| 摘要 |
PROBLEM TO BE SOLVED: To prevent an avalanche photo diode from breaking down by generating noise corresponding to the output of a full AGC circuit, namely, noise which corresponds to the bias voltage of the avalanche photodiode, superimposing it on a signal and suppressing th over-rise in the output of the full AGC circuit, even if optical input power becomes minute. SOLUTION: When optical input power inputted to the avalanche photodiode 1 becomes minute, the output of a level detection circuit 3 is reduced and the output voltage of the full AGC circuit 4 rises. The output voltage is supplied to a diode 6 via resistors 8a and 8b, an operational amplifier 7 and a resistor 8c. Thus when the optical input power becomes minute, the bias current of the diode 6 increases, and average noise power which the diode 6 generates becomes large. The noise is superimposed on the signal and is supplied to the level detection circuit 3. Thus, the output of the full AGC circuit 4 is prevented from rising excessively even if optical input power becomes mounts. |
