Stability problems in transistor power amplifiers

For devices to be useful, they must be applied in circuits. Expensive power transistors can be destroyed by undesired oscillations and instabilities. Furthermore, troublesome unwanted spurious frequencies must be avoided. The problem of stability is complicated by the fact that a class C amplifier represents the superposition of a linear class A amplifier and a nonlinear instabilities can occur. This paper discusses different physical transistor effects which could be responsible for such instabilities. It tries to clarify several aspects of the stability problem in RF power amplifiers. It is shown that this problem can be solved, at least to a certain extent, if one increases the stability of a hypothetical superimposed "class A" amplifier in the critical frequency range F1< F < F2. In this region the transistor is potentially unstable due to internal feedback. The upper limit F2of the critical frequency range is given approximately by one-half the transconductance cutoff frequency. F2may be much lower than the operating carrier frequency. Rules for the design of stable RF power amplifiers are given.