Transistor AC and DC Amplifiers With High Input Impedance

Abstract

A class of transistor amplifiers is described in which high input impedance is achieved with bias stability comparable with that of ordinary low input impedance amplifiers. Input impedances of several megohms shunted by one or two micromicrofarads are easily realized with simple circuits, and input resistances up to 100 megohms may be obtained with more elaborate circuitry. Other important properties are that input shunt capacitance can be almost completely eliminated, the voltage gain is stabilized, and the output impedance is low. Criteria for best noise performance are discussed. Typical practical measurements are given, and various illustrative circuits are shown for both a-c and d-c amplifiers