Hello. I am trying to understand how this circuit operates. I understand how the circuit works on the right side of the transistor, but the oscillation stage with the crystal confuses me. It appears that the crystal has no feedback from the output of the oscillator. I researched this and found out that the collector-base capacitance of the transistor provides a feedback path, but wouldn’t that only give a 90° phase shift instead of the 180° phase shift required for positive feedback?
I have seen similar circuits where a variable capacitor is included with the crystal to adjust frequency. Would that give the phase shift for the remaining 90°? Thanks, your help is appreciated.
Best Answer
Yes, it could oscillate, but in a SPICE simulator, it didn't. Not quite. A few component changes did start oscillations. The 7MHz crystal equivalent circuit is a guess (C1, L2, R5, C2):
The base-to-emitter capacitance of the 2N2222 is large enough that this is a Colpitts-type oscillator.