| 摘要 |
PROBLEM TO BE SOLVED: To dispense with a large scale AGC circuit, to reduce and simplify the scale of an optical receiving circuit, and to reduce its power consumption by using an amplitude limited amplifying circuit that consists of a simple differential amplifying circuit. SOLUTION: An amplitude limited amplification circuit 3 operates in a linear amplifying region, when an optical input level of an APD 1 is low, and the bias voltage of the APD 1 is automatically controlled so that the reference voltage is equal to the peak value by a feedback loop which consists of a peal value detection circuit 5 and a voltage control circuit 6. The output amplitude of an equalizing amplification/circuit 4 becomes constant in the operating range of the feedback loop, and the S/N ratio is determined by the electrical noise produced by a preamplifying circuit 2, the circuit 3 and the circuit 4 and the shot noises of the APD 1. When an optical input level of the APD 1 becomes large, the output amplitude of the circuit 4 is increased and the multiplication factor of the ADP 1 and also the shot noises become small respectively. The electrical noises are kept at constant levels, regardless of the output amplitude of the circuit 4, and accordingly the S/N ratio is high. |
