The following circuit is the transmit coil of an IB metal detector. I recognize most of this: an op amp astable with a CE push-pull output stage driving a resonant circuit. But, I have questions.
The frequency of the tank is about 27KHz, which I'm guessing is the fundamental frequency of the astable. I understand the general astable frequency formula, but I'm finding it difficult to calculate the positive feedback fraction in this instance. How to calculate the feedback fraction?
My other question relates to capacitors C1 and C2. They seem to act like a charge pump, pumping the centre-biased input signal up (and down) to either rail. My feeling is that it makes the push-pull pair more efficient by turning off one or other of the transistors, but I'm not sure. Can anyone shed any light?
Incidentally, here's the original project document (for context) Buccaneer Metal Detector.