JPH029508B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an amplifier circuit that amplifies a difference in photocurrent, and particularly to a photocurrent amplifier circuit in which the gain of the amplifier circuit can be changed at low contrast compared to the gain at high contrast. It is.

Figure 1 is a circuit diagram showing a conventional photocurrent amplification circuit, where D ₁ and D ₂ are photodetectors, A ₁₁ and A ₁₂ are conversion circuits that convert photocurrent into voltage, A ₂ is an operational amplifier, R ₁₁ ,
R ₂₁ , R ₁₂ , and R ₂₂ are resistors for determining the gain of operational amplifier A ₂ . FIG. 2 shows the operating characteristics of FIG. The operating principle of the photocurrent amplification circuit shown in FIG. 1 will now be explained.

The light amounts λ ₁ and λ ₂ that enter the light receiving elements D ₁ and D ₂ as input have different values, for example, the light amounts of images at the center and the periphery of a camera optical sensor (photodiode), and the light receiving elements D 1 and D 2 have different values. The currents are converted into currents I〓 ₁ and I〓 ₂ by elements D ₁ and D ₂ , respectively. These currents I〓 ₁ and I〓 ₂ are converted into voltages V _A11 and V _A12 by conversion circuits A ₁₁ and A ₁₂ , respectively. At this time, since the values of the light quantities λ ₁ and λ ₂ are different as described above, the currents I〓 ₁ and I〓 ₂ and, as a result, the voltages V _A11 and V _A12 are also different values. Furthermore, this voltage is converted to the differential voltage _ΔV (=V _A12 −
V _A11 ) is amplified to obtain the output voltage V ₀ .

However, the circuit of FIG. 1 has the following drawbacks. In other words, when the difference in light intensity is small (low contrast), the relationship between the difference in light intensity and the output changes linearly, as shown in Figure 2, so there may be no problem in signal processing. If there is a large difference, depending on how the gain of the amplifier _A2 is selected, a region (so-called saturated region) may occur where the relationship between the difference in light amount and the output is not linear. This does not pose a problem when the power supply voltage used is high, but it becomes a problem especially when the power supply voltage is low. Of course, by reducing the gain of the amplifier _A2 , it may be possible to use it even at high contrasts, but in this case, the absolute value of the output voltage _V0 at low contrasts will be small,
Signal processing in the next stage circuit that uses this output voltage V ₀ becomes complicated. In other words, the magnitude of meaningless noise other than the optical signal, such as circuit noise, external induction, and external noise, becomes a problem, and this has a serious negative impact on the linear relationship between light intensity difference and output at low contrast. affect In other words, in this conventional circuit, when using a low power supply voltage, there are cases where a linear relationship between the difference between the two input light amounts and the output does not hold at low contrast and high contrast, and it is inappropriate for signal processing. occurs.

An object of the present invention is to provide a photocurrent amplification circuit that does not have the above-mentioned drawbacks by changing the gain of the amplifier.

FIG. 3 shows an embodiment of the present invention. _R31 ,
R ₃₂ is a resistor, SW ₁ and SW ₂ are switches for changing the gain of operational amplifier circuit A ₂ , C ₁ and C ₂ are voltage comparators, and OR is a logical sum that determines the magnitude of the output of the voltage comparator. The circuit and other symbols are the same as in FIG.

FIG. 4A is a diagram showing the operating characteristics of the embodiment circuit of FIG. 3, and FIGS. 4B to 4E are characteristic diagrams of each part for explaining the operation. The operation of this embodiment will be explained with reference to FIGS. 4A to 4E. The amounts of light λ ₁ and λ ₂ that enter as input are converted into currents by the light receiving elements D ₁ and D ₂ , respectively.
Converted to I〓 ₁ and I〓 ₂ . These currents I〓 ₁ and I〓 ₂ are converted into voltages V _A11 and A _A12 by conversion circuits A ₁₁ and A ₁₂ , respectively. In the case of low contrast, that is, the absolute value of the difference between the output voltages of the conversion circuits A ₁₁ and A ₁₂ |ΔV|=
When |V _A12 −V _A11 | is smaller than the reference setting voltage difference ΔVd, the outputs of voltage comparators C ₁ and C ₂ both become low, as shown in FIGS. 4B and C. This reference setting voltage difference ΔVd is set by providing it as an offset voltage on the positive input sides of the voltage comparators C ₁ and C ₂ . The outputs of the voltage comparators C ₁ and C ₂ are input to the OR circuit, and in this case, the output of the OR circuit becomes low as shown in Figure 4D, and at this low signal, as shown in Figure 4E. Switch SW ₁ ,
SW ₂ connects contact c and contact a. In this case, the gain of operational amplifier circuit A ₂ is R ₂₁ /
R ₁₁ (R ₁₁ = R ₁₂ and R ₂₁ = R ₂₂ ).

In case of high contrast, i.e. conversion circuit A ₁₁ ,
Absolute value of the difference in output voltage of A ₁₂ |ΔV|=|V _A12 −
When V _A11 | is larger than the reference setting voltage ΔVd, as shown in FIG. 4B and C, the voltage comparators C ₁ ,
One output of _C2 will be low and the other will be high. The outputs of the voltage comparators C ₁ and C ₂ enter the OR circuit, and in this case, the output of the OR circuit becomes high as shown in Figure 4D, and when this high signal is present, the output as shown in Figure 4E , switches SW ₁ and SW ₂ connect contact point c and contact point b. The gain in this case is R ₃₁ /R ₁₁ (R ₁₁ = R ₁₂ and R ₃₁ = R ₃₂ ).

If we choose R ₂₁ /R ₁₁ > R ₃₁ /R ₁₁ , we get Figure 4A.
The following characteristics are obtained. That is, when the contrast is low, the slope of the output can be made large, and when the contrast is high, the slope of the output can be made small.

As described above, in this invention, the gain at low contrast is made larger than the gain at high contrast, so that the linearity of the signal at low contrast can be ensured, and the linearity at high contrast can be ensured. It has the advantage that it can be secured and signal processing can be facilitated. Therefore, according to this invention,
Processing of optical signals becomes easier, which simplifies the circuit. Particularly when applied to an integrated circuit, manufacturing becomes easier and cost increases can be prevented.

[Brief explanation of drawings]

Figure 1 is a circuit diagram showing a conventional photocurrent amplification circuit.
FIG. 2 is a diagram showing the operating characteristics of a conventional circuit, FIG. 3 is a circuit diagram showing an embodiment of the present invention, and FIGS.
E is a diagram explaining the operating principle of this embodiment circuit. In the figure, D ₁ and D ₂ are light receiving elements, A ₁₁ is a first conversion circuit, A ₁₂ is a second conversion circuit, C ₁ is a first voltage comparator, C ₂ is a second voltage comparator, OR is an OR circuit, _A2 is an operational amplifier, and _SW1 and _SW2 are gain switching switches. Note that the same reference numerals in the figures indicate the same or corresponding parts.

Claims (1)

[Claims] 1. First and second conversion circuits that convert photocurrents obtained from two light receiving elements into voltages corresponding to the respective photocurrent values, and the output voltages of these conversion circuits are inputted and perform the second conversion. a first voltage comparator that outputs a logic value "1" when the output voltage of the circuit is greater than the output voltage of the first conversion circuit by a predetermined value or more, and outputs a logic value "0" at other times; When the output voltages of the two conversion circuits are input and the output voltage of the first conversion circuit is greater than the output voltage of the second conversion circuit by a predetermined value or more, a logic value "1" is output; otherwise, the logic value is "1". a second voltage comparator that outputs the value "0"; an OR circuit that obtains the logical sum of the outputs of the first and second voltage comparators; and and an operational amplifier circuit which amplifies the difference and whose amplification gain is controlled such that it is large when the output of the OR circuit has a logic value of "0" and becomes small when the output of the OR circuit is a logic value of "1". photocurrent amplification circuit.