JPS59768B2 - Sotsukou Cairo - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a photometric circuit using a photodiode.

A photometric circuit using a photodiode is used to set the exposure in a camera, but from the viewpoint of linearity, the method of using a photodiode is to keep the voltage between both terminals of the photodiode approximately constant and to flow current. .

In this case, the output of the photodiode is amplified by a negative feedback amplifier, but after the power switch of the photometric circuit is turned on until the amplifier reaches a steady state, the capacitance inherent in the photodiode is charged to near the power supply voltage, and from then on It takes several seconds or more for this charged charge to be discharged as a photocurrent of the photodiode and for the voltage between both terminals of the photodiode to reach a predetermined constant value, and correct photometry cannot be performed during this time. The present invention attempts to eliminate the prolongation of the transient time at the time of starting a photometric circuit caused by the above-mentioned phenomenon in which the unique capacitance of the photodiode is charged.

To solve the above-mentioned problems, a method has been proposed in which the photodiode is short-circuited for a certain short period of time when the power of the photometric circuit is turned on to prevent the photodiode from being charged, and a circuit as shown in FIG. 1 has been proposed.

In Figure 115, PD is a photodiode and transistor T2
When the power is turned on, the photodiode is short-circuited for a short period of time to act as a switch. That is, the output of the photodiode PD is amplified by an amplifier Al made up of an FET, and the output is negatively fed back to the input side of A120 by a transistor Ti to keep the potential at the P1 point constant. If we consider the situation immediately after turning on the switch S, the voltage across the PD is o due to its own individual capacitance, and Ti conducts and a photoelectric current tries to flow into the PD's own capacitance, but the capacitor Cl Since there is no charged charge yet, the voltage of power supply E is connected to the base of T2 by resistor R.
The voltage divided by 1 and R2 acts, and T2 conducts to short-circuit both ends of the PD and prevent charging of the individual capacitance of the PD.
Thereafter, Cl is gradually photoelectrically charged, and soon the base potential of T2 becomes unable to keep T2 in a conductive state, and the normal operation of the photometric circuit begins. The time until T2 is cut off from conduction is determined by the time constant of the circuit consisting of Cl and resistors R1 and R2, and this time constant is approximately the time it takes for the amplifier Al to reach a steady state. In this configuration, even when the photometry circuit is in a steady state, a small amount of leakage current flows through the collector of the transistor T2, so this leakage current is ignored compared to the current flowing through the photodiode PD, especially in areas where the amount of incident light is small. As a result, the error becomes large and a measurement limit occurs in the low-luminance region. Additionally, this circuit has the following problems. If there is no transistor T2, the photodiode generates a photovoltaic force in the direction in which the point P1 becomes negative when both ends are open.
This photovoltaic force is amplified by A1 and applied to the base of T1 to increase the collector current of T1, and this current flows to PD as a photocurrent to keep the potential at point P1 at approximately 0.
Initially, the self-capacitance of D is charged by the photovoltaic force, and immediately after the switch S is closed, an excessive current flows through T1, and point P1 rises to near the power supply voltage. T1 has a configuration in which the P1 point is conductive in the negative region and cut off in the positive region, so when amplifier A1 enters a steady state, T1 is cut off and the charge that has been charged to the self-capacitance of photodiode PD close to the power supply voltage is PD is discharged through itself as a photocurrent,
When the potential at point P1 rises to O and becomes negative, T1 becomes conductive and normal photometry operation begins. Therefore, in order to stop this charging, PD is short-circuited with transistor T2, but P
Considering the case where there is little incident light on D, the potential at point P1 is 0 during the conduction period of T2 after switch S is turned on, and therefore T1 is in a cut-off state. Even after T2 is cut off, T1 remains cut off until the self-capacitance of the PD is photovolted by the photovoltaic force and the P1 point becomes negative to some extent, so even after T2 is cut off, photometry is still possible. It will take a considerable amount of time for the device to function normally. For example, the photocurrent generated by a photodiode is about 10 PA (picoampere) at a brightness where an F:1.4 exposure time of 2 seconds is the appropriate exposure for film ASAlOO, and the self-capacitance of the photodiode is Since it is about 500PF, assuming that the voltage across the photodiode is 0.1 volts in the normal operating state in this case, it will take about 5 seconds to charge from 0 to 0.1 volts with a photocurrent of 10 PA. Therefore, the exposure time becomes longer than the proper exposure time. To solve this problem, it is necessary to apply an appropriate bias to the FET of the amplifier A1 so that the transistor T1 remains conductive even when the potential at the P1 point is O, and the collector current of T1 at this time is equal to PD Since the dark current must be made equal to the dark current of , circuit adjustment becomes extremely troublesome. The present invention also solves the problems of conventionally proposed circuits as described above. The present invention will be explained below with reference to Examples. In the circuit shown in FIG. 2, A2 is a differential amplifier, the photoelectric output of the photodiode PD is applied to the inverting input terminal, and the output of A2 is fed back via the diode D for logarithmic conversion.

The collector-emitter of a transistor T3 is inserted in series with PD, and the base of the transistor is applied with the voltage of the power supply E divided by resistors R3 and R4 in a steady state, but a capacitor C2 is connected between the base and emitter. is connected, and R3 and C2 constitute a timekeeping means, so the moment the power switch S is closed, the base potential of T3 is O and T.
The collector current of T3 does not flow, and C2 is charged after a short period of time due to the timing operation in which capacitor C2 is charged through resistor R3, after which T3 becomes conductive. That is, the photodiode circuit remains open for a certain period of time from the moment switch S is closed, and after amplifier A2 reaches a steady state after startup, the PD circuit becomes conductive to prevent abnormal charging at startup. This results in a faster start-up transient. Note that the individual capacitances between the collector and base and between the collector and emitter of the transistor are sufficiently smaller than those of the photodiode, and the individual capacitances of the transistor T3 and the photodiode PD are connected in series into the input circuit of the amplifier A2. , A2, the capacitance component of the input circuit is extremely small compared to the case of using only a photodiode, and the photoelectricity generated when the power is turned on can be almost ignored. In addition, in the steady state of the photometric circuit, the current flowing through the photodiode PD flows through the series T3, so the current flowing through the diode D, which is the output current of the amplifier A2, is not shunted between the PD and the transistor as in the example of FIG. Since it is the photocurrent that flows through the sensor, there are no sources of error, and photometry can be performed accurately even at low brightness levels. The embodiment shown in FIG. 3 is basically the same as the example shown in FIG. 2, but has a different amplifier configuration and does not use a differential amplifier. Parts equivalent to those in FIG. 2 are given the same reference numerals. The output of photodiode PD is FETOI
) is amplified by T4 and at the same time the signal phase is inverted.
Amplifier A3 is an amplifier whose input and output are in phase, and the output of T4 is amplified and becomes an output. This output is negatively fed back to the input side of T4 through a feedback path in which a transistor T5 is inserted for logarithmic conversion, and the output voltage of the PD is held at approximately zero.
As described above, the photometric circuit of the present invention connects a switching element in series with a photodiode, controls this switching element with a time constant circuit that starts at the same time as the power switch is turned on, and shuts off the switching element for a certain period of time after the power switch is turned on. During this period, the amplifier enters a steady state, which prevents abnormal charging of the photodiode's own capacitance, shortens the transient time at startup, and allows the amplifier to enter the steady state as in the conventional case. The current flowing through the negative feedback path of the circuit is not shunted to the photodiode and other circuit members, and since the feedback current is an instantaneous photocurrent, it does not create a source of photometric error.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of a conventional example, and FIGS. 2 and 3 are circuit diagrams of the present invention, respectively. PD...Photodiode, T3...P
as a switching element in series with D. transistor,
C2, R3, R4... Capacitors and resistors forming the time constant circuit that controls T3, D... Diode in the feedback path that also serves as logarithmic conversion, T5... Above A transistor that has the same effect as.

Claims (1)

[Claims] 1. In a photometry circuit configuration that combines a photodiode and a negative feedback amplifier circuit and obtains a photoelectric output by keeping the voltage between both terminals of the photodiode constant through negative feedback action, the transistor connected in series with the photodiode and the It is connected to a power switch that supplies power to the photometric circuit and to the base of the transistor to control the cutoff and conduction of the transistor, and starts a timekeeping operation in response to the closing of the power switch, cutting off the transistor for a certain period of time. A photometric circuit characterized in that the photometric circuit is further provided with a timer for making the transistor conductive after the predetermined period of time has elapsed.