JPS6341410B2 - - Google Patents

Description

[Detailed description of the invention] The present invention relates to a photometry circuit for a camera.

A photovoltaic element such as a photodiode is a fast-responsive element, but when used in a photometry circuit of a camera, a power response delay occurs on the low volatility side due to reverse charging of its junction capacitance. To prevent this, the conventional method was to connect a diode in parallel in the opposite direction to the logarithmic compression diode in the photometry section to discharge some of the charge reversely charged to the junction capacitance of the photodiode. A device that temporarily increases the photocurrent of a photodiode and discharges the charge after the power is turned on.
No. 19067, Publication No. 53-19068, No. 31931 of Publication No. 50-31931
proposed a method to increase the power response speed by connecting something other than a photodiode and a logarithmic compression diode to the inverting input side of a photometric operational amplifier, as shown in No. However, in a photometric circuit, the diode connected in parallel with the logarithmic compression diode in the opposite direction has a small forward voltage and a small forward current, so the charge reversely charged in the photodiode's junction capacitance is discharged in a short time. Therefore, the effect of increasing the power response speed is small. Furthermore, the photometry circuit requires a light source for temporarily increasing the photocurrent of the photodiode and a circuit for turning on the light source for a short time in synchronization with the power switch, making the configuration complicated. Furthermore, in the photometric circuit, since something other than the photodiode and logarithmic compression diode is connected to the inverting input side of the photometric operational amplifier, leakage is likely to occur on the inverting input side of the photometric operational amplifier, and the leakage current may affect the photometric accuracy. This causes a decrease in When a capacitor is connected to the inverting input side of a photometric operational amplifier, the capacitor has a leakage current, and this leakage current also causes a reduction in photometry accuracy. Therefore, even if the power supply response on the low brightness side is made faster, photometry on the low brightness side is no longer possible due to leakage current.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a photometry circuit which improves the above-mentioned drawbacks and improves the power response on the low luminance side by simply adding a simple circuit to the photometry section.

The present invention will be described below by way of examples with reference to the drawings.

In the first embodiment of the present invention, the power response speed is increased by minimizing the amount of reverse charge to the junction capacitance of the photodiode, which causes a power response delay on the low brightness side. Furthermore, since the inverting input terminal of the photometric operational amplifier has an extremely high resistance, only the minimum required photodiode and logarithmic compression diode are connected to prevent the photometric accuracy from deteriorating.

FIG. 1 shows this first embodiment, and FIG. 2 is its timing chart. Operational amplifier OP ₁ consists of field effect transistors FET ₁ , FET ₂ , transistors Tr ₁ to _{Tr 6} , constant current sources CC ₁ to _{CC 4} , and capacitors.
C ₁ , resistors R ₁ and R ₂ , and together with the logarithmic conversion diode D ₁ and the photodiode D ₂ , constitute a photometry section MP. Photodiode D ₂ is operational amplifier OP ₁
The logarithmic compression diode _D1 is connected between the inverting and non-inverting input terminals of the operational amplifier.
Connected between the output terminal of OP ₁ and the inverting input terminal. The non-inverting input terminal and common terminal N of operational amplifier OP ₁ are grounded, and the power supply terminal P is connected to the power switch.
Connected to DC power supply V _B via SW ₁ . The delay circuit DL ₁ operates for a certain period of time after the power is turned on, and a timer or the like is used. This delay circuit DL ₁
is connected to point a, and point a is connected to a DC power supply _VB via a power switch _SW1 . The collector of the shorting transistor Tr ₇ is the output terminal of the photometry section MP.
The emitter is connected to V ₀ and grounded, and the base is connected to the output terminal b of the delay circuit DL ₁ via a resistor R ₃ . One end of the rise time control capacitor _C2 is connected to the output terminal _V0 , and the other end is connected to the point a through the resistor _R4 and the transistor _Tr8 in series, and to the operational amplifier _OP1 through the diode _D3 . It is connected to the base of transistor _Tr5 in. The base of transistor Tr ₈ is resistor R ₅ , transistor
Grounded through Tr ₉ in series, transistor Tr ₉
The base of is connected to point b via resistor _R6 .

Next, the operation of the first embodiment will be explained in detail.

When the power switch _SW1 is turned on, the output of the DC power supply _VB is supplied to point a, and the output of the delay circuit _DL1 becomes high level. Therefore, the short circuit transistor
Tr ₇ is turned on to short-circuit the output side of the photometer MP, and this state is continued by the time delay circuit DL ₁ from when the photometer MP turns on the power switch SW ₁ until it operates stably. Also, transistors Tr ₈ and Tr ₉ are turned on, and capacitor C ₂ is charged through resistor R ₄ . Thereafter, when the output of the delay circuit _DL1 becomes low level, the transistors _Tr7 to _Tr9 are turned off.
Then, the output of operational amplifier OP ₁ tries to rise to a high level due to the open circuit voltage of photodiode D ₂ , but the charge stored in capacitor C ₂ flows into operational amplifier OP ₁ via diode D ₃ and suppresses its output. The operational amplifier works to lower the level
The output of OP ₁ rises as shown in Figure 2 A while being regulated by the current flowing from capacitor O ₂ . As the output of the operational amplifier _OP1 increases in this way, current flows from the output terminal _V0 to the photodiode _D2 via the diode _D1 . This current discharges the charge in the junction capacitance of the photodiode D ₂ that has been charged by the open-circuit voltage of the photodiode D ₂ . It is sufficient if the current from the output terminal V ₀ stops when the photodiode D ₂ becomes zero bias, but since there is a delay in stopping the current by the response time between the input and output of the operational amplifier OP ₁ , the photodiode The junction capacitance of diode D ₂ is reversely charged.
However, this reversely charged charge is transferred to the capacitor C ₂
The current flows through the diode _D3 as described above, and the output is regulated, resulting in a small value. The amount of reverse charge increases as the voltage rise speed at the output terminal of the logarithmic compression diode _D1 , that is, the output terminal of the photometric amplifier increases, and the time required to reach the normal level increases. Therefore, if the time constant of the circuit consisting of capacitor C ₂ and resistor R ₄ in Figure 1 is made large, the speed at which the voltage rises at the output terminal of the photometric amplifier is reduced, and the amount of reverse charging can be made sufficiently small. The charge reversely charged to the junction capacitance of photodiode _D2 is discharged by the photocurrent of photodiode _D2 ,
When the output reaches the target value as shown in FIG. 2A, normal photometry operation is performed thereafter.

By the way, with this configuration, in order to reduce the amount of reverse charging, it is necessary to gradually allow the current to flow through the feedback diode _D3 for a long time, but this will lengthen the charging time and cause the photometric amplifier to become inoperable. As the time becomes longer, the overall response time becomes longer even if the subsequent normal level is reached quickly.

However, if the current of the capacitor _C2 is supplied through the diode _D3 to the base of the transistor _Tr5 as in this embodiment, when the output of the photometric amplifier approaches the normal value, the impedance of the diode _D3 will decrease. As the feedback current becomes higher and the feedback current decreases, the rising speed of the photometric amplifier becomes sufficiently slow when the capacitor _C2 finishes discharging. Even if the initial rising speed is fast, the final rising speed is slow in this way, so even if the capacitor C ₂ is discharged for a short time, the reverse charging of the photodiode D ₂ can be sufficiently small. In other words, the brightness of the light to be measured is converted into an electrical quantity by photodiode D ₂ and then sent to operational amplifier OP _1.
The signal is then logarithmically compressed by the logarithmic compression diode D ₁ and output from the output terminal V ₀ .

FIG. 3 shows a second embodiment of the invention.

In this second embodiment, the output of the delay circuit DL ₂ is inverted from that of the delay circuit DL ₁ , and the output is a transistor
Tr ₉ is omitted. The base of the transistor Tr ₈ is connected to point b via a resistor R ₅ , and the shorting transistor Tr ₁₀ is of PNP type.

FIG. 4 shows a third embodiment of the invention.

This third embodiment is an example in which the output has a polarity opposite to that of the above embodiment, and the photodiode D ₂ and the diodes D ₁ and D ₃ are oriented in opposite directions. operational amplifier
The non-inverting input terminal of OP ₁ is grounded via resistor R ₇ and connected to point a via resistor R ₈ , and the output terminal of operational amplifier OP ₁ is connected to output terminal V ₀ via resistor R ₉ . be done. The transistor Tr ₁₀ is connected between the output terminal V ₀ and the point a, and one end of the capacitor C ₂ is grounded through the resistor R ₄ and the transistor Tr ₈ in series, and is connected to the transistor Tr ₂ through the diode D ₃ . connected to the base.

As described above, according to the present invention, a photovoltaic element,
an operational amplifier to which the photovoltaic element is connected to the input side; a photometric section having a logarithmic compression diode connected to the feedback circuit of the operational amplifier; a delay circuit that operates for a certain period of time when the power is turned on; a first switch that is turned on when the delay circuit is activated by the output signal of the delay circuit to short-circuit the output side of the photometric section; a capacitor with one end connected to the output terminal of the photometric section; and a switch between the capacitor and the power supply. a second switch provided in the switch that is turned on when the delay circuit is operated by the output signal of the delay circuit to charge the capacitor; and a second switch between the other end of the capacitor and a predetermined location other than the input side of the operational amplifier. A rectifying element is connected to the photovoltaic device such as a photodiode, and the rectifying element regulates the output signal of the photometry section by discharging the capacitor to the photometry section when the first switch and the second switch are turned off. The amount of reverse charge to the capacitance of the element can be reduced. Therefore, even on the low brightness side, a photometric output can be obtained almost at the same time as the camera is powered on, the power can be turned on just before the camera is released, and the photometric range can be extended to low brightness. Furthermore, since it is only necessary to add a simple circuit to the photometry section, the cost is low and the performance can be improved.Furthermore, since the operation immediately after power is turned on and the photometry operation can be performed in a time-sharing manner, current consumption can be reduced. Further, since no circuit is added to the input side of the operational amplifier in the photometry section, there is no leakage on the input side of the operational amplifier, and the photometry accuracy is not degraded.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram showing one embodiment of the present invention, Figure 2 is a circuit diagram showing an embodiment of the present invention.
The figure is a timing chart of the same embodiment, and FIGS. 3 and 4 are circuit diagrams showing other embodiments of the present invention. DL ₁ , DL ₂ ...Delay circuit, C ₂ ...Capacitor, D ₃ ...
Rectifying element, Tr ₇ to Tr ₁₀ ...switch, MP...photometering section,
D ₁ ... Logarithmic compression diode, D ₂ ... Photovoltaic element,
OP ₁ ...Operation amplifier.

Claims (1)

[Claims] 1. A photometry unit that includes a photovoltaic element, an operational amplifier to which the photovoltaic element is connected to the input side, and a logarithmic compression diode connected to the feedback circuit of the operational amplifier, and operates for a certain period of time when the power is turned on. a delay circuit; a first switch that is turned on when the delay circuit is operated by an output signal of the delay circuit to short-circuit the output side of the photometric section; and a capacitor having one end connected to the output terminal of the photometric section; A second switch is provided between the capacitor and the power source and is turned on when the delay circuit is operated by the output signal of the delay circuit to charge the capacitor, and a switch other than the other end of the capacitor and the input side of the operational amplifier. and a rectifying element connected between a predetermined location of the switch and a rectifying element that regulates the output signal of the photometric section by discharging the capacitor to the photometric section when the first switch and the second switch are off. .