| 摘要 |
PURPOSE:To set easily a switching level of an AGC system by using a comparison output between a mean value detecting voltage of an output current of an avalanche photodiode and a reference voltage to switch an optial AGC system and an electric AGC system. CONSTITUTION:A voltage proportional to the mean value of the output current of the avalanche photodiode APD1 is detected by a detection circuit 7 and its output V0 is inputted to a comparison circuit 8. The circuit 8 compares the reference voltage determined in response to a prescribed optical level of the APD1 with the voltage V0 and when V0<Vr exists, an output voltage Ve of the circuit 8 goes to an L level, and a signal detected by a peak detecting circuit 3 from an output signal amplifying (2) an output of the APD1 at a switching circuit 9 is switched into the position of the optical AGC circuit 4. The output of the circuit 4 changes a bias voltage of the ADP1 via a bias generating circuit 6 to actuate the optical AGC system thereby limiting the operation of the electric AGC system. When V0>Vr, the voltage Ve goes to an H level, the circuit 9 is controlled to switch the output of the circuit 3 to the position of the electric AGC circuit 5 to actuate the electric AGC system thereby limiting the operation of the optical AGC system. |
