Low-noise extended-frequency response with cooled silicon photodiodes.

With appropriate preamplifier design and cooled silicon photodiode and input field effect transistor, broad frequency bandwidth (e.g., 0.01 Hz to 100 kHz) performance comparable or better than that of photon noise limited photomultipliers can be achieved. The FET input preamplifier is operated current mode with a large feedback resistance. Consequent limited frequency response of the preamplifier is compensated by treble boosting at the postamplifier. Cooling the photodiode and input FET to 200 K eliminates photodiode dark current and FET gate reverse current shot noises as significant contributors. Remaining noise sources are FET voltage noise, minimized by small input capacitances, and feedback and FET load resistances Johnson noise. At an output SNR of 10, for example, the internal noise and the photon noise are about the same level. Thus the performance gain attributable to the photodiode's high quantum efficiency can be significant.