Transimpedance amplifier circuit

#The present invention relates to a transimpedance amplifier (TIA) circuit and, more particularly, to the automatic gain control function of a TIA circuit.

#In an optical receiving device, an optical signal received by a photodiode (PD) is converted into a current signal, and further converted by a transimpedance amplifier (TIA) circuit into a voltage signal. FIG. 9 is a block diagram showing an example of the arrangement of a general TIA circuit. A TIA circuit 100 is a multistage amplifier circuit including amplifiers 22, 40, and 50. The TIA circuit 100 also includes an automatic gain control (AGC) circuit 200 that includes a gain control unit 21 and controls the gain of the amplifier 22 of the first stage. The gain control unit 21 controls a feedback resistor Rf of the amplifier 22 by a control voltage Vagc, and thus controls the gain of the amplifier 22 such that the amplitude of a signal output from the amplifier 22 maintains a predetermined value.

#In optical communication, since the received light intensity changes depending on the communication distance, it is necessary to amplify any signals ranging from a weak (dark) optical signal to a strong (bright) optical signal while ensuring low noise and low distortion. For this purpose, a TIA circuit for optical communication often has a function of controlling the amplification gain in accordance with the magnitude of an input signal strength, that is, the magnitude of a received light intensity. The TIA circuit makes the gain large when the received light intensity is high, and makes the gain small when the received light intensity is low. An AGC circuit that automatically executes such gain control has been input into practical use in various architectures.

#FIG. 1 is a block diagram showing an example of the arrangement of a TIA circuit according to the first embodiment of the present invention;

#FIG. 2 is a block diagram showing another example of the arrangement of the TIA circuit according to the first embodiment;

#FIG. 3 is a block diagram showing a modification of the TIA circuit according to the first embodiment;

#In the present invention, to implement a plurality of time constants of an AGC function, for example, at least one of the resistance value of a resistive element and the capacitance value of a capacitive element, which determine the value of a time constant of the AGC function, is changed based on a control signal.

#More specifically, a first selection circuit that selects the time constant of the AGC function from a plurality of predetermined values can include a plurality of resistive elements or a plurality of capacitive elements included in the AGC circuit, and a switch element connected in series or parallel with at least some of the plurality of resistive elements or the plurality of capacitive elements and turned on or off based on the control signal to change one of the resistance value of the resistive element and the capacitance value of the capacitive element which determine the value of the time constant.

#For example, a switch is placed for the resistive elements or the capacitive elements or at least some of both elements in the circuit which decide the time constant of the AGC function, and on/off-controlled to enable or disable the elements, thereby changing the resistance value or capacitance value.