The noise of input stages with low parasitic capacitance

The noise of two types of input stages, that of a high-input-impedance buffer and that of a current-to-voltage converter, operated with high-impedance sources at relatively high frequencies, was analysed. It was found that the noise may achieve, at first sight, unexpectedly high values. In buffers the biasing circuit's noise is multiplied by the bootstrap. The input capacitance and the parasitic capacitance of the biasing circuit reduce the bandwidth but without affecting the signal-to-noise ratio. The input capacitance or any other capacitance connected to the input of the current-to-voltage converters may increase the noise gain by orders of magnitude and significantly reduce the signal-to-noise ratio. The results throw doubt on the frequently accentuated advantage of using current-to-voltage converters for amplifying low-level signals from high-impedance sources, at least at high frequencies.