OPTICAL SIGNAL DETECTING CIRCUIT

摘要

PURPOSE:To detect an optical signal with a fine level even dring high-speed transmission by comparing a reference voltage obtained from a reverse bias voltage applied to a photodiode with a DC voltage developed across a resistance with the optical signal flowing through the photodiode. CONSTITUTION:The current flowing through the photodiode 1 is converted by a current/voltage converting circuit 2 into a voltage proportional to the input current. One terminal of a resistance R3 is connected to the cathode of the diode 1 and the other terminal is connected to a line l to which the reverse bias voltage for the diode 1 is applied. The reverse bias voltage is divided by R6 and R8 and applied to the plus terminal of a comparator 4 as the reference voltage. The comparator 4 compares the DC voltage proportional to the light signal current applied to its minus terminal with the reference voltage and outputs a negative photodetection signal when the DC voltage is higher than the reference voltage. Thus, this circuit used during high-speed transmission and even a photodetected signal having a fine level is detected.