JPH1019675A - Photometry circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the number of part items of a measuring circuit and reduce the circuit scale by integrally forming arithmetic circuits for converting a photoelectromotive current into voltage and time on the same semiconductor base. SOLUTION: A voltage converting arithmetic circuit 3 to which the photoelectromotive current of a light receiving sensor 1a formed of a silicon photodiode (SPD) is inputted outputs the photoelectromotive current as voltage integral from a first output terminal 5. A time converting arithmetic circuit 4 to which the photoelectromotive current of a light receiving sensor 1b formed of SPD is inputted outputs the photoelectromotive current as time integral from a second output terminal 6. The voltage converting arithmetic circuit 3 and the time converting arithmetic circuit 4 are integrally formed on the same semiconductor base 2. Thus, the number of part items can be reduced to reduce the circuit scale.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a photometric circuit,
In particular, the present invention relates to a photometric circuit such as an automatic exposure meter for a camera using a semiconductor light receiving element as a light receiving sensor.

[0002]

2. Description of the Related Art FIGS. 6 and 7 are explanatory diagrams of a photometry circuit conventionally used. The circuit of FIG.
(A) converts a photoelectromotive current generated by receiving light into a voltage. The output of the light receiving sensor 1 (a) composed of a silicon photodiode (hereinafter referred to as SPD) is input to the log amplifier 7 (a), and the output of the log amplifier 7 (a) is canceled by a cancel circuit (hereinafter referred to as a cancel circuit). , Is
8, and the output of the Is cancellation circuit 8 is input to the amplification circuit 9. Log amp 7
(A) The Is cancellation circuit 8 and the amplification circuit 9 constitute the voltage conversion operation circuit 3. After the photovoltaic current output from the light receiving sensor 1 (a) is input to the log amplifier 7 (a) and converted into a voltage, an Is cancellation circuit 8 cancels a reverse saturation current having a large temperature fluctuation, and an Is cancellation circuit. 8
Is amplified by the amplifier circuit 9 to obtain a solution as a voltage value.

The circuit shown in FIG. 7 converts a photoelectromotive current generated by receiving light from the optical sensor 1 (b) into time. The output of the light receiving sensor 1 (b) is input to the log amplifier 7 (b),
The output of the log amplifier 7 (b) is input to the expansion circuit 10, the output of the expansion circuit 10 is input to the integration circuit 11, and the integration circuit 1
1 is input to the comparator circuit 12. The log amplifier 7 (b), the expansion circuit 10, the integration circuit 11, and the comparator circuit 12 constitute the time conversion operation circuit 4. The photovoltaic current output from the light receiving sensor 1 (b) is
(B), once converted into a voltage, and then expanded
At 0, the current value is converted again and input to the integration circuit 11. The comparator circuit 12 reads the output voltage of the integration circuit 11, and when the output voltage exceeds the threshold, the output is inverted. A solution is obtained by measuring the time from the start of integration until the output of the comparator 12 is inverted.

[0004]

The conventional photometric circuit has the following problems since it is configured as described above. The first problem is that since the voltage solution light measurement circuit and the time solution light measurement circuit are separately provided, when both the voltage solution light measurement circuit and the time solution light measurement circuit are used, a light receiving sensor and an arithmetic circuit are required respectively. Therefore, the number of parts was increased. As a second problem, in a conventional photometry circuit that solves time, since there is no means for amplifying the photoelectromotive current output from the light receiving sensor, it is difficult to adjust the proportional coefficient when converting light into time. Was.

SUMMARY OF THE INVENTION The present invention has been made to solve such a problem, and it is possible to reduce the number of components of a photometric circuit, reduce the circuit scale, and adjust a proportional coefficient when converting light into time. It was done.

A first invention is to provide a photometric circuit in which a voltage conversion operation circuit and a time conversion operation circuit are integrally formed on a common semiconductor substrate to reduce the number of parts and reduce the circuit scale. It is.

A second aspect of the present invention is to provide a photometric circuit in which a light receiving sensor is integrally formed on a semiconductor substrate to reduce the number of components and reduce the circuit scale.

According to a third aspect of the present invention, a voltage conversion operation circuit includes a light receiving sensor, a log amplifier, a reverse saturation current canceling circuit, and an amplification circuit.
An object of the present invention is to provide a photometric circuit that is composed of the log amplifier, the expansion circuit, the integration circuit, and the comparator circuit, and that can reduce the number of components of the photometric circuit and reduce the circuit scale.

According to a fourth aspect of the present invention, the voltage conversion operation circuit and the time conversion operation circuit are composed of an operational amplifier and a transistor, so that the number of components of the light measurement circuit can be reduced, and a light measurement circuit capable of reducing the circuit scale can be obtained. Things.

According to a fifth aspect of the present invention, the emitter terminal of the second PNP transistor is connected to a variable voltage to reduce the number of components of the photometric circuit, thereby reducing the circuit scale. It is an object of the present invention to obtain a photometric circuit which can be obtained and whose amplification factor can be freely set.

[0011]

In a photometric circuit according to a first aspect of the present invention, a first arithmetic circuit for converting a photovoltaic current obtained from a light receiving sensor into a voltage, and a second arithmetic circuit for converting the photovoltaic current into a time. And the first and second arithmetic circuits are integrally formed on a common semiconductor substrate.

A photometric circuit according to a second aspect of the present invention is characterized in that a light receiving electromotive force sensor for performing photoelectric conversion is integrally formed on the semiconductor substrate.

In a photometric circuit according to a third aspect of the present invention, the voltage conversion operation circuit cancels a log amplifier having a light receiving sensor connected to an input terminal and a reverse saturation current connected to an output terminal of the log amplifier. The time conversion operation circuit includes a cancel circuit and an amplifier circuit connected to a connection terminal of the output terminal of the cancel circuit. The time conversion operation circuit includes an expansion circuit connected to an output terminal of the log amplifier, and an output terminal of the expansion circuit. , And a comparator circuit connected to the output terminal of the integration circuit.

In a photometric circuit according to a fourth aspect of the present invention, a cathode terminal of a light receiving sensor comprising a silicon photodiode for performing photoelectric conversion and a first reference voltage are connected to a non-inverting input terminal, and the light receiving sensor of the light receiving sensor is connected to an inverting input terminal. A first operational amplifier having an anode terminal connected to the emitter terminal of the first PNP transistor, an output terminal connected to a base terminal and a collector terminal of the first PNP transistor, and a non-inverting input terminal having the first PNP transistor connected thereto. Connect the base terminal of the transistor, connect the cathode terminal of the diode to the inverting input terminal, draw a constant current from the cathode terminal of the diode, connect the output terminal to the anode terminal of the diode, and connect it to the input terminal of the amplifier circuit. A second operational amplifier to be connected, and an output terminal of the amplifier circuit. The first output terminal, a second P connected to the base terminal of the first PNP transistor
A second PNP connected to a base terminal of the NP transistor and a non-inverting input terminal of the first operational amplifier;
A comparator connected to an emitter terminal of the transistor and a collector terminal of the second PNP transistor; an inverting input terminal of the comparator; a capacitor and a changeover switch provided in parallel between the inverting input terminal of the comparator and ground; A second reference voltage is connected to a non-inverting input terminal of the comparator, and a second output terminal is provided to an output terminal of the comparator.

[0015] In the photometric circuit according to a fifth aspect of the present invention, the emitter terminal of the second PNP transistor is connected to a variable voltage.

The embodiments of the present invention have the following means for achieving the object. A photometric circuit according to an embodiment of the present invention includes an arithmetic circuit for converting a photovoltaic current obtained from a light receiving sensor into a voltage, and an arithmetic circuit for converting the photovoltaic current into time, which are integrally formed on the same semiconductor substrate. It is characterized by being formed.

Further, a light receiving sensor for performing photoelectric conversion is integrally formed on the semiconductor substrate.

Next, the voltage conversion circuit includes a log amplifier having a light receiving sensor connected to an input terminal, an Is cancel circuit connected to an output terminal of the log amplifier,
An amplifier circuit connected to the output terminal of the cancel circuit, wherein the time conversion circuit includes an expansion circuit connected to the output terminal of the log amplifier, an integration circuit connected to the output terminal of the expansion circuit, and an integration circuit. And a comparator circuit connected to the output terminal of the circuit.

Further, the first operational amplifier 13
The cathode terminal of the SPD and the reference voltage 15 are connected to a non-inverting input terminal (hereinafter referred to as a VP terminal) of the first operational amplifier 13, and an anode terminal of the SPD is connected to an inverting input terminal (hereinafter referred to as a VN terminal) of the first operational amplifier 13. First PNP
The emitter terminal of the transistor 14 is connected, and the output terminal of the first operational amplifier 13 is connected to the first P-terminal.
The base terminal of the NP transistor 14 is connected to the collector terminal, the VP terminal of the second operational amplifier 16 is connected to the base terminal of the first PNP transistor 14, and the second operational amplifier 16
The cathode terminal of the diode 17 is connected to the VN terminal of
A constant current 18 is drawn from the cathode terminal of the diode 17, the output terminal of the second operational amplifier 16 is connected to the anode terminal of the diode 17, and then connected to the input terminal of the amplifier circuit 19, and The first output terminal 5 is provided with a first output terminal 5, and the base terminal of the second PNP transistor 20 is connected to the base terminal of the first PNP transistor 14;
The emitter terminal of the second PNP transistor 20 is connected to the VP terminal of the first operational amplifier 13, the collector terminal of the second PNP transistor 20 is connected to the VN terminal of the comparator 21, It is characterized in that a capacitor 23 and a changeover switch 24 are provided in parallel between the VN terminal and GND, and the reference voltage 25 is connected to the VP terminal.

Further, the connection destination of the emitter terminal of the second PNP transistor 20 is a variable voltage 26, and the variable voltage 26
By adjusting this voltage, the current injected into the VN terminal of the comparator 21 can be freely amplified.

In the present invention, an arithmetic circuit for converting a photovoltaic current obtained from a light receiving sensor into a voltage and an arithmetic circuit for converting the photovoltaic current into time are integrally formed on the same semiconductor substrate. In addition, since a light receiving sensor for performing photoelectric conversion can be formed integrally with the semiconductor substrate, the number of components can be reduced. Further, since a part of the arithmetic circuit is shared with the light receiving sensor, the circuit scale is reduced, and the voltage conversion and the time conversion of the photovoltaic current generated by one SPD can be performed simultaneously.
Further, since a means for amplifying the photovoltaic current generated by the SPD is provided, it is possible to freely adjust the proportional coefficient when converting light into time.

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiment 1 FIG. FIG. 1 is a block diagram of a photometric circuit according to a first embodiment of the present invention. In the figure, 1 (a) · 1
(B) is a light receiving sensor made of SPD, 2 is a semiconductor substrate,
Reference numeral 3 denotes a voltage conversion operation circuit, 4 denotes a time conversion operation circuit, 5 denotes a first output terminal, and 6 denotes a second output terminal.

The light receiving sensor 1 (a) made of SPD has an output terminal connected to the voltage conversion operation circuit 3, and the light reception sensor 1 (b) made of SPD is connected to the time conversion operation circuit 4. The first output terminal 5 of the voltage conversion operation circuit 3 to which the photovoltaic current of the light receiving sensor 1 (a) made of SPD is input.
Outputs the photovoltaic current as a voltage solution, and the second output terminal 6 of the time conversion operation circuit 4 to which the photovoltaic current of the light receiving sensor 1 (b) composed of SPD is input, converts the photovoltaic current into a time solution. Output. Here, the voltage conversion operation circuit 3 and
By integrally forming the time conversion operation circuit 4 on the same semiconductor substrate 2, the number of components is reduced.

Embodiment 2 FIG. Further, FIG. 2 shows that the semiconductor substrate 2 has light receiving sensors 1 (a) and 1 (b) made of SPD.
Is a block diagram of a second embodiment in which the number of parts is further reduced by integrally forming 1 (a) ・ 1 (b) is SP
D is a light receiving sensor, 2 is a semiconductor substrate, 3 is a voltage conversion operation circuit, 4 is a time conversion operation circuit, 5 is a first output terminal, and 6 is a second output terminal.

Embodiment 3 The block diagram shown in FIG. 3 is a configuration effective when the voltage conversion circuit 3 and the time conversion circuit 4 described in FIGS. 1 and 2 are integrally formed on the same semiconductor substrate 2, and is composed of SPD. The third embodiment aims at reducing the circuit scale by sharing a part of the arithmetic circuit with the light receiving sensor.

1 is a light receiving sensor made of SPD, a first output terminal, 6 is a second output terminal, 7 is a log amplifier, and 8 is an I / O
s cancel circuit, 9 is an amplification circuit, 10 is an expansion circuit, 1
1 is an integrating circuit, and 12 is a comparator circuit.

The configuration has a set of I / V conversion circuits in which the output of the SPD 1 is input to the log amplifier 7, and an Is cancel circuit 8 connected to the output terminal of the log amplifier 7, A voltage conversion operation circuit section composed of an amplification circuit 9 connected to the output terminal of the Is cancellation circuit 8, an expansion circuit 10 connected to the output terminal of the log amplifier 7, and an output terminal of the expansion circuit 10 It comprises a time conversion operation circuit section comprising an integrated circuit 11 connected thereto and a comparator circuit 12 connected to an output terminal of the integrated circuit 11.

As described above, by sharing the SPD 1 and the log amplifier 7, one set of the SPD and the log amplifier can be reduced, so that the circuit scale can be reduced. In the first and second embodiments, especially, effective.

FIG. 4 is a circuit diagram which implements the third embodiment described in FIG. First, the configuration will be described. In FIG. 4, reference numeral 1 denotes a light receiving sensor made of SPD, 5 denotes a first output terminal, 6 denotes a second output terminal, 13 denotes a first operational amplifier, 14 denotes a first PNP transistor, 15
Is a first reference voltage, 16 is a second operational amplifier, 17 is a diode, 18 is a pull-out current, 19 is an amplifier circuit, 20 is a second PNP transistor, 21 is a comparator, 23 is a capacitor, 24 is a switch,
25 is a second reference voltage.

VP of the first operational amplifier 13
A cathode terminal of the light receiving sensor SPD1 that performs photoelectric conversion is connected to a terminal and the reference voltage 15; a VN terminal of the first operational amplifier 13 is connected to an anode terminal of the SPD1 and an emitter terminal of the first PNP transistor 14; The base terminal and the collector terminal of the first PNP transistor 14 are connected to the output terminal of the first operational amplifier 13.

Subsequently, the second operational amplifier 1
6, the base terminal of the first PNP transistor 14 is connected to the VP terminal, the VN terminal of the second operational amplifier 16 is connected to the cathode terminal of a diode 17, and a constant current 18 is supplied from the cathode terminal of the diode 17. And the second operational amplifier 1
After the output terminal 6 is connected to the anode terminal of the diode 17, it is connected to the input terminal of the amplifier circuit 19, and the output of the amplifier circuit 19 is provided with the first output terminal 5.

The first PNP transistor 14
The base terminal of the second PNP transistor 20 is connected to the base terminal of the second PNP transistor 20, the emitter terminal of the second PNP transistor 20 is connected to the VP terminal of the first operational amplifier 13,
Is connected to the VN terminal of the comparator 21. Further, a capacitor 23 and a changeover switch 24 are provided in parallel between the VN terminal and GND of the comparator 21, a reference voltage 25 is connected to a VP terminal, and a second output terminal 6 is provided at an output of the comparator 21.

With this configuration, as in the block diagram of FIG. 3, the number of SPDs and log amplifiers is reduced by one, and a voltage solution and a time solution can be obtained simultaneously from the photovoltaic current generated by one SPD. Can be.

Next, the operation will be described. The photoelectromotive current generated by the SPD 1 connected between the inputs of the operational amplifier 13 does not flow into the VN terminal but is injected into the emitter terminal of the first PNP transistor 14 because the input terminal of the operational amplifier 13 has a high impedance. Is done. At this time, the first PNP transistor 14
Is expressed by (Equation 1).

[0035]

(Equation 1)

V1 is the operational amplifier 16
Of the second operational amplifier 16, a diode 17 is provided between the VN terminal and the output terminal of the second operational amplifier 16, and a constant current I2 is drawn from the cathode terminal of the diode 17. The voltage V2 input to the amplifying circuit 19 at the subsequent stage of the operational amplifier 16 is represented by (Equation 2).

[0037]

(Equation 2)

Therefore, the final output voltage VOUT obtained from the first output terminal 5 is obtained by (Equation 3).

[0039]

(Equation 3)

Subsequently, the first PNP transistor 1
Since the fourth and second PNP transistors 20 form a current mirror circuit, the transistor areas are equal,
Assuming that the current amplification factor of both PNP transistors is large and the base current is negligible with respect to the emitter current, the collector current of the second PNP transistor 20 is equal to the emitter current of the first PNP transistor 14. It is equivalent to the current. That is, at this time, the collector current of the second PNP transistor 20 is equivalent to the photovoltaic current generated by the SPD1.

The thus obtained collector current of the second PNP transistor 20 does not flow into the VN terminal of the comparator 21 having high impedance, but flows to the third output terminal 22 side. Since the third output terminal 22 is provided with a capacitor 23 and a changeover switch 24 in parallel between GND, the changeover switch 24 is turned off, and the third output terminal 22 is released from the ground. From the point in time, the collector current of the second PNP transistor 20 is injected into the capacitor 23 and integration is started.

After the start of integration, the potential of the third output terminal 22 rises and exceeds the potential of the reference voltage 25 connected to the VP terminal of the comparator 21, and the second output terminal of the comparator 21 An inverted signal is output to the output terminal 6. Here, by counting the time from the start of integration to the inversion of the second output terminal 6, the light received by the SPD 1 can be obtained as the time solution TOUT. The calculation formula is as shown in (Formula 4).

[0043]

(Equation 4)

Further, the second PNP transistor 20
The photovoltaic current generated by the SPD1 can be amplified by changing the transistor size. As an example, when the transistor size of the second PNP transistor 20 is set to α times the first PNP transistor 14, the time solution TOUT is given by (Equation 5).

[0045]

(Equation 5)

This is particularly effective when a light-receiving sensor having a low power generation capacity has a photoelectromotive current that can be amplified at a certain magnification.

Embodiment 4 FIG. 5 is a circuit diagram of a photometric circuit according to a fourth embodiment of the present invention. In FIG. 5, reference numeral 1 denotes a light receiving sensor made of SPD, 5 denotes a first output terminal, 6 denotes a second output terminal, 13 denotes a first operational amplifier, 14 denotes a first PNP transistor, and 15 denotes a first reference. Voltage, 16 is the second operational amplifier, 17 is the diode, 18 is the extraction current, 19 is the amplifier circuit, 20
Is a second PNP transistor, 21 is a comparator, 2
3 is a capacitor, 24 is a changeover switch, 25 is a second switch
Is a reference voltage, and 26 is a variable voltage.

This configuration differs from the circuit diagram of the third embodiment in that the emitter terminal of the second PNP transistor 20 is connected to the variable voltage 26. The operation of the circuit diagram of the fourth embodiment of the photometric circuit of the present invention thus configured is different from the operation of the circuit diagram of the third embodiment in that the output current of the SPD is amplified by the area ratio of the transistor. That is, what is done is amplified using the Ic-VBE characteristic of the transistor. The time solution TOUT in this configuration is obtained by (Equation 6).

[0049]

(Equation 6)

That is, by adjusting the potential of the variable voltage 26, the photoelectromotive current of the SPD can be freely amplified. An example of the amplification factor is shown in the following table.

[0051]

[Table 1]

Therefore, a larger amplification factor can be obtained with a smaller circuit scale than using the area ratio of the transistor, and the amplification factor can be set freely. This is effective for a photometric circuit for a camera or the like, and makes it possible to adjust the photometric time according to the film sensitivity.

As described above, according to the embodiment of the present invention, the voltage conversion operation circuit and the time conversion operation circuit including the SPD can be integrally formed on the same semiconductor substrate, so that the number of components can be reduced. . Further, since one set of the SPD and the log amplifier can be reduced, the circuit scale can be reduced, and the voltage solution and the time solution can be simultaneously obtained from one SPD. Furthermore,
Since a means for amplifying the output current of the SPD is provided, the photometric time can be freely adjusted.

[0054]

According to the first aspect of the present invention, the voltage conversion operation circuit and the time conversion operation circuit are integrally formed on a common semiconductor substrate to obtain a photometric circuit capable of reducing the number of parts and reducing the circuit scale. be able to.

According to the second aspect of the present invention, a photometric circuit can be obtained in which a light receiving sensor is integrally formed on a semiconductor substrate to reduce the number of components and the circuit scale.

According to the third aspect of the present invention, the voltage conversion operation circuit comprises a light receiving sensor, a log amplifier, a reverse saturation current canceling circuit, and an amplification circuit, and the time conversion operation circuit is the light receiving sensor, the log amplifier, and the expansion circuit. A photometering circuit configured with an integrating circuit and a comparator circuit can reduce the number of components of the photometering circuit and can reduce the circuit scale.

According to the fourth aspect, the voltage conversion operation circuit and the time conversion operation circuit are constituted by an operational amplifier and a transistor, so that the number of parts of the light measurement circuit can be reduced and a light measurement circuit capable of reducing the circuit scale can be obtained. Can be.

According to the fifth aspect, the emitter terminal of the second PNP transistor is connected to a variable voltage, so that the number of components of the photometric circuit can be reduced, and the circuit scale can be reduced.
A small and large amplification factor can be obtained with a small circuit scale, and
A photometric circuit whose amplification factor can be freely set can be obtained.

[Brief description of the drawings]

FIG. 1 is a block diagram of a photometric circuit according to a first embodiment of the present invention.

FIG. 2 is a block diagram of a photometric circuit according to a second embodiment of the present invention;

FIG. 3 is a block diagram of a photometric circuit according to a third embodiment of the present invention;

FIG. 4 is a circuit diagram of a photometric circuit according to a third embodiment of the present invention.

FIG. 5 is a circuit diagram of a photometric circuit according to a fourth embodiment of the present invention.

FIG. 6 is a block diagram showing an example of a conventional circuit.

FIG. 7 is a block diagram showing another example of a conventional circuit.

[Explanation of symbols]

1.1 (a) ・ 1 (b) light receiving sensor composed of SPD,
2 semiconductor substrate, 3 voltage conversion operation circuit, 4 time conversion operation circuit, 5 first output terminal, 6 second output terminal, 7
Log amp, 8Is cancel circuit, 9 amplifying circuit,
Reference Signs List 10 expansion circuit, 11 integration circuit, 12 comparator circuit, 13 first operational amplifier, 14 first PNP transistor, 15 first reference voltage, 16
A second operational amplifier, 17 diodes,
18 extraction current, 19 amplifier circuit, 20 second PN
P transistor, 21 comparator, 23 capacitor, 24 changeover switch, 25 second reference voltage,
26 Variable voltage.

Claims (5)

[Claims] A first arithmetic circuit for converting a photoelectromotive current obtained from a light receiving sensor into a voltage; and a second arithmetic circuit for converting the photoelectromotive current into a time, wherein the first and second arithmetic circuits are provided. A photometric circuit, wherein the arithmetic circuit is formed integrally on a common semiconductor substrate. 2. The photometric circuit according to claim 1, wherein a light receiving sensor for performing photoelectric conversion is integrally formed on the semiconductor substrate. 3. The voltage conversion operation circuit includes a log amplifier having a light receiving sensor connected to an input terminal, a cancel circuit connected to an output terminal of the log amplifier for canceling a reverse saturation current, An amplifier circuit connected to an output terminal, wherein the time conversion operation circuit includes a decompression circuit connected to an output terminal of the log amplifier, an integration circuit connected to an output terminal of the decompression circuit, and an integration circuit. The photometric circuit according to claim 1, comprising a comparator circuit connected to an output terminal. 4. The non-inverting input terminal is connected to a cathode terminal of a light receiving sensor comprising a silicon photodiode for performing photoelectric conversion and a first reference voltage, and the inverting input terminal is connected to an anode terminal of the light receiving sensor and a first PNP transistor. A first operational amplifier connecting the base terminal and the collector terminal of the first PNP transistor to the output terminal, the base terminal of the first PNP transistor to the non-inverting input terminal, A second operational amplifier that connects a cathode terminal of the diode to the inverting input terminal, draws a constant current from the cathode terminal of the diode, connects an output terminal to the anode terminal of the diode, and connects to an input terminal of the amplifier circuit; A first output terminal provided at an output terminal of the amplifier circuit; A base terminal of a second PNP transistor connected to a base terminal of the second PNP transistor, an emitter terminal of a second PNP transistor connected to a non-inverting input terminal of a first operational amplifier, and the second PNP transistor A comparator and an inverting input terminal of a comparator connected to the collector terminal of the comparator. A capacitor and a changeover switch are provided in parallel between the inverting input terminal of the comparator and the ground. Connect the voltage,
The photometry circuit according to claim 3, wherein a second output terminal is provided at an output terminal of the comparator. 5. The photometric circuit according to claim 4, wherein an emitter terminal of the second PNP transistor is connected to a variable voltage.