On the application of Thevenin and Norton equivalent circuits and signal flow graphs to the small-signal analysis of active circuits

Small-signal Thevenin and Norton equivalent circuits seen looking into each terminal of the BJT and the FET are described. The application of these circuits to writing by inspection the expressions for gain, input resistance, and output resistance of multistage amplifiers is demonstrated. The application of the circuits to the noise analysis of devices is illustrated by the calculation of the noise input voltage and current of the BJT and the noise input voltage of the MOSFET. The circuits are useful for the analysis of feedback amplifiers where Mason's signal flow graph can be used to solve the simultaneous equations that are obtained. Several examples are presented which illustrate flow-graph solutions for feedback circuits.