Practical analysis of wideband feedback amplifiers

Abstract
A powerful new method of assessing feedback on individual branches representing a transistor is based upon the use of computer-controlled network analysers. The s-parameters of a transistor, and of the remainder of the amplifier as viewed from the transistor terminals, are measured. These results are converted to admittance matrices. Relations are established between these and the matrix of a multinode circuit diagram, thereby extending the mathematical theory of feedback due to Bode to this method of characterising a physical amplifier operating linearly. The method is described and illustrated by practical applications. The theory is outlined, and aspects of stability margins are considered. The new method advances the practice of feedback assessment. It provides reliable gain and phase margins against self oscillation, total return ratios and components thereof, and sensitivities over frequency ranges as broad as 1-1300 MHz. It does not require special test points, nor circuit diagrams and transistor models. The information obtained gives greater assurance in circuit design, and a tool for use in developments.

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