15.34 The LC oscillator in Fig. P15.34 is based on connecting a positive-gain amplifier (formed by Q1, Q2, and RC) with a bandpass RLC circuit in a feedback loop.

(a) Replace the BJTs with their small-signal models while neglecting rπ and ro (to simplify matters).

(b) By inspection of the circuit found in (a), find the frequency of oscillation and the condition required for oscillations to start. Express the latter as the minimum required value of (IRC).

(c) If IRC is selected equal to 1 V, show that oscillations will start. If oscillations grow to the point that Vo is large enough to turn the BJTs on and off, show that the signal at the collector of Q2 will be a square wave of 1 V peak-to-peak. Estimate the peak-to-peak amplitude of the output sine wave Vo.

(Hint)

## 1 Answer