Abstract 57 The front end of an optical receiver circuit (10) is of the type having a field-effect device transimpedance amplifier (16, 21) which receives at its input (14) the photocurrent of a photodiode (12). A field-effect device shunt impedance (26) to protect against amplifier overloading is connected between the input and ground through a decoupling capacitor (28). The shunt (26) is controlled by a controller (30), which has its input connected to the output (24) of the amplifier (16) and its output connected to the gate of the shunt (26). The controller (30) compares the output (24) of the amplifier (16) to a threshold reference voltage for determining whether to activate the shunt (26) and regulates the gate voltage of the shunt (26) by means of an AGC amplifier. A direct current feedback resistor (32) is connected between the output (24) of the amplifier (16) and the source of the shunt (26). This prevents the d.c. component of large photocurrents from significantly changing the input bias voltage level of the amblitier (16).
A transimpedance amplifier circuit, comprising: an amplifier having an input and an output a first feedback impedance element connected between the input and the output of the amplifier; a field-effect shunt transistor having one side of its conduction path connected to the input of the amplifier: a decoupling capacitor connected at one side to the other side of the conduction path of the transistor and connected at its other side to a reference potential means, and a shunt transistor control means having an input connected to the output of the amplifier and having its output connected to the control electrode of the shunt transistor, CHARACTERIZED BY a second feedback impedance element connected between the output of the amplifier and the common node of the shunt transistor and the capacitor.
Claim 2
Dependent Claim
129 words
A transimpedance amplifier circuit, comprising: an amplifier having an input and an output a first feedback impedance element connected between the input and the output of the amplifier; a field-effect shunt transistor having one side of its conduction path connected to the input of the amplifier: a decoupling capacitor connected at one side to the other side of the conduction path of the transistor and connected at its other side to a reference potential means, and a shunt transistor control means having an input connected to the output of the amplifier and having its output connected to the control electrode of the shunt transistor, CHARACTERIZED BY a second feedback impedance element connected between the output of the amplifier and the common node of the shunt transistor and the capacitor.
Claim 3
Dependent Claim
22 words
The circuit defined in claim 1 CHARACTERIZED BY a photodiode connected between the input of the amplifier and a bias voltage means.
Claim 4
Dependent Claim
17 words
The circuit defined in claim 2 CHARACTERIZED IN THAT the amplifier is of the field-effect device type.
Claim 5
Dependent Claim
33 words
The circuit defined in claim 2 CHARACTERIZED IN THAT the second impedance element has a value of about the minimum "on" resistance of the shunt transistor times the voltage gain of the amplifier.
Claim 6
Dependent Claim
28 words
The circuit defined in claim 4 CHARACTERIZED IN THAT the second feedback impedance has a resistance value in the range of from about 500 to about 3000 ohms.
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