Optimum preamplification and shaping of signals delivered by photodetectors without internal amplification: a theoretical analysis

The use of photodetectors without internal amplification is an appealing choice in the design of low-cost solid-state optical instrumentation. However, once photodiodes are used in the detection unit, the instrument sensitivity is related not only to the photodetector characteristics, but mainly to the front-end electronics noise performance. Therefore, great care has to be taken in the design of the preamplification and shaping electronics in order to minimize their noise contribution. The present paper introduces, from the theoretical point of view, the optimum front-end electronics to process signals delivered by photodetectors without internal amplification. In the analysis performed, with simple formalism, the optimum signal-to-noise ratio and the minimum detectable signal have been calculated. Moreover some practical realizations, which approximate the optimum signal processor, are presented.