| 摘要 |
PURPOSE:To prevent the deterioration of the characteristic of a photo-electric conversion element by providing a protection circuit, which suppresses an inverse bias voltage, impressed to the photo-electric conversion element, below a prescribed voltage, based on a timing component, extracted from an output signal when a received light signal falls lower. CONSTITUTION:When a light received level falls lower or an input comes to naught, the output V1 of a peak detection circuit 19 comes to V1<Vref3. At this time, according to the result of a comparison between the reference voltage Vref3 of a comparator 18 and the output V1 of the circuit 19, the base potential of a transistor (TR) 17 changes. For the light receiving level, higher than the prescribed level, it comes V1>Vref3, and the TR 17 comes to a cut-off state. On the other hand, at the time of no input, it comes to V1<Vref3, and the TR 17 operates, and a voltage Vref1 falls lower by the portion of a current to flow in the TR 17, by a resistor 15. Thus, when the light receiving level falls lower or the input comes to naught, the reference voltage Vref1 falls lower than the initial voltage, and the output of a gain control circuit 12 operates in a direction to lower the inverse bias voltage of an APD 1. |
