Field-Effect Transistor Reactance Circuits.

Abstract

Efforts to miniaturize the reactance circuits of vacuum-tube technology by using junction transistors have been only partially successful. Large equivalent inductances and capacitances are obtainable, but the effective Q is limited to low values by the low input impedance of these devices. Unipolar field-effect transistors offering high gain and high input impedance are investigated for this application. Analytical models, based on the equivalent circuit of the FET, are developed for reactance circuits employing R-C feedback networks. A simple capacitive reactance circuit, utilizing a first-order feedback network and operating at a frequency of 250 KHz, yielded a low-Q effective capacitance but served to verify the model employed. Computer-aided design of a reactance circuit employing a second-order feedback network resulted in a stable effective capacitance offering Q multiplication. Sensitivity of the feedback voltage to component tolerances precludes packaging this circuit in integrated form, but the feasibility of designing an FET reactance circuit to meet rather precise specifications is demonstrated.