JPS6120830A - Optical integration circuit - Google Patents

Abstract

PURPOSE:To detect accurately the quantity of photodetection and to reduce the size of the circuit by providing plural transistors (Tr) for expansion. CONSTITUTION:Plural Trs 11' and 11'' which have a common base, emitter, and collector with a Tr11 are provided. Therefore, the signal of a photocurrent generated by a photodiode PD1 and converted into a voltage through Trs 11, 11', and 11'' connected to an operational amplifier 10 through a diode 12 is charged in a capacitor 12' as a current expanded through the plural Trs 11, 11', and 11''. Therefore, a logarithmic relation is born between the terminal voltage of the capacitor 12' and the quantity of light incident on the PD1, and even when the quantity of light incident on the PD1 is extremely small, the optical integration circuit is combine with a dimming circuit which raises the voltage of a reference voltage source by a specific value every time a dimming level is doubled, thereby reducing an error.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical integration circuit that converts the amount of light into an electrical signal.

The dynamic range of conventional basic optical integration circuits was not sufficient for changes in the amount of light.

This will be explained below using FIG. The conventional optical integration circuit shown in FIG. 1 stores the photocurrent generated in the photodiode 1 due to light incident on the photodiode 1 in a capacitor 2, and compares the terminal voltage of the capacitor 2 with the voltage of a reference voltage source 5 using a comparator 4. It was configured to detect the amount of light incident on the photodiode.

If the voltage generated by the reference voltage source 5 is set from a minimum of 18 mV to about 2 V, the dynamic range of the optical integrator circuit shown in FIG. 1, that is, the ratio of the minimum voltage to the maximum voltage, is 2 V/1 S.
It becomes 27 in mv, which is 7 when expressed in apex value.
You will only be able to get a grade.

By the way, when trying to use a strobe device with TTL flash control, the light integrating circuit of the flash control device of the strobe device has film sensitivity types ranging from 1806 to 806,400, and apex values of the photographic lens aperture ranging from Fl to F32. When converted into an apex value, a dynamic range of 10 steps is required. Therefore, when both the film sensitivity and aperture change independently, a dynamic range of 20 steps is required when converted to an apex value.

Therefore, if an attempt was made to control the amount of light by using the above-mentioned optical integration circuit in a strobe device, the dynamic range would be insufficient, making it impossible to control the amount of light. For this reason, a circuit is known, as shown in FIG. 2, in which the dynamic range of the optical integration circuit is increased by logarithmically compressing the photocurrent outputted by the photodiode.

In the optical integration circuit shown in Fig. 2, the compression diode 1
2 into the negative feedback transition path of operational amplifier 10 and
A capacitor 2 is connected to the emitter of a transistor 11 that extends the output of 0.

Here, if the photocurrent flowing through the photodiode 1 is Hp, it is expressed by the following equation below the voltage of the base of the transistor 11.

T 1p Vb=-gin-(1) 1s 18: For the reverse saturation current of the diode: Portsma/coefficient T: Absolute temperature q: Charge of electrons Also, let the emitter current of transistor 1 be 1(t),
Letting the terminal voltage of the capacitor 2 be Vc, it is expressed by the following equation.

Here, 1 (1) is determined by the voltage between the base and emitter of the transistor 110, and is expressed by the following equation.

1(t) == is 8Xp (-(Vb-Vc)
) (3) T To state only the conclusion, the terminal voltage Vc of capacitor 2 is expressed by the following equation.

Vc = αln (1; L+1)
(4) αQ is determined by αkneeQ=Of Ipdt (O is a constant).

Here, α is approximately 26 mV at room temperature based on known constants. Here, if Q is very large compared to 1, the VQ(7) value will be 18m by doubling Q.
It will increase by V (= 0.026/n2). That is, when the amount of light incident on the photodiode 1 doubles, the terminal voltage Vc of the capacitor 2 increases by 18 mV.

Here, the output voltage of the reference voltage generator 5 is changed from 18 mV to 1
If the apex value is changed to 800 mV, a dynamic range of 100 steps can be obtained.

However, since equation (4) showing the terminal voltage of the capacitor of the optical integrating circuit shown in Figure 2 includes GL+1 in the antilog term, Q is sufficient for 1 as shown in 1 in Figure 4. If it is large, a logarithmic relationship holds between the amount of light incident on the photodiode 1 and the terminal voltage of the capacitor 2, but
If Q is not sufficiently large relative to 1, a logarithmic relationship will not hold between the amount of light incident on the photodiode 1 and the terminal voltage of the capacitor 2. Figure 4(a) is (4)
The graph of the equation is shown in FIG. 4 (1,), which is a table showing the numerical values. For example, as shown in 1 in Figure 4, in a region where Q is sufficiently large, when Q doubles, the terminal voltage of the capacitor increases by 18 mV, but when Q is close to 1,
In the case of doubling, for example, when Q changes from 2 to 4, the terminal voltage of the capacitor does not increase by 13.2 mVl.

Generally, the light control level of a flash device is converted into an apex value, and when the apex value increases by one step, the reference level indicating the light control level is increased to a constant value. Therefore, when the above-mentioned optical integration circuit is used in the dimming circuit of a strobe device, when the dimming level is low, that is, when the flash is stopped, the amount of light received by the photodiode and the terminal voltage of the capacitor are Since there is no logarithmic relationship between the two, a large error occurs in the light control level, making it impossible to obtain proper exposure during strobe photography. By the way, in the case of a strobe device that emits relatively weak light towards the subject and controls the main flash according to the reflected light level of the pre-flash, the reflected light level of the blur-flash is small because the amount of pre-flash is small. Since this is extremely low, if an optical integration circuit such as the one described above is used, the reflected light level cannot be accurately measured, and the main light emission cannot be accurately controlled, which becomes an even bigger problem.

SUMMARY OF THE INVENTION An object of the present invention is to provide an optical integrator circuit which eliminates the drawbacks of the conventional optical integrator circuit described above and can accurately detect the amount of light received by a photodiode even when the amount of light received by a photodiode is extremely low.

The present invention will be explained in detail below using the drawings.

FIG. 3 is a block diagram of one embodiment of the present invention. In FIG. 3, elements having the same functions as the elements shown in FIGS. 1 and 2 are given the same reference numerals, and their explanations will be omitted.

In this figure, reference numerals 11' and 11' indicate transistors having a common base, emitter, and collector with the transistor 11, and at least two transistors are provided. Instead of providing a plurality of transistors that share a base, emitter, and collector with the operational amplifier 11, a transistor with an area multiple times the junction area between the base and emitter of the diode-connected transistor provided in the return path of the operational amplifier 10 is used. The same effect can be expected by providing a transistor having a junction area between pace emitters.

The operation of the embodiment of the present invention configured as above will be explained.

In this embodiment, a plurality of transistors having a common base, emitter, and collector with the transistor 11 are provided.

Therefore, the photocurrent signal generated in the photodiode 1 which is current-to-voltage converted and compressed by the operational amplifier 10 and the diode-connected transistor becomes a current extended by each of the plurality of transistors K, and charges the capacitor 2. Therefore, the terminal voltage V (t) of the capacitor 2 is expressed by the following equation.

V (t) = Q /n (nQ, +1) where n is the number of transistors whose base emitters and collectors are commonly connected to the transistor 11.

Therefore, if n is set such that Q > 1 in the above equation, that is, if an appropriate transistor is installed that shares the base, emitter, and collector with transistor 11, the terminal voltage v (t) of capacitor 2 will be as follows. It is approximated by Eq.

V(t)4=α1nnQ.

That is, the terminal voltage V(t) of the capacitor 2 and the amount of light incident on the photodiode 1 are expressed as (I) in FIGS. 4(a) and (b).
A logarithmic relationship holds as shown in I).

Therefore, even if the amount of light incident on the photodiode 1 is very small, the terminal voltage of the capacitor 2 will increase by 18 mV every time the amount of incident light is doubled.

That is, if the optical integration circuit of this embodiment is combined with a dimming circuit that controls the dimming level using the apex value, in other words, increases the voltage of the reference voltage source by a fixed value every time the dimming level is doubled. Errors can be made extremely small.

As explained above, according to the present invention, the photocurrent generated by the light incident on the light receiving element is logarithmically compressed, and the logarithmically compressed signal is expanded by the expansion transistor. Since a plurality of the extension transistors are connected in parallel in the optical integration circuit that integrates the amount of light incident on the light receiving element by charging the capacitor connected to the emitter of the In a region where the amount of light incident on the element is small, the disadvantage that a logarithmic relationship does not hold between the amount of light incident on the light receiving element and the terminal voltage of the capacitor is solved, and the amount of light incident on the light receiving element is small. It was possible to realize an optical integrator circuit in which a nearly logarithmic relationship is established between the amount of light incident on the light receiving element and the terminal voltage of the capacitor even in the area.

Furthermore, according to the present invention, the charging current of the extension sensor becomes larger in the conventional optical integrating circuit, and a relatively inexpensive photodiode of L type can be used as the light receiving element, and the optical integrating circuit can be miniaturized. play.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram of a conventional optical integrator circuit, Fig. 2 is a circuit diagram of a conventional optical integrator circuit that is an improved version of the optical integrator circuit in Fig. 1, and Fig. 3 is a circuit diagram of an embodiment of the present invention. FIGS. 4(a) and 4(b) are diagrams showing the relationship between the terminal voltage of the capacitor shown in FIGS. 2 and 3 and the amount of received light. 1... Photodiode '11, 11', 11'... Extension transistor 12
・・・Compression diode

Claims (1)

[Claims] The photocurrent generated by the light incident on the light receiving element is logarithmically compressed, and the logarithmically compressed signal is expanded by an expansion transistor, and the generated current charges a capacitor connected to the emitter of the expansion transistor. In the optical integration circuit that integrates the amount of light incident on the light receiving element,
An optical integration circuit characterized in that a plurality of the extension transistors are connected in parallel.