becomes Vref. This cancels out the offset voltage from the output voltage.

[0041] Subsequently, the adjustment switch 22 and the state switch 21 switch from the conductive state to the non-conductive state, transitioning to the charged state. Furthermore, even in the charged state, the charge stored in the adjustment capacitor 32 and the cancellation capacitor 33 is not released, so their voltages are maintained. Therefore, even in the charged state, the effect of the offset voltage of the amplifier 12 can be canceled, and the zero-bias state of the photodiode 11 can be maintained.

[0042] Note that it takes a certain amount of time for the output voltage to drop from Vref+Voff to Vref. Therefore, in this embodiment, a second reset stage is provided, after which the system transitions to the charge state. However, this process is not mandatory; for example, the second reset stage may be omitted, and the conduction and non-conduction states of the first switch 23, the second switch 24, and the third switch 25 may be swapped simultaneously with the transition to the charge state.

[0043] The charge amplifier circuit 1 of the second embodiment has a zero-bias function and an offset voltage rejection function. Therefore, it is possible to simplify the circuit design of the charge amplifier circuit 1 while suppressing the reduction in accuracy caused by the offset voltage.

[0044] Next, the charge amplifier circuit 1 of the third embodiment will be described with reference to Figures 7 and 8.

[0045] In the third embodiment, one capacitor (specifically, the adjustment capacitor 32) is used to achieve both a zero-bias function and an offset voltage rejection function. That is, the adjustment capacitor 32 in the third embodiment is charged with a charge to zero-bias the photodiode 11 and a charge to cancel the offset voltage of the amplifier 12.

[0046] The charge amplifier circuit 1 of the third embodiment includes, in addition to the elements of the charge amplifier circuit 1 of the first embodiment, a first switch 23 and a second switch 24. Figures 7 and 8 show the switching states of these switches. The first switch 23 is located in a circuit that branches off from the feedback circuit between the amplifier 12 and the feedback capacitor 31. The branched portion is connected to a constant voltage source 40, and the first switch 23 is located between the branched portion and the constant voltage source 40. The second switch 24 is located in a circuit that branches off from the feedback circuit between the amplifier 12 and the state switch 21. The branched portion is connected between the adjustment capacitor 32 and the inverting input terminal. The constant voltage source 40 is always connected to the non-inverting input terminal of the amplifier 12.

[0047] As shown in Figure 7, in the reset state of the charge amplifier circuit 1 of the third embodiment, the adjustment switch 22, the first switch 23, and the second switch 24 are in a conductive state, and the state switch 21 is in a non-conductive state. As a result, one plate of the adjustment capacitor 32 is connected to the ground point, and the other plate of the adjustment capacitor 32 is connected to the inverting input terminal with an offset voltage. As a result, the feedback capacitor 31 is charged with a charge corresponding to the constant voltage Vref. In addition, the adjustment capacitor 32 is charged with a charge corresponding to the offset voltage while maintaining the photodiode 11 at zero bias.

[0048] Subsequently, the state switch 21, adjustment switch 22, first switch 23, and second switch 24 switch between conductive and non-conductive states, transitioning to a charged state. As a result, in the charged state, the charge stored in the adjustment capacitor 32 is not released, and their voltages are maintained. Therefore, the effect of the offset voltage of the amplifier 12 can be canceled, and the zero-bias state of the photodiode 11 can be maintained.

[0049] The charge amplifier circuit 1 of the third embodiment has a zero-bias function and an offset voltage rejection function. Therefore, it is possible to simplify the circuit design of the charge amplifier circuit 1 while suppressing the reduction in accuracy caused by the offset voltage. Furthermore, in the charge amplifier circuit 1 of the third embodiment, the non-inverting input terminal of the amplifier 12 is always connected to the constant voltage source 40. Therefore, compared to a circuit that changes the voltage input to the non-inverting input terminal (for example, the charge amplifier circuit 1 of the second embodiment), the input dynamic range of the amplifier 12 is not narrowed, thus reducing the effect of the dynamic range.

[0050] As shown in the first embodiment, the offset voltage removal function is not an essential function and can be omitted. Furthermore, the circuits shown in the first to third embodiments are examples, and the same function may be achieved using a circuit configuration other than those disclosed herein.

[0051] As described above, the charge amplifier circuit 1 of the above embodiment comprises a photodiode 11, an amplifier 12, a feedback capacitor 31, a state switch 21, an adjustment switch 22, and an adjustment capacitor 32. The photodiode 11 generates a photocurrent when light is incident on it. The amplifier 12 has an inverting input terminal connected to the cathode side of the photodiode 11, a non-inverting input terminal, and an output terminal. The feedback capacitor 31 is placed in the feedback circuit of the amplifier 12 and charges the output of the photodiode 11 to produce an output voltage. The state switch 21 is placed in parallel with the feedback capacitor 31 and switches between a state in which charge is charged to the feedback capacitor 31 and a state in which the charge is reset. The adjustment switch 22 is placed in parallel with the photodiode 11. The adjustment capacitor 32 holds a charge to bring the photodiode 11 to a zero-bias state when the adjustment switch 22 is conducting, and can hold a charge at least when the adjustment switch 22 is not conducting.

[0052] As a result, the photodiode 11 can be brought to a zero-bias state by providing the adjustment capacitor 32 and the adjustment switch 22. Consequently, it is no longer necessary to set the voltage at the non-inverting input terminal of the amplifier 12 to 0V. Therefore, the increased difficulty in circuit design and device selection for the charge amplifier circuit 1 when operating with a single power supply can be suppressed.

[0053] In the charge amplifier circuit 1 of this embodiment, a charge is provided to hold a charge for canceling out the offset voltage of the amplifier 12 (a cancellation capacitor 33 in the second embodiment, and an adjustment capacitor 32 in the third embodiment).

[0054] This allows you to obtain the correct output voltage.

[0055] In the charge amplifier circuit 1 of the second embodiment, a cancellation capacitor 33 is provided, which is a different capacitor from the adjustment capacitor 32. The cancellation capacitor 33 holds a charge to cancel out the offset voltage of the amplifier 12.

[0056] This allows the two capacitors (adjustment capacitor 32 and cancellation capacitor 33) to perform their individual functions.

[0057] In the charge amplifier circuit 1 of the third embodiment, the adjustment capacitor 32 holds a charge to bring the photodiode 11 into a zero-bias state and a charge to cancel out the offset voltage of the amplifier 12.

[0058] This allows one capacitor (regulating capacitor 32) to perform two functions.

[0059] In the charge amplifier circuit 1 of the third embodiment, a constant voltage source 40 is connected to the non-inverting input terminal of the amplifier 12.

[0060] This prevents the input dynamic range of amplifier 12 from becoming narrower. [Explanation of symbols]

[0061] 1. Charge amplifier circuit 11 Photodiode 12 Amplifiers 21 Status switch 22 Adjustment switch 23. Switch 1 24 Second switch 25. Third switch 31 Feedback Capacitor 32 Adjustment Capacitors 33 Cancellation Capacitor 40 Constant voltage source

Claims

1. In a charge amplifier circuit that converts a charge based on photocurrent into a voltage and outputs it, A photodiode generates a photocurrent when light is incident on it, An amplifier having an inverting input terminal connected to the cathode side of the photodiode, a non-inverting input terminal, and an output terminal, A feedback capacitor is placed in the feedback circuit of the amplifier and charges the output of the photodiode to produce an output voltage, A state switch is arranged in parallel with the feedback capacitor and switches between a state in which the feedback capacitor is charged and a state in which the charge is reset. An adjustment switch is arranged in parallel with the aforementioned photodiode, An adjustment capacitor that holds a charge to bring the photodiode to a zero-bias state when the adjustment switch is in a conductive state, and that can hold said charge at least when the adjustment switch is in a non-conductive state, A charge amplifier circuit characterized by comprising the following features.

2. A charge amplifier circuit according to claim 1, A charge amplifier circuit characterized by being provided with a capacitor that holds charge to cancel out the offset voltage of the amplifier.

3. A charge amplifier circuit according to claim 2, It is equipped with a cancellation capacitor, which is a different capacitor from the aforementioned adjustment capacitor. A charge amplifier circuit characterized in that the cancellation capacitor holds a charge for canceling out the offset voltage of the amplifier.

4. A charge amplifier circuit according to claim 2, A charge amplifier circuit characterized in that the adjustment capacitor holds a charge for bringing the photodiode to a zero-bias state and a charge for canceling out the offset voltage of the amplifier.

5. A charge amplifier circuit according to claim 4, A charge amplifier circuit characterized in that a constant voltage source is connected to the non-inverting input terminal of the aforementioned amplifier.

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