Frequency Response of Theoretical Models of Junction Transistors

For a grown-junction transistor, the concept of a constant base-spreading resistance may not be valid at high frequencies, owing to the distributed nature of the transistor parameters in the transverse direction of the base. However, results of a theoretical analysis of an appropriate two-dimensional model have shown that this type of transistor may be represented by the same type of model as that normally used for the fused-junction transistor, but with the constant base spreading resistance of the latter model replaced by a complex frequency-dependent base impedance. These two types of models represent limiting cases which should be useful for calculating circuit performance of practical junction transistors. In this paper, a method of comparing circuit performance of these two types of transistor models is described for both grounded-base and grounded-emitter configuration, using the series-parallel, or parameters. Under simplifying conditions, either type of transistor model in either configuration can be described by three normalized functions of frequency relative to -cutoff frequency plus three additional constants. Simple relations are shown to exist between grounded-base and grounded-emitter parameters. Polynomial representations are given for the h parameters for both grounded-base and grounded-emitter operation, and simplified equivalent circuits are presented. To illustrate this method of circuit analysis, numerical examples are given for power gain and input resistance for a one-stage amplifier terminated in a pure resistance. Finally, the subject of maximum available power gain also is discussed briefly.