A simplified approach to predicting negative resistance in a microwave transistor with series feedback

This paper addresses the question of how to predict the series feedback termination required to generate negative resistance in a microwave transistor in different circuit configurations. Although this question is usually approached through computer modelling and simulation, we present here a simple closed form equation that predicts the value of feedback reactance most likely to generate negative resistance at the input of a 2-port sub-circuit comprising a transistor plus series feedback, based solely on the 2-port S-parameters of the transistor. This simple equation provides new insight into the effect of series feedback on a transistor and leads us to propose a general rule, namely that the series feedback reactance required to generate negative resistance in a given transistor in a given configuration depends on the sign of the imaginary part of the sum of the 2-port S-parameters for the device in that configuration.