Photodiode preamplifier systems: low-noise positive-feedback.
Positive feedback circuits have been developed that allow an NEP (1.42, 500, 1) of 10(-14) W Hz(-(1/2)) or better to be achieved for cooled intrinsic germanium photodiodes of up to 6-mm diam. These circuits have also been tested at room temperature with silicon photodiodes and with test capacitors; it is reasonable to suppose that they may be used with other transducers that have a capacitance as their dominant circuit component. Circuits that had one-half-power frequencies of 2-3 Hz with no feedback have routinely achieved bandwidth enhancement factors larger than 400 without degrading the original NEP. The technique is not limited to low frequency.
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