Unilateralization of Junction-Transistor Amplifiers at High Frequencies

Abstract

In designing an amplifier with a bilateral device, unilateralization is one approach which permits use of techniques already developed for unilateral devices, in particular vacuum tubes. A general method for deriving a unilateral circuit based on an equivalent circuit of the device is described, emphasis being placed on junction transistors operating at high frequencies. The principle is: application of external feedback which neutralizes the internal feedback of the device so that the signals at the output end no longer yield a signal at the input end. In general, two types of circuits are used for junction-transistor high-frequency amplifiers: emitter input and base input connections. Measurements of the input characteristics of two singly-tuned amplifiers have verified their unilateral properties. Tolerances of the neutralizing network depend on such factors as: transistor parameter spread, impedance level of the collector circuit, and performance deviations which a designer will accept. Gain and input and output characteristics of the unilateralized high-frequency amplifier may be predicted easily from the transistor parameters (both emitter input and base input connection) in a frequency range below the alpha cutoff frequency.