FEEDBACK AMPLIFIER CIRCUIT

PURPOSE: To adjust gain against a change in frequency characteristics due to the dispersion of elements by controlling the frequency band width of a feedback amplifier circuit by a 1st transistor(TR) and varying the level of an output signal by a 2nd TR. CONSTITUTION: When a photodetector 1 is irradiated with light, a current flows into FETs 41, 42 in a feedback circuit 40, the drain current of an FET 31 in a feedback amplifier circuit 30 is increased and the potential of the gate G and source S of an FET 34 and the potential of the gate G of an FET 35 are reduced. Thereby output voltage Vout obtained from an output terminal T is changed in a direction reverse to the gate potential of the FET 31. Since the potential of the terminal T is reduced due to the drop of the voltage Vout, the rise of the gate potential of the FET 31 is suppressed. When the cathode current of the photodetector 1 is reduced, the output voltage Vout is increased. Thereby, when the resistance value Rf of the circuit 40 is reduced, a feedback amount is increased, high band interruption frequency is increased and the frequency band of the circuit 30 is extended and vice versa.