JPH1123619A - Signal current detection circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To acquire a low noise signal current detection circuit by introducing a differentiation circuit into a charge storage type circuit and providing a normal integration circuit additionally. SOLUTION: A current from a detector is converted through an input capacitor 4 into a voltage which is then amplified through an amplifier 6 and passed through a differentiation circuit (comprising a capacitor 7 and a resistor 8 for differentiation circuit). In such a circuit, signal current of a photodetector is stored in the input capacitor 4 dependent on the capacitances of the photodetector and the amplifier and the voltage thereof appears at the output through the amplifier 6. A depletion type p-channel MOSFET is employed as a reset switch 5 at the input section. A differentiation circuit follows the amplifier 6 and a reset switch 9 is connected in parallel with the resistor 8 for differentiation circuit. According to the arrangement, a photodetection circuit can be constituted without requiring a computer and the signal/noise ratio can be improved in proportion to 3/2 power of the time when the noise has no frequency dependence.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a signal current detection circuit for detecting an output signal current of various sensors and the like.

[0002]

2. Description of the Related Art Conventionally, as a photodetection circuit which does not require particularly high sensitivity and high response speed, there is a circuit widely used as a transimpedance type shown in FIG.
In this circuit, the current generated by the current generating type sensor 1 is converted into a voltage by the feedback resistor 3, so that the performance such as the sensitivity of the detector is high and the noise is low,
The thermal noise of the feedback resistor determines the final noise of the circuit. The output voltage is always constant if the generated current is constant (see the graph of output voltage versus time in FIG. 7).

FIG. 8 shows a charge storage type circuit as a photodetection circuit having a low speed but high sensitivity. In this circuit, the current of the current generating type sensor 1 is accumulated in the capacitance (input capacitance 4) of the entire input circuit including the sensor itself, and the amount of generated current is known by measuring a voltage change generated in the capacitance through an amplifier. It has become.

As shown in the graph of output voltage versus time in FIG. 9, if the current generated by the sensor is constant, the output voltage increases at a constant rate. The original generated current value can be obtained using the ratio between the voltage change and the time change at this time. Since this circuit does not use a resistor like the above-mentioned transimpedance type circuit, the thermal noise of the feedback resistor can be avoided.However, to know this voltage change, the voltage is taken in by a computer, and the voltage change and the time change The ratio had to be calculated.

In addition, in order to occasionally release (reset) the charge accumulated in the input capacitor 4 in the input circuit, it is necessary to introduce a reset switch 5 in the normal input circuit. After the switch is turned on and the charge is released, when the switch is turned off again to start the charge accumulation, a noise called kTC noise is generated. That is, at the moment when the switch is turned off, charges enter the input capacitor 4 randomly, and a random voltage is generated.

The shift of the output voltage from the origin in FIG. 9 indicates the presence of this noise. Because of this noise, it is necessary to obtain the difference between the output voltages at two points immediately after the start of accumulation and immediately before the end of the accumulation in the processing by the computer. When data is taken, noise is always added, so if two-point data is taken, the noise increases by √2 times, and the signal / noise ratio becomes worse.

Usually, a circuit used in a very high frequency region (VHF band or higher) is connected to a 50 ohm resistor regardless of the impedance of the detector as shown in FIG. Thermal noise determined the noise. In addition, the 50 ohm resistor not only performs impedance matching, but also converts the current from the sensor to a voltage with this low resistance, so the signal /
The noise ratio was very bad.

The graph of output voltage versus time in FIG. 11 shows the output voltage for a sensor that generates a pulsed current to show the time response in such a circuit. Assuming that the response of the amplifier 6 is sufficiently fast, the rise time of the output pulse is determined by the response time of the sensor.
The fall time is determined by a time constant determined by the resistance of 50 ohms and the capacitance of the sensor. In the figure, the response time of the sensor is assumed to be sufficiently short.

[0009]

As described above, the conventional signal current detection circuit has various problems in amplifying and detecting the signal of the sensor.

[0010]

SUMMARY OF THE INVENTION The present invention has been made in view of the above, and has a first reset switch for enabling conduction between an internal resistance of a signal source, a first capacitor, and both electrodes of the first capacitor. An amplifier for amplifying an output signal of the integration circuit; a second capacitor for differentiating the output signal of the amplifier; a resistor; and a second reset for enabling conduction of both electrodes of the resistor and the resistor. And a signal current detection circuit configured by a differentiation circuit including a switch.

According to the present invention, there is provided an integrating circuit comprising an internal resistance of a signal source and a first capacitor, a first amplifier for amplifying an output signal of the integrating circuit, and differentiating the output signal of the first amplifier. A differentiating circuit including a second capacitor and a resistor; a second amplifier for inverting and amplifying an output signal of the first amplifier; and an output signal of the second amplifier via the negative feedback resistor to the first amplifier. And a low-frequency negative feedback circuit to be inputted to the amplifier of (1).

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The configuration of an embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a circuit diagram showing a first embodiment of the present invention.

This circuit is basically an improvement of the charge storage type circuit. The current from the detector is converted into a voltage by the input capacitor 4, then amplified by the amplifier 6, and the voltage is differentiated by the differentiating circuit (differential circuit in FIG. 1). (Comprising a circuit capacitor 7 and a differential circuit resistor 8). With only this, the detection circuit becomes a circuit that can directly read a signal voltage as a DC voltage without taking a signal into a computer.

The charge storage type circuit integrates a current, and a voltage proportional to the original current can be obtained by differentiating the current by a differentiating circuit (see the graph of output voltage versus time in FIG. 2). Therefore, an output signal similar to the circuit using a resistor (FIG. 6) described in the prior art can be achieved without thermal noise of the resistor. Also, when a pulse-like signal current is generated, an output voltage similar to that of FIG. 11 can be obtained without thermal noise of the resistor.

In order to remove kTC noise, a reset switch 9 is provided in parallel with the resistor 8 of the differentiating circuit. This switch is turned off shortly after the reset switch 9 of the input circuit is turned off. This eliminates kTC noise since the output of the differentiating circuit always starts at 0 volts (see graph of output voltage versus time in FIG. 2). Needless to say, kTC noise is also generated by the reset switch 9 of the differentiating circuit, but since the signal has already been amplified, the effect can be reduced to a negligible level.

FIG. 1 shows a specific circuit configuration of the first embodiment.
Shown in In this circuit, the signal current of the photodetector is stored in the input capacitance determined by the capacitance of the photodetector and the amplifier, and the voltage appears at the output through the amplifier 6. As the reset switch 5 of the input section, a depletion type p-channel M
OSFET is used. A differentiating circuit is inserted after the amplifier 6,
A reset switch 9 similar to the input unit is turned on in parallel with the differentiating circuit resistor 8. At this time, it is necessary to release the reset slightly later than the reset at the input unit as described above. This is performed by a reset pulse generator. The output voltage from the circuit of FIG. 1 may be taken by any voltmeter. It is most efficient to read out the output after a time that is larger of the time constants of the integration circuit and the differentiation circuit, whichever is greater.

Next, a second embodiment of the present invention will be described with reference to the drawings. Since the charge storage type circuit cannot take a signal during resetting, it becomes a problem in applications such as communication. In order to avoid this, a low frequency negative feedback circuit is inserted in the second embodiment of the present invention shown in FIG. In this method, a low-frequency negative feedback resistor 11 larger than 50 ohms is put in an input circuit, and negative input is applied to the input circuit only in a low-frequency region through this resistor and the low-frequency feedback inverting operational amplifier 10.

This circuit looks like a conventional transimpedance circuit instead of a charge storage type circuit at first glance, but operates in a charge storage type in a high frequency region such as a pulse signal. This is because the time constant of the input circuit is much larger than the time constant corresponding to the required high frequency, and the input circuit is capacitive. Therefore, in order to obtain an output proportional to the photocurrent, a differentiating circuit is necessary at the subsequent stage. In this circuit, since the input resistance is determined by the low-frequency feedback feedback resistor 11, a large input resistance value can be obtained, and the signal /
A circuit having a large noise ratio can be configured.

Since impedance matching becomes a problem in the high frequency region, an input resistance of 50 ohm is usually used. However, if the input resistor is disposed sufficiently close to the input amplifier as compared with the wavelength of the radio wave corresponding to the required frequency, impedance matching is not necessarily required. Therefore, a problem in the case of a high input resistance is that the input voltage of the amplifier increases even for light of the same intensity, and the input operating point of the amplifier fluctuates.

The fluctuation of the input operating point of the amplifier is caused when the amplifier has non-linearity (this is almost the case with ordinary elements).
The nonlinearity of the output voltage with respect to the light intensity leads to a decrease in the dynamic range. Negative feedback circuits are the most effective circuits for suppressing such fluctuations in input voltage, but have not been used in the high frequency region because of the inability to obtain the necessary open-loop gain and adverse effects such as oscillation.

However, by limiting the negative feedback to only the low frequency, the above-mentioned disadvantage is solved, and the fluctuation of the input voltage can be reduced even in the high frequency region. In this circuit, the high-speed response has no effect, so that the response speed is the same as when no low-frequency negative feedback is applied.

The voltage versus time graphs of FIGS. 4 and 5 show the voltage change at point B after the input section, point A and the differentiating circuit in FIG. 3 when a pulse current is generated. In the input section,
First, a voltage is generated in the input capacitor 4 due to accumulation of the pulse current. At this time, since the inverting operational amplifier 9 for low frequency feedback cannot yet respond, the voltage change is determined only by the response of the amplifier 6 alone. However, as the time elapses, the low-frequency feedback inverting operational amplifier 9 begins to respond, and the average voltage gradually approaches 0 volt. On the other hand, after the differentiating circuit, a long voltage fluctuation is dropped, and a quick response corresponding to the time constant of the differentiating circuit is started (FIG. 5).

More specifically, for a low-frequency negative feedback circuit example in the photodetector, a photodiode may be inserted in the current generating sensor 1 in FIG. The input capacitance 4 is determined by the capacitance of the photodiode. The high speed of the inverting operational amplifier 9 for the negative feedback circuit affects the time during which the fluctuation of the input circuit stops. The value of the negative feedback resistor 10 is determined by the light intensity, the voltage fluctuation of the input section, and the degree to which thermal noise is desired to be suppressed. To reduce thermal noise, a large negative feedback resistor may be provided, but in order to suppress voltage fluctuations at the input section, the output voltage of the negative feedback operational amplifier becomes high, which has a limit. If the time scale of the voltage fluctuation at the input section is to be shortened, the negative feedback resistance must also be small. The time constants of the differentiating circuit capacitor 7 and the resistor 8 may be determined in consideration of the required response speed.

As described above, the present invention has been described based on the embodiments. However, the present invention is not limited to the above-described embodiments, and may be implemented in any manner unless the configuration described in the claims is changed. it can. For example, the present invention can be applied not only to an optical sensor but also to any element having a relatively high output impedance with a current output.

[0025]

As described above, in the signal current detection circuit of the present invention, in the first embodiment, not only a photodetection circuit can be constructed without a computer, but also if the noise has no frequency dependency, the time is not required. The signal / noise ratio can be improved in proportion to the power of 3/2. This is a theoretical limit. In particular, by making the time constants of the differentiating circuit and the integrating circuit equal, the signal / noise ratio can be maximized. In addition, if this circuit is used, an output signal needs to be obtained only once after an appropriate time has elapsed. Further, this advantage is very effective in a high frequency region where the calculation speed of the computer cannot keep up, or when many detector signals have to be acquired simultaneously. Further, the charge storage type circuit with a low frequency negative feedback circuit (FIG. 3) according to the second embodiment of the present invention has a great effect such as maintaining a high-speed response while reducing thermal noise.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a configuration of a charge storage type circuit with a differentiating circuit according to a first embodiment of the present invention.

FIG. 2 is a characteristic diagram showing an output voltage at a point A with respect to a DC current input in FIG.

FIG. 3 is a circuit diagram showing a configuration of a charge storage type circuit with a low frequency negative feedback circuit according to a second embodiment of the present invention.

FIG. 4 is a characteristic diagram showing a voltage at a point A with respect to a pulse current input in FIG. 3;

FIG. 5 is a characteristic diagram showing a voltage at a point B with respect to a pulse current input in FIG. 3;

FIG. 6 is a circuit diagram showing a configuration of a conventional transimpedance circuit.

7 is a characteristic diagram showing an output voltage at a point A with respect to a DC current input in FIG.

FIG. 8 is a circuit diagram showing a configuration of a conventional charge storage type circuit.

9 is a characteristic diagram showing an output voltage at a point A with respect to a DC current input in FIG.

FIG. 10 is a circuit diagram showing a configuration of a conventional high-speed current measurement circuit.

11 is a characteristic diagram showing a voltage at point A with respect to a pulse current input in FIG.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 current generating sensor 2 operational amplifier 3 feedback resistor 4 input capacitance 5 reset switch 6 amplifier 7 differentiator circuit capacitor 8 differentiator resistor 9 delay reset switch 10 low frequency feedback operational amplifier 11 low frequency feedback resistor

Claims (2)

[Claims] 1. An integrating circuit comprising an internal resistance of a signal source, a first capacitor, and a first reset switch for making both electrodes of the first capacitor conductive, and amplifying an output signal of the integrating circuit. An amplifier, a second capacitor for differentiating an output signal of the amplifier, a resistor, and a differentiating circuit including a resistor and a second reset switch for enabling both electrodes of the resistor to conduct. Signal current detection circuit. 2. An integration circuit comprising an internal resistance of a signal source and a first capacitor, a first amplifier for amplifying an output signal of the integration circuit, and a second amplifier for differentiating an output signal of the first amplifier. , A second amplifier for inverting and amplifying the output signal of the first amplifier, and the output signal of the second amplifier via the negative feedback resistor to the first amplifier. And a low frequency negative feedback circuit for inputting the signal to the signal current detection circuit.