I've been research metal detector designs for a few days now and I can't quite wrap my head around the amplification they are using for the first stage right out of the coil. Below is an example schematic from one of the paper explaining how to build a pulse induction metal detector:
I understand the most of the circuit, the problem I am having is understanding what the inputs and outputs should look like on IC4. As far as I understand it the amplifier is essentially amplifying the difference between the coil and the 9V rail. As far as I can tell the coil voltage should fly highly positive when TR4 opens. This voltage is clamped by D3 and D4 until it gets below the forward voltage threshold. At this point it is then amplified by IC4. The part I don't get is why they made it an inverting amplifier, because from what I can tell the output of IC4 should be pegged at 0 the entire time the coil is discharging since the voltage will always be higher than the 9V rail.
If anyone has insight as to what I am missing or how that section of the circuit works I would really appreciate it.
Here is the link to the full pdf paper which contains an explanation for most of the circuit: PI Metal Detector - Mark Stuart (1989)