A biasing circuit for a photodiode. The novel biasing circuit includes a first system for setting a reference gain threshold, a second system for setting an operating gain threshold, and a third system for adjusting a bias of the photodiode until a ratio of the operating gain threshold to the reference gain threshold is equal to a predetermined factor Z. In an illustrative embodiment, the reference gain threshold corresponds to a given probability of an output of the photodiode crossing the reference gain threshold when the photodiode is operating at a reference gain bias, and the operating gain threshold corresponds to a given probability of the photodiode output crossing the operating gain threshold when the photodiode is operating at an operating gain bias. The predetermined factor Z is a ratio of noise at a desired operating gain of the photodiode to noise at the reference gain of the photodiode.