An avalanche photo-diode (APD) circuit includes a first APD and a bias circuit. The first APD is configured to detect light. The bias circuit is configured to control a gain of the first APD. The bias circuit includes a second APD, a reference voltage source, a bias voltage generation circuit, and a metal layer configured to shield the second APD from the light. The reference voltage source is configured to bias the second APD. The bias voltage generation circuit is configured to generate a bias voltage for biasing the first APD based on dark current output by the second APD.