JPS6020652A - Photoelectric conversion circuit - Google Patents

Abstract

PURPOSE:To eliminate the effect of parasitic capacitance on a frequency and the effect of input capacitance by making a change in a voltage impressed across a photodetector equal and further making a voltage change at a source terminal of the 1st amplifier means equal to an input voltage change. CONSTITUTION:When a light signal is inputted to a photo diode 1, a converted current flows to a resistor R3 so as to increase the voltage of a gate G of an FET1 as the 1st amplifier to flow a current, a signal voltage is impressed to a base B of a pnp transistor (TR)1 as the 2nd amplifier and a signal output is extracted. Since the voltage gain of the photoelectric conversion circuit comprising the 1st and 2nd amplifiers is the unity, both ends of the photo diode 1 are reverse-biased by a prescribed voltage value at all times. Further, the change in the cathode voltage of a Zener diode 3 is equal to the change in the anode voltage of the photo diode 1 thereby allowing the voltage at the source S of the FET1 to be change identially to the voltage change in the gate G.

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Technical Field The present invention relates to a photoelectric conversion circuit that converts an optical signal into an electrical signal in an optical communication system.

(B) Prior Art Conventionally, the circuit configuration shown in FIG. 1 has been known as the photoelectric conversion circuit. This photoelectric conversion circuit has a photodiode 1 as a light receiving element and a resistor R1 connected in series between a DC power supply voltage line and a ground line. The light incident on the photodiode 1 is converted into a current, and the voltage across the resistor R1 generated by this current flowing through the resistor R1 is extracted as an electrical signal output.

Figure 2 shows another conventional photoelectric conversion circuit, in which one photodiode is connected to the DC power supply voltage line.
and an anti-phase amplifier 2 are connected in series, and a resistor R2 is connected between the input and output terminals of the anti-phase amplifier 2 as a negative feedback resistance. Then, the light incident on the photodiode 1 is converted into a current, and this current is taken out as a signal output via the anti-phase amplifier 2, and a part of the output voltage is passed through the resistor R2 to the input of the anti-phase amplifier 2. It has been returned to the edge.

(c) Problems to be Solved by the Invention In the photoelectric conversion circuit shown in FIG. There was a drawback that the signal band was limited.

In addition, the photoelectric conversion circuit shown in Fig. 2 uses an anti-phase amplifier 2 with a sufficiently wide band and high amplification, so it has the advantage of a wide receiving signal band. However, it is difficult to obtain an anti-phase amplifier with a high frequency, and it is also difficult to operate it stably over a wide receiving signal band.

This invention has been made based on the above-mentioned circumstances, and its purpose is to eliminate the reception signal band from being limited by the parasitic capacitance of the light-receiving element, and to eliminate the reception signal band from being limited by the parasitic capacitance of the light receiving element, and without using an anti-phase amplifier. An object of the present invention is to provide a photoelectric conversion circuit that can widen the band. To achieve this purpose, an amplifying element (field effect transistor) is used, and the input capacitance of the amplifying element and the parasitic capacitance of the light receiving element are made unaffected by frequency, thereby widening the received signal band. It is.

B) Embodiment An embodiment of the present invention will be described below with reference to FIG. The anode of the photodiode 1 as a light receiving element is connected to the other end of a resistor R6 whose one end is grounded and to the gate G terminal of a field effect transistor (hereinafter abbreviated as F'ET) 1 as a first amplifier. . This FE
The ground S terminal of T1 is connected to the other end of a resistor R4 whose one end is grounded and to the anode of a Zener diode 3 as a constant voltage element. The drain D terminal of the FET 1 is connected to the other end of a resistor R5 whose one end is connected to a power supply voltage line and to the base terminal of a PnP h-run raster TR1 serving as a second amplifier. One end of the emitter terminal of this PnP ) transistor TR1 is connected to the other end of the resistor R6, which is connected to the power supply voltage line, and its collector terminal is connected to the cathode of the photodiode 10 (and the cathode of the Zener diode 3) to provide a signal. Output is retrieved.

Therefore, the reverse bias voltage applied across the photodiode 1 is equal to the sum of the constant voltage determined by the Zener diode B and the reverse bias voltage between the gate G terminal and the source S of the FET 1.

The mutual conductance of the PET 1, which is a parameter that determines the amplification factor, is not sufficiently large, and when a source follower amplifier is configured as shown in FIG. 6, the voltage amplification factor cannot be made sufficiently close to 1.

Therefore, by connecting the PnP transistor TR1, the apparent transconductance of FET1, that is, the ratio of the change in the current flowing through the resistor R6 to the change in the applied voltage of FET By setting R5/R6 times the true transconductance of , the configuration is such that the apparent voltage gain of the FETI, in other words, the voltage gain in the photoelectric conversion circuit consisting of the FETI and the PnP transistor TR1, can be made sufficiently close to 1. There is.

In the photoelectric conversion circuit configured as described above, when no optical signal is input to photodiode 1, the current flowing through resistor R3 is equal to the dark current of photodiode 1 and F.
This is the leakage current of the gate G of ET1.

When an optical signal is input to the photodiode 1, the converted current flows through the resistor R3 and increases the voltage at the gate G. Then, a current flows through the resistor R5, FET1, and resistor R4, and a signal voltage is applied to the base B of the PnP transistor TR1. As a result, a current flows through the resistor R6 and the PnP transistor TR1, and is taken out as a signal output.

By the way, since the voltage gain in the photoelectric conversion circuit is 1, both ends of the photodiode 10 are always reverse biased with a constant voltage value. That is, the amount of change in the anode voltage of the photodiode 1, in other words, the amount of change in the light incident on the photodiode 1, is applied to the cathode of the photodiode 10 as the same amount of change.

However, the parasitic capacitance existing in the photodiode 1 becomes irrelevant to changes in the received light, and as a result, the received signal band is not affected by the parasitic capacitance. Similarly,
Since the cathode voltage of the Zener diode B is also the same as the change in the anode voltage of the photodiode 1, and the voltage across the Zener diode 6 is always constant, F
The voltage at the S terminal of ET1 changes in the same way as the voltage at the G terminal. Therefore, FE'I'l
The input capacitance present at is also unaffected by changes in the received light, and as a result the received signal band is unaffected by the input capacitance. Further, by setting the reverse bias voltage applied to the photodiode 11 to be higher than that of the Zener diode 6, the inherent parasitic capacitance of the photodiode 10 can be suppressed to a small value.

(e) Effects As explained above, according to the present invention, the changes in the voltage applied to both ends of the light receiving element are equalized to eliminate the influence of parasitic capacitance on the frequency, and furthermore, the influence of the parasitic capacitance on the frequency is eliminated. The influence of the input capacitance of the first amplifying means can be removed by making the voltage change at the terminal equal to the input voltage change output from the light receiving element. Therefore, since the cause of limiting the optical reception signal band is removed, the reception signal band can be widened and stable operation can be performed over a wide band.

[Brief explanation of drawings]

FIG. 1 is a diagram of a conventional photoelectric conversion circuit, FIG. 2 is a diagram of another conventional photoelectric conversion circuit, and FIG. 6 is a diagram showing an embodiment of the photoelectric conversion circuit of the present invention. 1...Photodiode 6...Zener diode FET1...Field effect transistor TR1...PnP) Transistor R3, R
4, R5, R6...Resistance. Figure 1 Tin 2 Figure

Claims (1)

[Claims] A photodetector and an f! 1 amplification means, and the apparent voltage gain of this first amplification means is approximately 1.
a second amplifying means connected to the first amplifying means in a similar manner; and a constant voltage element connected between the output terminal of the second amplifying means and the source terminal of the first amplifying means. The voltage difference between the output voltage of the second amplification means m1 and the input terminal is applied to the light receiving element as a reverse bias voltage, and the change in the input voltage is applied to the voltage of the source terminal and the second amplification means. By being equal to the change in the output voltage of the means, the parasitic capacitance existing in the light receiving element and the first
A photoelectric conversion circuit characterized in that it eliminates the influence of frequency based on human power capacity present in an amplification means.