Improved methods of assessing feedback in wide-band multiloop amplifiers

A pressing problem in modern, wide-band feedback amplifiers is to obtain reliable information on margins against self-oscillation. The application of Bode's general and exact theory of feedback to practical circuits of considerable complexity, such as a complete repeater, has been achieved by a new method based upon measurements over a very wide band of frequencies. Transistor models and circuit diagrams are not required above the traffic band. The theory of the method is presented in nodal analysis terms. The principal result is a set of relationships between determinants of a 4-node matrix derived from measurements and corresponding determinants of a hypothetical, multinode matrix of the complete circuit. Extensive software was developed to translate the theoretical results and insights into algorithms. These have been applied to several practical amplifiers with outstanding results. Apart from stability assessment, feedback also affects sensitivities and some results due to Bode are extended to cover large changes. Return ratios and related quantities have been evaluated for transistors in practical amplifiers over the range 1-1300 MHz, with accuracies comparable with those of the basic
s
-parameters. Reliable gain and phase margins, and values of total feedback and sensitivities, have been computed without the need to split a node as required for loop gain. More information, greater assurance in circuit design, and a potential tool in amplifier development have been achieved.