Laser-receiver circuits must bias their avalanche photo diodes (APD) to achieve optimal gain. Unfortunately, an APD
"s gain depends on the operating temperature. The circuit controls the operating voltage of an APD over a large temperature range to maintain the gain at the optimal value. The circuit uses Dl as a temperature sensor, thermally matched with the APD. A voltage regulator, IC1, supplies the necessary reference voltage to the circuit. IC2A and Ql bias Dl at a constant current. IC2B, IC2C, IC3A/IC3B, and IC3C amplify Dl"s varying voltage and set Q2 to the optimal-gain corresponding value. Potentiometer Rl controls the amplification over a range of 5 to 15. R2 controls the voltage level, which corresponds to the optimal gain of the APD at 22Â°C (the temperature is specific to the type of APD). The circuit shown was tested with an RCA C 30954E APD. The tests covered -40 to + 70Â°C and used a semiconductor laser. The laser radiation was transmuted on the APD"s active surface in the climatic room via fiberoptic cable. The gain varied by, at most, Â±0.2 dB over the entire temperature range.