Resistors to suppress parasitic oscillation of emitter follower circuit
Emitter follower circuits are often used to realize low output impedance amplifiers. But they can oscillate parasitically unless proper attention is paid. When this phenomenon is overlooked, unexpected trouble may be caused, e.g. EMI. Moreover, it is difficult to check oscillation because the condition is so delicate as to be changed by touching with a probe of an oscilloscope. Still more, oscillation can be stopped even with changing temperatures.
An effective solution for this kind of oscillation will be to decouple the collector from the Vcc and to put a resistor Rb, ranging from several 10 to several 100, to the base circuit as illustrated. When the emitter load is capacitive, inserting a low resistance resistor into the load in series also has an effect.
For power amplifiers, this series resistance becomes very small. And it might bring about unexpected heating or going up in smoke depending on the transistor failure mode. In such situations, Fusing Resistors RF or RF73 which has a fuse function is recommended. Rush current should be taken into consideration together when choosing Fusing Resistors.