Stability conditions in tuned common-emitter transistor amplifiers

Some of the important factors which determine the stability of common-emitter tuned amplifiers are discussed. It is shown that the minimum value of feedback capacitance required to produce instability depends on the number of stages in the amplifier and that it varies appreciably with the phase shift in the transistor. In any particular case two values are obtained corresponding to whether the circuit is under- or over-neutralized, and these values define the boundaries of the stability region. At frequencies very much lower than the ¿-cut-off frequency of the transistor, these two limits are equal in magnitude, but as the frequency is increased there is an increasing difference between them. In practice these two limits are made sufficiently large so that component and transistor variations do not greatly affect the stability of the amplifier. It is shown, however, that variations from the perfectly neutralized condition may still produce considerable distortion in the amplifier frequency-response if the actual feedback capacitance is greater than about 20% of the values required for oscillation.