JPS60157307A - Logarithmic amplifier circuit - Google Patents

Abstract

PURPOSE:To obtain a logarithmic amplifier circuit having an illumination detecting function to a temperature change, by separating the illumination light into signals of each color and giving the logarithmic compression and then the differential amplification those separated signals. CONSTITUTION:The illumination light is separated into each color by filters 17 and 18. These separated colors are supplied to a differential amplifier 32 via temperature compensation registers 27 and 28 after undergoing the photoelectric conversion by photodiodes 14 and 15 and then the logarithmic compression by logarithmic amplifier 23 and 24 respectively. Therefore a signal output is obtained at an output terminal 23 according to the ratio of signal quantity of each color. Here the output of amplifiers 23 and 24 are turned into differential outputs by the amplifier 32. Thus the reverse saturation current components of diodes 19 and 20 which affect the temperature characteritics of amplifiers 23 and 24 are offset. The temperature characteristics of resistors 27 and 28 are set at kT/q (k: Boltzman constant; T: absolute temperature; q: characteristics for offset of electron charge amount). In such a constitution, a logarithmic amplifier circuit can attain its desired purpose.

Description

[Detailed Description of the Invention] Industrial Application Field The present invention relates to a logarithmic amplifier circuit that can be used in a photodetection circuit for detecting the properties of illumination light and that can realize stable photodetection against temperature changes. be.

Conventional Structure and Problems Therewith FIG. 1 is a circuit diagram showing the structure of a generally known logarithmic amplifier circuit. Generally, a logarithmic amplifier circuit is a semiconductor PN
This utilizes the property that the relationship between the forward current i in the junction and the terminal voltage V exhibits a relatively good exponential characteristic. The formula is 1-Is・(exp complex-1) where q is the amount of charge of the electron,
is the Boltzmann constant, T is the absolute temperature, and ■8 is the reverse saturation current.

Below, FIG. 1 will be explained. Input current I from input terminal 1 is input to operational amplifier 2. Resistor 3゜Transistor 4
is supplied to a logarithmic compressor consisting of transistor 4
On the other hand, the transistor 6 is supplied with a constant current I0 from the current source 6, and becomes the base of the transistor 6. On the other hand, the operational amplifier 7 has a resistor R
18, the resistor R29 constitutes a non-inverting amplifier, and since a signal voltage of V2-Vl is applied to the non-inverting input terminal 1o, the output signal V. 11 is, Here, ■6 is normally very small compared to I, Io, and if it is used under conditions where it can be ignored even if it changes due to temperature, If a resistor whose resistance value increases proportionally is used, the output signal v
0 can be stabilized against temperature changes.

Usually, the device for detecting the color properties of Il@ bright light is as follows:
Even if the amount of light changes, if the color of the illumination light does not change, it is necessary to output the same detection signal. Considering the case where the simplest illumination light detection device separates illumination light into two colors, red and blue components, and detects the ratio, the detection circuit consists of two sets of the circuits shown in Figure 1. A differential amplifier is required to detect the difference between the output signals of the two edges.
The circuit configuration becomes complicated, resulting in a significant increase in cost.

OBJECTS OF THE INVENTION An object of the present invention is to provide a logarithmic amplifier circuit that can detect the ratio of two signal currents and realize an illumination light detection function that is stable against temperature changes with a relatively simple circuit configuration. be.

Configuration of the Invention The logarithmic amplifier circuit of the present invention has the following features: a second input terminal, a first input terminal consisting of a diode and an operational amplifier; a second logarithmic compressor; a first logarithmic compressor; a second temperature compensation resistor and a differential amplifier; The signal from the second input terminal is
Each of the above 1. the first logarithmic compressor; The output signals of the second logarithmic compressor are respectively the first and second logarithmic compressors.
the second temperature compensation resistor; The two signals passed through the second temperature compensation resistor are configured to be supplied to the differential amplifier, and the first . A signal responsive to the ratio of the amounts of two input signals from the second input terminal is obtained.

DESCRIPTION OF THE EMBODIMENT FIG. 2 is a circuit diagram showing an embodiment of the present invention. 1st
．． The second input terminals 12.13 are each supplied with a photocurrent R2IB of a photodiode 14.16. FIG. 2 shows a case in which the present invention is applied to an illumination light detection device that detects the ratio of red and blue components contained in illumination light. The illumination light is collected by a condenser 16, passes through a red transmission groove filter 17 and a blue transmission filter 18, reaches a photodiode, and is photoelectrically converted. The photocurrent IR2■B is transmitted through diodes 19, 20 . A first circuit consisting of operational amplifiers 21 and 22. Second logarithmic compressor 2
Logarithmically compressed by 3.24. 1. Assuming that the output signals of the second logarithmic compressors 23 and 24 are VR25 and VB26, respectively, where ■ indicates the reverse saturation current of the diode 19.20, IR>>IK, IB>>IK 1. Assuming that the second temperature compensation resistors 27 and 28 are the same and their resistance value is R8, and the resistors 29 and 30 are the same and their resistance value is R, then the resistors 29 and 30 and the operational amplifier As is clear from the soil formula, the output signal v033 of the differential amplifier 32 constituted by the differential amplifier 31 does not affect the output due to (1), which is one of the factors that influence the temperature characteristics of the logarithmic compressor. On the other hand, kT/q increases linearly at a rate of about 3350 ppm per 1 degree Celsius increase in temperature, so the temperature coefficient of the temperature compensation resistor 27.28 is also R.

Taking into account the temperature coefficient of , the value is similarly 3200 to 3600, resulting in a signal that responds only to the ratio of the input signal current IR2■B. In other words, it is possible to obtain a detection signal that does not respond to changes in the amount of illumination light but only responds to the color properties of the illumination light and is stable against temperature changes.

Note that the power supply 34 has a function of providing a reference potential for the logarithmic compressor, and the terminal 36 is a terminal for providing a potential for the differential amplifier 32, and at the same time, a terminal for performing offset adjustment.

Note that the polarities of the diodes 19 and 20 are the same as those of the photodiodes i4. When using i5 with its polarity reversed, it is also necessary to reverse it.

Effects of the Invention As described above in detail, the present invention realizes a logarithmic amplifier that is stable against temperature changes using a relatively conventional circuit configuration of three operational amplifiers.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing a conventional logarithmic amplifier, and FIG. 2 is a circuit diagram showing an embodiment of the present invention. 1.12.13...Input terminal, 2,7,21°22.31...Operation amplifier, 3,9,29.3
0...Resistor, 4.6...Transistor, 6...Current source, 8,27.28...
・Temperature compensation resistor, 14° 16...Photodiode, 16...Concentrator, 17...Red light transmission filter, 18...Blue light transmission Filter, 19.20...Diode, 23°24
...logarithmic compressor, 32...differential amplifier.

Claims (2)

[Claims] (1) First. A first input terminal consisting of a second input terminal, a diode and an operational amplifier. a second logarithmic compressor and a first logarithmic compressor; a second temperature compensation resistor and a differential amplifier;
．． The signals at the second input terminals respectively correspond to the first and second input terminals. Second
is supplied to the logarithmic compressor of the first. The output signals of the second logarithmic compressor are respectively the first and second logarithmic compressors. the second temperature compensation resistor; A logarithmic amplifier circuit characterized in that the two signals passing through the second temperature compensation resistor are supplied to the differential amplifier. (2) The logarithmic amplifier circuit according to claim 1, wherein the first and second temperature compensating resistors are constructed of resistors having the same temperature characteristics.