| 摘要 |
A photodetector circuit having a power supply and an avalanche photodiode coupled electrically thereto. The power supply includes a first power supply portion, which supplies a substantially constant current, and a second power supply portion, which supplies a substantially constant voltage. At relatively low to moderate optical input signal levels, the substantially constant current that is applied to the avalanche photodiode generates a biasing potential that provides a signal gain, the magnitude of which varies depending upon the optical input signal level. A bias filter between the power supply and the avalanche photodiode slows changes in bias voltage so that the instantaneous photocurrent follows the modulation on the input optical signal. With constant average current bias, the bias potential automatically adjusts with temperature and sample variations to give the same average current flow. A primary feature of this biasing scheme is the benefit of automatic bias optimization that occurs with constant current bias. The optimum bias current is set to an optimum that depends on the type of avalanche photodiode and the bandwidth and noise characteristics of the following preamplifier. A large change in the actual set current then causes very little change in performance. At higher optical input signal levels, the substantially constant minimum voltage is applied to the avalanche photodiode to maintain acceptable operation of the photodetector circuit over a very large dynamic range of input optical signal level.
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