Computer-aided-design techniques based on feedback concepts

Abstract

Proven feedback theory applied to linear and nonlinear microwave active circuits enables designers to predict true stability margins in amplifiers. H.W. Bode's return ratio can always be defined for transistors, and some applications in analysis and design are clarified by examples. Combined with a semigraphical technique, it shows the influence of feedback on reflection properties of singly terminated transistors. Return ratio is a powerful CAD concept for oscillators. The first known combination with nonlinear simulation using the direct method is both effective and accurate, as demonstrated for a nonlinear VCO. Possible extensions to matching and interstage design and to other nonlinear microwave circuits are noted. The current worldwide drive towards more realistic characterisation and modelling of active and passive components and of circuits will provide data from which more accurate assessments of feedback can be achieved using the return ratio concept