JPH02174409A - Optical reception circuit - Google Patents

Abstract

PURPOSE:To expand the dynamic range of an optical reception circuit with respect to an optical signal input by decreasing an input impedance if a preamplifier circuit for the output of a photodetector when the incident strength of an optical signal is strong so as to prevent the preamplifier circuit from being in the saturation state. CONSTITUTION:An output corresponding to the incident strength of light is inputted to an equalization amplifier circuit 4, an output signal of a preamplifier circuit 2 is equalized and outputted and part of the output signal is inputted to an AGC feedback circuit 5. When the incident strength is strong, a gate voltage VGP, to a transistor(TR) Q8 is decreased via a level shift circuit 6 and a voltage VGN to a TR Q9 via a level shift circuit 7 is increased. When the gate-source voltage VG of the TRs Q8, Q9 exceeds a threshold level VT, the source-drain resistance is decreased linearly, and a current flows in response to the voltage VG. Each resistor is selected so as to match the input bias level of the preamplifier circuit 2 thereby selecting the gate aspect ratio. The input impedance of the preamplifier circuit 2 is decreased due to the reduction in the source-drain resistance, the saturation is evaded and the dynamic range of the reception circuit is expanded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical receiving circuit, and more particularly to an optical receiving circuit having an equalizing amplifier circuit with an AGC feedback circuit for amplifying the output from a light receiving element.

[Conventional technology]

Conventionally, in this type of optical receiving circuit, as shown in an example block diagram in FIG.
1, a preamplifier circuit 2 that amplifies the output signal of the photodetector 1, a feedback resistor 3 that is connected in parallel to the preamplifier circuit 2, and an output signal of the preamplifier circuit 2. Equalization amplifier circuit for input 4. and an AGC feedback circuit 5 for keeping the level of the output signal of the equalization amplifier circuit 4 constant. It was composed of.

In FIG. 3, the power supply voltage +VCC is output from the power supply terminal 51.
is supplied, and the output of the light receiving element 1 is sent to the preamplifier circuit 2.
A feedback resistor 3 is connected to the input side of the preamplifier circuit 2 and is provided between the input and output of the preamplifier circuit 2 to constitute a transimpedance type amplifier.

The output signal of the transimpedance amplifier is sent to an equalizing amplifier circuit 4. is equalized and output via the output terminal 52, and a part of the output is sent to the AGC feedback circuit 5. The signal is fed back to the equalization amplifier circuit 41 via. AGC feedback circuit5. equalizing amplifier circuit 4. corresponds to the change in the output of equalizing amplifier circuit 41. Gain control is performed so that the output of the equalization amplifier circuit 41L, which fluctuates depending on the intensity of light input to the light receiving element 1, always has a constant amplitude. It is being said.

[Problem to be solved by the invention]

In the conventional optical receiver circuit described above, when the intensity of light incident on the light receiving element increases, the current in the light receiving element increases, so the preamplifier circuit becomes saturated, and therefore the waveform of the output signal of the preamplifier circuit is distorted. Occur. Furthermore, when the intensity of incident light increases, it exceeds the operating range of the preamplifier circuit and reception becomes impossible, making it difficult to expand the so-called dynamic range for the amount of incident light.

[Means to solve the problem]

The optical receiving circuit of the first invention includes a light receiving element that converts an optical signal into an electrical signal, a preamplifier circuit that amplifies the output signal from the anode of the light receiving element, and an output signal of the preamplifier circuit. An optical receiving circuit comprising an equalization amplifier circuit that performs equalization amplification, and an AGC feedback circuit that controls the gain of the equalization amplifier circuit so as to keep the output level of the output signal of the equalization amplifier circuit constant. The first A output from the feedback circuit
a first level shift circuit that inputs a GC control voltage and outputs a predetermined first output voltage; and a second level shift circuit that inputs a second AGC control voltage output from the AGC feedback circuit and outputs a predetermined second output voltage. a second level shift circuit that
A first circuit whose gate inputs the output voltage of
and a second MOS transistor whose gate inputs the second output voltage, whose source is connected to the ground terminal, and whose drain is connected to the anode of the light receiving element.

The optical receiving circuit of the second invention includes a light receiving element that converts an optical signal into an electrical signal, a preamplifier circuit that amplifies the output signal from the anode of the light receiving element, and an output signal of the preamplifier circuit. An optical receiving circuit comprising an equalizing amplifier circuit that performs equalization amplification and an AGC feedback circuit that controls the gain of the equalizing amplifier circuit so as to keep the output level of the output signal of the equalizing amplifier circuit constant. a current-voltage conversion circuit that detects the current flowing through the light receiving element and converts it into voltage, which is inserted between the cathode of the element and the power supply terminal; and a first level shift circuit whose input terminal is connected to the cathode of the light receiving element. a second level shift circuit whose input terminal is connected to the cathode of the light receiving element and whose input/output characteristics are inverted; and a gate thereof is connected to the output terminal of the first level shift circuit and whose source is connected to the power supply terminal. a first MOS transistor whose drain is connected to the anode of the light-receiving element, whose gate is connected to the output terminal of the second level shift circuit, whose source is connected to a ground terminal, and whose drain is connected to the anode of the light-receiver. and a second MOS transistor.

〔Example〕

Next, the present invention will be explained with reference to the drawings.

FIG. 1 is a block diagram of an embodiment of Sui's invention.

As shown in Fig. 1, the cathode of the light receiving element 1 is connected to the power supply +■
. 0 is supplied to the terminal 51 , and the anode of the light receiving element 1 is connected to the input side of the preamplifier circuit 2 . When an optical signal is incident on the light receiving element 1, the current of the light receiving element 1 changes according to the incident intensity of the optical signal, and the optical signal is transmitted through a transimpedance amplifier including a preamplifier circuit 2 and a feedback resistor 3. The output signal of the preamplifier circuit 2 is equalized in the equalization amplifier circuit 4 in which an output signal corresponding to the incident intensity is input to the equalization amplifier circuit 4, and the output signal of the preamplifier circuit 2 is equalized and output via the output terminal 52. Part of the AGC feedback circuit 5
and output from the AGC feedback circuit 5.

When the output voltage of the equalization amplifier circuit 4 increases, the AGC control voltage is outputted from the Loss terminal which outputs the second AGC control voltage which increases.In contrast, the output from the 0AIN terminal outputs the first AGC control voltage which decreased. and is fed back to the equalization amplifier circuit 4, and the first
The AGC control voltage is input as the first output voltage to the gate of the p-channel first MOS transistor 8 via the first level shift circuit 6, and the second AGC control voltage from the LO3S terminal is input as the first output voltage. The voltage is input as a second output voltage to the gate of a second n-channel MOS transistor 9 via a level shift circuit 7 .

The drains of the transistors 8 and 9 are connected to the anode of the light receiving element 1, that is, the input terminal of the preamplifier circuit 2.
S) The source of the transistor 8 is connected to a power supply terminal 51 that supplies power +Vcc, and the source of the MOS transistor 9 is connected to a ground terminal.

In this configuration, as the output voltage of the equalization amplifier circuit 284 increases, the first AGC control voltage from the GAIN terminal decreases, and the second AGC control voltage from the Loss terminal increases. As in the case of the optical receiver circuit shown in FIG. control the operating state of the

When the incident intensity of the optical signal is weak, the level shift circuit 6
The voltages of the first and second output voltages input to the gates of transistors 8 and 9, respectively (through MOS and 7) are:
Compared with the threshold voltages ↑ of MOS transistors 8 and 9, the relationship shown in equation (1) is obtained. However, Va is the gate-source voltage.

V t <V o (1) In this case, both MOS transistors 8 and 9 become non-conductive, and the operation of the optical receiver circuit shown in FIG. 1 is similar to the operation of the conventional example shown in FIG. 3. Become.

Next, when the incident intensity of the optical signal increases, the first AGC control voltage decreases, the second AGC control voltage increases, and the gate voltage is output to the MOS transistor 8 via the level shift circuit 6 as the first output voltage. Voltage VGp decreases, and similarly, gate voltage VIIN as the second output voltage to MOS transistor 9 via level shift circuit 7 increases.

Here, due to the characteristics of the MOS transistor, VO-V t
'>Vo, resistance is exhibited between the source and drain, and the resistance RON at this time is expressed as shown in equation (2). However, β is a gain coefficient.

RoN=+=[β(Vo Vt))−'・12) Formula (
From 2), the gate-source voltage of MOS transistors 8 and 9 ■. When the voltage becomes larger than the threshold voltage VT, the MO
When the S transistor is in operation, the source-drain resistance decreases linearly, and current flows between the sources and drains of MOS transistors 8 and 9 in accordance with the gate voltage value. However, the source of p-channel type MOS transistor 8
The gate aspect ratio of the drain-to-drain resistance ROMTP) and the source-drain resistance ROM(a) of the n-channel MOS transistor 9 must be set to match the input bias potential of the preamplifier circuit 2 described above. The relationship between the gate aspect ratio and the source-drain resistance ROM is expressed by the formula (3
). However, μ・ is mobility + CO
is the gate oxide film capacitance, and W/L is the gate aspect ratio.

β=μCo(W/L)...(3) From the above results, as the incident intensity of the optical signal increases, the source of Mo3) transistors 8 and 9 increases depending on the intensity.
Since the resistance between the drains is reduced and current flows between the source and drain, the input impedance of the preamplifier circuit 2 is reduced and saturation in the preamplifier circuit 2 is avoided.
The dynamic range of the optical receiving circuit relative to the incident light intensity can be expanded.

FIG. 2 is a block diagram of an embodiment of the second invention.

As shown in FIG. 2, the cathode of the light-receiving element 1 is connected to a power supply terminal 51 that supplies power +vcc via a resistor 10 as a current-voltage conversion circuit, and the input terminals of the level shift circuits 6 and 11 are connected to the light-receiving element, respectively. 1 cathode. Other connections are the same as in the embodiment shown in FIG. 1 described above.

In such a configuration, when the incident intensity of the optical signal increases, a current flows through the light receiving element 1, and the anode voltage of the light receiving element 1 decreases due to the voltage drop across the resistor 10. According to this change in voltage, the level shift circuit 6 shifts the p-channel type M
Controls the gate voltage of OS transistor 8. In addition, the level shift circuit 11 has an inverted output so that the output increases as the input voltage decreases, and is an n-channel type MO
Controls the gate voltage of S transistor 9. Therefore, it has the same operating characteristics as the embodiment of FIG. 1 described above.

〔Effect of the invention〕

As explained above, the present invention prevents the preamplifier circuit from becoming saturated by lowering the input impedance of the preamplifier circuit that amplifies the output of the light receiving element when the incident intensity of the optical signal is strong. This has the effect of expanding the dynamic range of the optical receiving circuit with respect to optical signal input.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an embodiment of the first invention, FIG. 2 is a block diagram of an embodiment of the second invention, and FIG. 3 is a block diagram of an example of a conventional optical receiving circuit. . 1... Light receiving element, 2... Preamplifier circuit, 3... Feedback resistor, 4.4. ... Equalization amplifier circuit, 5, 5.・・・
- AGC feedback circuit, 6, 7.11... Level shift circuit, 8.9... Mo3) transistor, 10... Resistor. ff history series, z front 1L width times, 3 Maro 11 Mr. L.
4-temperature U-width circuit, 5 AGC parts! Times 2 given, 6,7 level shift times 4.8,'? MO:5 ト5;＞'l'
3.5ftJJF+”y, sz*force m child.

Claims (2)

[Claims] (1) A photodetector that converts an optical signal into an electrical signal, a preamplifier circuit that amplifies the output signal from the anode of the photodetector, and an equalization amplifier circuit that equalizes and amplifies the output signal of the preamplifier circuit. and an AGC that controls the gain of the equalization amplifier circuit so as to keep the output level of the output signal of the equalization amplifier circuit constant.
a first level shift circuit that receives a first AGC control voltage output from the AGC feedback circuit and outputs a predetermined first output voltage; a second level shift circuit which inputs the second AGC control voltage to be output and outputs a predetermined second output voltage; the gate inputs the first output voltage, the source is connected to the power supply terminal, and the drain is connected to the a first MOS transistor connected to the anode of the light receiving element; and a second MOS transistor having the second output voltage as a gate input, a source connected to a ground terminal, and a drain connected to the anode of the light receiving element. An optical receiving circuit comprising: (2) A photodetector that converts an optical signal into an electrical signal, a preamplifier circuit that amplifies the output signal from the anode of the photodetector, and an equalization amplifier circuit that equalizes and amplifies the output signal of the preamplifier circuit. and an AGC that controls the gain of the equalization amplifier circuit so as to keep the output level of the output signal of the equalization amplifier circuit constant.
a current-voltage conversion circuit that detects a current flowing through the light-receiving element and converts it into a voltage, which is inserted between the cathode of the light-receiving element and a power supply terminal; The first connected to the cathode of
a second level shift circuit whose input terminal is connected to the cathode of the light-receiving element and whose input/output characteristics are inverted, and whose gate is connected to the output terminal of the first level shift circuit and whose source is connected to the first level shift circuit; a first MOS transistor connected to a power supply terminal and having a drain connected to the anode of the light receiving element; a first MOS transistor having a gate connected to the output terminal of the second level shift circuit, a source connected to a ground terminal, and a drain connected to the light receiving element; a second MOS transistor connected to the anode of the optical receiver circuit.