| 摘要 |
PURPOSE: To restore a reception circuit to the normal state by boosting the bias voltage of an avalanche photo diode APD, which is temporarily reduced for the purpose of preventing malfunction, by a reset signal at the time of input of a normal optical signal. CONSTITUTION: When the level of the input optical signal is a prescribed value or lower, an alarm signal is generated and is inputted to the base of a transistor TR 9, and the applied voltage from a reference voltage VREF1 to an error amplification circuit 5 is reduced by the voltage drop of a resistor R2 due to the collector current to control an APD bias voltage generation circuit 6, and the multiplication factor of an APD 1 is reduced to prevent the malfunction due to the noise. Meanwhile, the alarm signal is inputted to a reset signal generation circuit 11, and the reset signal is periodically generated and is supplied to the base of a TR 10, and the applied voltage from a reference voltage VREF2 to a comparator circuit 8 is reduced by the voltage drop of a resistor R4 to release the alarm signal. At this time, the reception circuit is normally operated if the optical input signal is normal, but the alarm signal is generated again to interrupt the operation if it is abnormal. |
