Fig. C.1 represents a transistor with input and output tuned circuits damped by parallel-connected resistances. As explained on page 182 feedback occurs via the internal collector-base capacitance cre and this can cause oscillation at a frequency below the resonance value for the tuned circuits. Oscillation is most likely when the phase shift introduced by the tuned circuits totals 90 °: if the circuits are similar and resonant at the same frequency oscillation occurs at the frequency for which each circuit gives 45 ° phase shift. From this information it is possible to arrive at a simple expression for the stability of the amplifier.
Fig. C.1 Essential features of a transistor tuned amplifier ...
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