On the noise optimum of gigahertz FET transimpedance amplifiers

The main figure of merit for transimpedance amplifiers used in amplifying photocurrents in fiber-optic systems is the optical sensitivity. This sensitivity is determined by the equivalent input noise current of the amplifier. To obtain the best noise performance, most transimpedance amplifiers with FET input stages are designed using a result that prescribes making the capacitance of the input FET equal to the photodiode capacitance. It is shown here that this is not necessarily the most favorable practice when using FETs in scaled submicrometer technologies because it may compromise the stability of the amplifier. The tradeoff in sensitivity to obtain better stability is examined.