INVERSE BIAS CIRCUIT FOR AVALANCHE PHOTODIODE

PURPOSE:To drastically simplify a circuit constitution by providing resistance insert-connected between an avalanche photodiode and a constant voltage power source and an emitter diode parallel-connected to the resistance at a poarity inverse-biased by the constant voltage power source. CONSTITUTION:An avalanche photodiode is inverse-biased by a contant voltage power source 12. Then, resistance 14 is insert-connected between the avalanche photodiode 10 and the constant voltage power source 12, and a Zener diode 16 is connected to the resistance 14 parallel. First, on the minimum light receiving power of the avalanche photodiode 10, a light current I hardly flows, and the voltage lowering of the resistance 14 can be ignored. Then, since the output voltage of the constant voltage power source 12 is regarded as the voltage on the minimum light receiving power, when the avalanche photodiode 10 is inverse-biased at the power source voltage, an output voltage V of a front end amplifier 15 becomes the best SN ratio.