JPS62209923A - Light receiving circuit - Google Patents

Abstract

PURPOSE:To detect only the modulating signal light of a high frequency, which is a purpose, without fail by providing a time constant circuit to set so as to make a fly-back quantity to a direct current or a low frequency larger than the fly-back quantity to a high frequency at the fly-back circuit of an amplifier. CONSTITUTION:Since in case of the photoelectric current of a high frequency due to a modulating signal light, a capacitor C shows a low impedance to the voltage of a high frequency, by setting suitably the time constant of a time constant circuit TC, the fly-back voltage of the high frequency is bypassed and the fly-back is not loaded through the first resistance R1. Namely, the time constant circuit TC is operated just like a high-cutting filter. Consequently, in the photoelectric current of a high frequency, the voltage is flown back only through a resistance Rf for fly-back. Here, since the resistance value of the resistance Rf for the fly-back is larger than the sum of respective resistances of the first and second resistances R1 and R2, the fly-back quantity to the photoelectric current of the low frequency is larger than the fly-back quantity to the photoelectric current of the high frequency, only the high frequency modulating signal, which is a purpose, is outputted without being influenced by the photoelectric current of the low frequency to a disturbance light, etc., and saturation due to the disturbance light can be prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a light receiving circuit for an electronic switch for detecting an object that operates, for example, an automatically opening/closing door.

<Prior Art> The biggest problem in the light receiving circuit of this type of optoelectronic switch is disturbance from external light that is stronger than the intended signal light, such as sunlight or general illumination light. Normally, when interference by disturbance light is expected in a photoelectronic switch, an intermittent light beam, that is, a modulated signal light, is projected from the light projector of the optoelectronic switch in order to separate the disturbance light and the signal light.

In this case, a difference is created between the modulation frequency of the signal light and the modulation frequency included in the disturbance light such as general illumination light to remove the influence of the disturbance light.

<Problems to be Solved by the Invention> However, if the light receiving circuit including the load of the light receiving element becomes saturated due to strong disturbance light entering the light receiving element, the signal output by the signal light will decrease. This makes it impossible to separate the signal due to the disturbance light and the signal light in an electric circuit connected after the light receiving circuit.

As a means to solve the above-mentioned problems, there are optical methods that narrow the field of view of the photodetector to prevent as much disturbance light other than signal light from entering the photodetector, and a low-frequency transformer that can be used as a shield for the photodetector. However, the former method cannot be applied to photodetectors that require a wide field of view, and the latter method connects the photodetector to the low-impedance side of a low-frequency transformer. At the same time, the high impedance side is the output side, and the load impedance of the light receiving element is approximately constant, but when strong disturbance light enters the light receiving element, the transformer core becomes magnetically saturated and a signal output is obtained. Another drawback is that the iron core vibrates due to impact, and the vibrations become electrical output and are transmitted to the output side, or receive electromagnetic induction from other electromagnetic devices.

<Purpose of the Invention> The present invention has been made in view of the above-mentioned problems, and it is possible to output only a signal based on the target signal light, with almost no effect even when receiving strong interference from external light. The purpose of this invention is to provide a light receiving circuit that can be used.

Means for Solving the Problems> In order to achieve the above object, the light receiving circuit of the present invention includes a light receiving element that receives high frequency modulated signal light and converts it into an electric signal, and an output current of the light receiving element. An amplifier for converting to voltage and for amplification, whose virtual ground point is connected to the light-receiving element to reduce the load impedance of the light-receiving element, and a feedback circuit of this amplifier that converts the amount of feedback for direct current or low frequency into high frequency. The gist of the present invention is to include a time constant circuit that is set to be larger than the amount of feedback for.

Effect> With the above configuration, when the target high-frequency modulated signal light and the DC or low-frequency disturbance light are simultaneously received by the light receiving element, the DC or low-frequency photocurrent due to the disturbance light is transmitted to the amplifier. By inputting the input, the feedback amount of the amplifier increases or decreases according to the intensity of this photocurrent, and the output level of the photocurrent due to this disturbance light (VO=-IX (rl+r2)
) becomes approximately constant. On the other hand, when high-frequency modulated signal light is amplified by an amplifier, it is hardly fed back by the time constant circuit of the feedback circuit, and is directly amplified and output (VO = -I -
r f) be done. Therefore, only the output signal from the signal light can be obtained, and since the load impedance of the light receiving element is small, the influence of the internal resistance of the light receiving element can be ignored. However, the aforementioned ■o is the output level, ■ is the photocurrent, rl,
r2 indicates the resistance value of each resistor in the feedback circuit, and rf indicates the resistance value of the feedback resistor.

<Example> Hereinafter, an example of the present invention will be described in detail based on the drawings.

In FIG. 1 showing one embodiment, a light receiving element PD consisting of a photodiode is connected to a time constant circuit TC consisting of a first resistor R1 and a capacitor C and an inverting input terminal of an operational amplifier op. Since the non-inverting input terminal of the amplifier OP is grounded, it is equivalent to grounding the inverting input terminal 1 as well as the non-inverting input terminal. That is, it is virtually grounded, and this virtual ground point B is connected to the light receiving element PD. Further, the output terminal of the operational amplifier OP is led out as the signal output terminal VO, and the output terminal is connected to the first resistor R1 of the time constant circuit TC via the second resistor R2.
It is connected to the series connection point of capacitor C and capacitor C, forming a feedback circuit. As the feedback resistor Rf of the operational amplifier OP, one having a resistance value greater than the sum of the resistance values of the first and second resistors R1 and R2 is used.

Next, the operation of the embodiment circuit configured as described above will be explained with reference to FIG.

When low-frequency disturbance light and high-frequency modulated signal light simultaneously enter the light-receiving element PD, the light-receiving element PD
As shown in FIG. 2(a), a photocurrent flows in which a low-frequency photocurrent A due to disturbance light and a high-frequency photocurrent aperture due to modulated signal light are superimposed. When this photocurrent is input to the operational amplifier OP, the low-frequency photocurrent A is fed back through the feedback resistor Rf and the first and second resistors R1 and R2, respectively, and the amount of feedback is shown in Fig. 3(b). As shown, the current waveform is almost similar to photocurrent A. Therefore, the output due to this photocurrent A remains almost constant and is not output as a signal. On the other hand, in the case of a high-frequency photocurrent caused by modulated signal light, the capacitor C shows a low impedance with respect to the high-frequency voltage. The feedback voltage is bypassed, and no feedback is applied through the first resistor R1. That is, the time constant circuit TC functions as a high-cut filter. Therefore, in this high frequency photocurrent,
It is fed back only through the feedback resistor Rf. Here, since the resistance value of the feedback resistor Rf is larger than the sum of the respective resistance values of the first and second resistors R1 and R2 as described above, the amount of feedback for the low frequency photocurrent is greater than the high frequency photocurrent. (As shown in Figure C1, only the desired high-frequency modulation signal is output without being affected by low-frequency photocurrent caused by disturbance light, etc., and moreover, it is possible to avoid saturation caused by disturbance light. In addition, the light receiving element PD
is connected to the virtual ground point A of the operational amplifier OP, so the resistance value of the feedback resistor Rf is the first. Second resistor R1,R
Even if the resistance value of the feedback resistor Rf is larger than the sum of the resistance values of the operational amplifier OP, the load impedance Z of the photodetector PD against the high-frequency photocurrent can be
When the loop gain is A, it is expressed as Z=rf/A, and this impedance Z is much smaller than the resistance value of the first resistor R1, ignoring the influence of the internal resistance of the photodetector PD. can.

FIG. 3 shows another embodiment of the present invention, and in FIG. 3, the same or equivalent parts as in FIG. 1 are given the same reference numerals. Then, the time constant circuit TC is connected to the feedback resistor Rf.
A series circuit of a transistor Q and a second resistor R2 is connected in parallel to this time constant circuit TC, and the base of the transistor Q is connected to the first resistor R2 of the time constant circuit TC.
It differs from the one shown in FIG. 1 in that it is connected to the series connection point of resistor R1 and capacitor C.

Next, the operation of this embodiment will be explained. When the DC or low-frequency photocurrent due to disturbance light increases, the operational amplifier OP is a reverse phase amplifier and the input and output have opposite polarities, so the output signal level of the output terminal v□ shifts to a negative level from the ground level. Accordingly, the emitter of the transistor Q also becomes a negative level, the base current of the transistor Q increases through the first resistor R1, and the collector current increases. That is, as the collector current increases or decreases in response to the increase or decrease in photocurrent due to disturbance light, the output terminal V
The output signal level of o becomes constant. On the other hand, when the photocurrent due to the high frequency modulated signal light increases, the output terminal vo
The output signal level of transistor Q becomes negative level and
The emitter of the transistor Q also becomes a negative level, but at the same time, the base of the transistor Q also becomes a negative level through the capacitor C, which has a low impedance to high frequencies. Therefore, even if the photocurrent due to the high-frequency signal light increases, neither the base current nor the collector current of the transistor Q increases. In other words, feedback is not applied by the time constant circuit TC. Therefore, as in the embodiment described above, only the desired high frequency modulation signal can be extracted without being affected by disturbance light.

Note that the present invention is not limited to the above embodiments,
Of course, various embodiments are possible based on the scope of the claims. For example, although a case has been described in which a photodiode is used as the light receiving element PD, it goes without saying that similar effects can be obtained by using other light receiving elements such as a semiconductor device detection element or a phototransistor.

<Effects of the Invention> As detailed above, according to the light receiving circuit of the present invention, the light receiving element is connected to the virtual ground point of the amplifier, and the feedback circuit of the amplifier is connected to the feedback amount for direct current or low frequency waves. The configuration includes a time constant circuit that is set to be larger than the amount of feedback, so even if there is strong interference from external light, there is little saturation or effect on the high frequency signal, and the desired high frequency modulation can be achieved. Only the signal light can be reliably detected. Further, since the load impedance of the light receiving element is small, the influence of the internal resistance of the light receiving element can be ignored, and there is an advantage that good responsiveness can be obtained.

[Brief explanation of drawings]

FIG. 1 is an electrical wiring diagram of an embodiment of the light receiving circuit of the present invention, FIGS. 2(a) to (C1 are respectively operational waveform diagrams of FIG. 1,
FIG. 3 is an electrical wiring diagram of another embodiment of the present invention. PD - Light receiving element OP - Amplifier TC temporary constant circuit

Claims (1)

[Claims] (1) A light-receiving element that receives high-frequency modulated signal light and converts it into an electrical signal, and a virtual ground point connected to the light-receiving element for converting and amplifying the output current of this light-receiving element to voltage. It is characterized by comprising an amplifier with a reduced load impedance of the light-receiving element, and a time constant circuit provided in the feedback circuit of the amplifier and set so that the amount of feedback for direct current or low frequencies is larger than the amount of feedback for high frequencies. light receiving circuit.