A high slew rate, high input impedance, low noise differential amplifier is most suitable for diverse applications in fast signal processing. The present paper describes a specially designed a low noise currant amplifier for Photo Multiplier Tube (PMT). The circuit includes special features like wide handwidth, selectable gain and high slew rate. Use of such circuit has been demonstrated for an indigenously developed spectroscopic ellipsometer where the light intensity is very weak. The amplified signal is further processed by a data acquisition system developed for the ellipsometer. The circuit is calibrated for averaging as well as photon counting of PMTs.
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