OPTICAL RECEIVING AND AMPLIFYING DEVICE

PURPOSE:To simplify a circuit constitution and to cut off a noise output by detecting the output current of an avalanche photodiode and the gain of a variable gain amplifier circuit and ANDing them. CONSTITUTION:A dark current flowing to the avalanche photodiode 1 at no signal is detected by a no signal detecting means comprising a resistor 8 and a differential amplifier 9, and a gain set value of the variable gain circuit 2 is detected by a gain detecting circuit 10. Further, the AND between outputs of the circuits 9 and 10 is obtained and it is applied to a signal cut-off circuit 6 from an AND gate 11 as a signal cut-off indication signal. As a result, the circuit constitution is simplified and also the circuit 6 at the output end is controlled to be cut off at no signal, thereby allowing to cut off a noise output appearing at the output.