Electronic – Questions Regarding Metal Detector Tx Coil Circuit

astableoscillatorpush-pulltransistors

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.

enter image description here

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.

Best Answer

The frequency of the tank is about 27KHz, which I'm guessing is the fundamental frequency of the astable.

No, the op-amp isn't acting as an astable multivibrator; the 100 nF capacitor fed from R5 (100 kohm) is there to self bias the op-amp so that it produces a pretty decent 50:50 square wave. The 100 kohm and 100 nF provide a cut-off at about 16 Hz so are way, way below the operating frequency of the tank.

The transistor output stage is inverting (collectors at the output point feeding the tank) and this means the sinewave signal from the tank gets turned into a square wave by the op-amp and the transistors force the tank in the opposite direction. This causes sustained oscillation.

Your feeling about the way the output transistors are driven appears correct - it's just a way of saving a few mA and extending battery life.