JPH06140843A - Optical reception circuit - Google Patents

Abstract

PURPOSE:To provide the optical reception circuit enlarging a dynamic range to an optical input signal. CONSTITUTION:This optical reception circuit is composed of a photodetector 10, preamplifier circuit 20 and attached circuit 30. The photodetector 10 converts the optical signal to an electric signal. The electric signal from the photodetector 10 is amplified by the preamplifier circuit 20 and outputted. The attached circuit 30 is mainly composed of a pair of transistors 31 and 32 constituting a current mirror circuit, resistor 33, and transistor 34 connected to the input terminal of the preamplifier circuit 20. A current from the output step of the preamplifier circuit 20 can be detected by the transistor 31. The output current of the preamplifier circuit 20 to be changed by the intensity of incidental light to the photodetector 10 is detected by the transistor 31, and an input impedance is varied by operating the transistor 32 on the mirror side of the current mirror circuit and operating a transistor 35.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical receiver circuit, and more particularly to an optical receiver circuit having a preamplifier circuit for a light receiving element.

[0002]

2. Description of the Related Art Conventionally, an optical receiving circuit of this kind has been provided as a circuit for receiving an optical signal and converting it into an electric signal of a certain magnitude.

FIG. 2 is a circuit diagram showing a conventional optical receiving circuit. The light receiving circuit shown in this figure includes a light receiving element 110,
It is composed of a transimpedance type preamplifier circuit 120. Here, the light receiving element 110 has its cathode connected to the power supply V _CC and its anode connected to the preamplifier circuit 1.
It is connected to the base of 20 transistors 121.

Here, the preamplifier circuit 120 is constructed as follows. That is, the emitter of the transistor 121 is grounded and the collector thereof is connected to the power supply V _CC through the resistor 122. The collector of the transistor 121 is connected to the base of the transistor 123. The collector of the transistor 123 is connected to the power source V _CC , and the emitter of the transistor 123 is the level shifting diode 1.
It is connected to 24 anodes. The cathode of the level shift diode 123 is connected to the base of the transistor 121 via the resistor 125 and is grounded via the resistor 126.

According to the conventional optical receiving circuit having such a configuration, when an optical digital signal is input to the light receiving element 110,
The optical digital signal is converted into an electric signal by the light receiving element 110, and the transimpedance type preamplifier circuit 1
It is input to 20. In the preamplifier circuit 120, the electric signals of the transistors 121 and 123 are amplified, converted into current-voltage, and output.

[0006]

In such a conventional optical receiving circuit, when the intensity of the incident light on the light receiving element 110 increases, the current in the light receiving element 110 increases, and the preamplifying circuit 120 of this preamplifying circuit 120 increases. As the base potential of the front-end transistor 121 rises,
The transistor 121 is saturated. As a result, the output waveform of the preamplifier circuit 120 is distorted, and when the intensity of the incident light is further increased, the output waveform is further distorted, and reception may be impossible at all. As described above, the conventional optical receiver circuit has a drawback that the light receiving dynamic range in the intensity of incident light is narrow.

Therefore, an object of the present invention is to provide an optical receiving circuit capable of expanding the light receiving dynamic range.

[0008]

In order to achieve this object, an optical receiving circuit according to the present invention comprises a light receiving element for converting an optical signal into an electric signal, and the light receiving element connected to an input terminal,
In a light receiving circuit including a preamplifier circuit that amplifies an electric signal from a light receiving element, one of a pair of transistors forming a current mirror circuit is connected so that the output current of the preamplifier circuit can be detected. Connect the series circuit of the other of the pair of transistors and the resistor to the power source and the constant potential point,
And an auxiliary circuit connected to the input terminal of the preamplifier circuit so that the voltage generated by the resistor can be input to the base of the transistor that changes the input impedance of the preamplifier circuit. It is a thing.

According to the present invention, since the light receiving circuit is configured as described above, when the incident intensity of the optical signal input to the light receiving element is high, the output current of the preamplifying circuit for amplifying the electric signal from the light receiving element increases. To do. This is detected by a pair of transistors forming a current mirror, and the impedance of the transistor connected to the input terminal of the preamplifier circuit is lowered. As a result, the electric signal from the light receiving element is partially bypassed via the above-mentioned transistor, so that the electric signal is not input to the preamplifier circuit and the saturation state of the preamplifier circuit can be prevented.

Further, when the incident intensity of the optical signal input to the light receiving element is small, the output current of the preamplifier circuit decreases. Therefore, this is detected by a pair of transistors forming a current mirror and the preamplifier is used. The transistor connected to the input terminal of the circuit is turned off. As a result, all electric signals from the light receiving element are input to and amplified by the preamplifier circuit.

[0011]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a circuit diagram showing an embodiment of an optical receiving circuit according to the present invention. The light receiving circuit shown in this figure comprises a light receiving element 10, a preamplifier circuit 20, and an accessory circuit 30 thereof.

First, the light receiving element 10 is an element which receives an optical signal and converts it into an electric signal, and its cathode is a power source V _CC.
Of the transimpedance type preamplifier circuit 20, and its anode is connected to the base of a transistor 21 forming a part of the transimpedance type preamplifier circuit 20.

The preamplifier circuit 20 is constructed as follows. That is, the collector of the transistor 21 is connected to the power supply V _CC through the resistor 22 and the emitter thereof is grounded. The collector of the transistor 21 is connected to the base of the transistor 23. The transistor 23 has its collector connected to the power supply V _CC and its emitter connected to the anode of the level shifting diode 24. The cathode of the diode 24 is connected to the base of the transistor 21 via the resistor 25, and is also grounded via the resistor 26. The cathode of the diode 24 and the resistor 2
The connection point between 6 and connect the node N _a.

Further, the accessory circuit 30 is constructed as follows. That is, the transistors 31 and 32 form a current mirror circuit, and in order that the output current of the preamplifier circuit 20 can be detected by the transistor 31,
Connect the collector of the transistor 31 and the base connection node N _a, and is grounded to the emitter of the transistor 31. The base of the transistor 31 is connected to the base of the transistor 32, and the transistor 3
2 has its emitter grounded, and its collector has a resistor 33
A current mirror circuit is configured by connecting to the power supply V _CC via the. A capacitor 34 is connected between the collector and emitter of the transistor 32. The collector of this transistor 32 and the capacitor 34
A connection point with and is referred to as a connection node N _b . The base of the transistor 35 is also connected to the connection node N _b . The collector of the transistor 35 is connected to the base of the transistor 21, and the emitter of the transistor 35 is grounded. Transistor 35 has the voltage at the connection node N _b, which changes according to the current mirror circuit,
The impedance changes so that the impedance of the input terminal of the preamplifier circuit 20 can be changed.

In such an optical receiver circuit, first, an optical digital
The light signal is input to the light receiving element 10.
If the incident intensity of this optical digital signal is weak,
Point N _aSince the potential at the point is rising, the transistor 31
The current according to the signal is flowing through. At this time,
The transistor 31 and the transistor 32 have a current mirror circuit structure.
Since it is composed, it is the same as flowing to the transistor 31.
Will flow into the transistor 32 as well. did
Therefore, the connection node N_bThe potential at the point drops and the transistor
Reference numeral 35 indicates a cutoff state. That is, it is incident on the light receiving element 10.
When the amount of incident light is small, it is converted by the light receiving element 10.
The electric signal is amplified by the preamplifier circuit 20 and is converted into a current voltage.
It will be converted and output.

Next, when the incident intensity of the optical digital signal is increased to the light receiving element 10, the potential of the connection node N _a point is lowered,
No current flows through the transistor 31. Then, no current flows through the transistor 32 forming the current mirror circuit. As a result, the potential at the connection node N _b rises, the transistor 35 is turned on, and the input impedance of the base of the transistor 21 can be lowered. Therefore, the light receiving element 10
A part of the electric signal converted by is bypassed via the transistor 35. As a result, the amount of current input to the preamplifier circuit 20 decreases, and the transistor 21 does not saturate.

Needless to say, the same effect can be obtained even if the level shift diode 24 is used instead of the emitter follower of the NPN transistor to perform the level shift or the level shift is simply performed by the resistance.

[0019]

As described above, according to the present invention, when the incident intensity of the optical signal is high, the input impedance of the preamplifier circuit for amplifying the output of the light receiving element is lowered to saturate the preamplifier circuit. Since the state is prevented, there is an effect that the dynamic range of the optical receiving circuit with respect to the optical input signal can be expanded.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of an optical receiving circuit of the present invention.

FIG. 2 is a circuit diagram of a conventional optical receiver circuit.

[Explanation of symbols]

 10 light receiving element 20 preamplifier circuit 21, 23, 31, 32, 35 transistor 22, 25, 26, 33 resistor 24 diode 30 auxiliary circuit 34 capacitor

Claims (1)

[Claims] 1. A light receiving circuit comprising a light receiving element for converting an optical signal into an electric signal, and a preamplifier circuit for connecting the light receiving element to an input terminal and amplifying an electric signal from the light receiving element. One of a pair of transistors forming the current mirror circuit is connected so that the output current of the preamplifier circuit can be detected, and a series circuit of the other of the pair of transistors and a resistor is connected to a power source and a constant potential point, An optical circuit characterized in that an auxiliary circuit connected to the input terminal of the preamplifier circuit for inputting the voltage generated by the resistor to the base of the transistor for changing the input impedance of the preamplifier circuit is provided. Receiver circuit.