Stability and Power Gain of Tuned Transistor Amplifiers

The transistor is a nonunilateral device which, if appropriately terminated, can become unstable at frequencies where its "internal feedback" is sufficiently large. At such frequencies, the maximum power gain is infinite and the transistor may oscillate. This paper discusses the maximum power gain realizable as a function of a required degree of stability. A "stability factor" is defined in terms of the transistor parameters and terminations (the admittance matrix is used as an example, but the approach is analogous using other representations). The maximum stable power gain of an isolated amplifier stage and the terminating admittances required for the realization of this maximum power gain are then computed as functions of the stability factor. The computations are extended to include bandwidth requirements and limitations. (It is found that, although bandwidth requirements may impose limitations on the power gain, there is no simple relationship tying together bandwidth and power gain.) The treatment of multistage amplifiers is outlined with the conclusion that the gain realizable in an n-stage amplifier is smaller than n times the gain of a one-stage amplifier having the same stability factor as the stages of the n-stage amplifier. The respective advantages of different representations for different circuit configurations are discussed. In an appendix, the theoretical considerations are applied to tuned transistor amplifiers in common-emitter and common-base configurations. The stability factor is related to the tolerances in transistor parameters and terminating impedances. Examples are given for the maximum realizable stable gain as function of parameter tolerances.