APPLICATION NOTE 1831

Low-Noise APD Bias Circuit


Abstract: This circuit generates and controls a low-noise bias voltage for avalanche photodiodes (APDs) used in optical communications. The variable voltage controls the avalanche gain of the APD to optimize sensitivity in a fiber-optic receiver. The circuit employs a low-noise fixed-frequency PWM boost converter with an inductor operating in discontinuous-current mode. Slow switching of the internal MOSFET reduces high-frequency spikes for low-noise performance. A complete circuit is provided, and an extended circuit is suggested. The latter employs an ADC for digital control allowing a microcontroller to read a thermistor to provide temperature compensation by referring to a lookup table.