| 摘要 |
PURPOSE:To prevent damage to an avalanche photo diode or to avoid the lowering of the S/N ratio, by decreasing a bias voltage of the avalanche photo diode when an optical signal is lowered to a prescribed range or below. CONSTITUTION:If no optical signal is incident to an avalanche photo diode APD1, a bias voltage of the APD1 is considerably increased. When this bias voltage is a constant voltage Ec slightly higher than the operating range of an AGC circuit 6, the bias voltage is detected with an APD bias voltage detecting means 10. The means 10 outputs a constant output voltage with the detection of the Ec voltage, turns on an external DC voltage supplying means 11 and supplies a DC voltage to a load resistor 3. Even if no signal current flows to the APD1, as if a photo signal current were flowed to the circuit 6, allowing to control the bias voltage of the APD1 so as not to rise the bias voltage beyond the voltage Ec. |
