LIGHT RECEIVING CIRCUIT

摘要

In a light receiving circuit wherein the output of an amplifier (13) is kept at a constant level independent of the level of light inputs by controlling the multiplication factor of an avalanche photodiode (12) and the gain of the amplifier (13), the multiplication factor M of the avalanche photodiode (12) and the gain G of the amplifier (13) are successively changed according to polygonal line-approximation in order to make the signal-to-noise ratio in the output of the amplifier (13) approach toward and optimum value by use of a simple control circuit. For this purpose, when the light input level (11) is low, VAPD is adjusted through a differential amplifier (52), a diode (56) and a high voltage generating circuit (54) so as to decrease M and, when the light input level heightens and V2 becomes negative, VAGC is adjusted so as to decrease G. When the light input level becomes still higher and VAGC becomes equal to V reff3, M is still decreased by adjusting a reference voltage supplied to a differential amplifier (51) to a lower level. When the output V5 of a differential amplifier (53) is saturated, VAPD is fixed to as to keep M constant and only VAGC changes so as to decrease G. Thus, M and G are controlled to follow optimum values.