are substantially unaffected by the operation of the servo loop in settled operation of the amplifier circuit this is possible because of the very low frequency roll off of the servo loop as determined by a pole originating from the combination of cmic and the large resistance of the polysilicon diodes 30 and 30 b in settled operation since the servo loop based on servo amplifier 20 is essentially noiseless during settled operation another beneficial aspect of the present invention can be recognized by noting the dc bias voltage of the preamplifier output 45 is set via the servo loop and therefore any noise voltage emanating from the reference voltage generator 25 is efficiently attenuated through very effective low pass filtering in the servo loop another advantageous effect provided by the present invention is that the servo loop makes the preamplifier input node 35 insensitive to parasitic leakage currents flowing into or out from the preamplifier input 35 the servo loop will automatically and efficiently nil leakage currents and maintain a valid operating point of the amplifier circuit even though the input resistance may be lower than the design goal under such adverse circumstances the present amplifier circuit has improved the traditional trade off between low noise and fast settling time by applying a bias control means that helps settling the high impedance preamplifier input node 35 without loading the input 35 with low impedances during normal operation of the amplifier circuit in another embodiment of the invention the.