Transistor-feedback-amplifier stabilisation using admittance measurements

A method is described for stabilising a single-loop negative-feedback transistor amplifier based on measurements of a chosen short-circuit-stable closed-loop driving-point admittance between two terminals within the amplifier. A correcting network is synthetised, the admittance of which, when added to the measured admittance, prevents zero encirclement by the locus of the total admittance by an adequate margin. Formulas are given enabling margins against zero encirclement to be interpreted in terms of movements of corner frequencies associated with a frequently used internal amplifier configuration. The method is demonstrated to be straightforward for an amplifier requiring measurements up to 20MHz, and the admittance margins obtained are compared with margins obtained from loop-gain measurements.