I'm back on this for more study

[Mark-VA]

Decided to go through my original electronics classes using the books I had back in the day. NRI
Well anyway the picture below is basically the same as what Carl sent me a few months ago and the instructions of making a current probe to measure the current from a detector coil.
My book gives a complete analysis of the voltages, current flow and gain with examples.
Assuming correctly I would replace R1 with my receiving coil.

[Mark-VA]

Actually further study indicates R2 is feeding back part of the output back into the amp. This tends to stabilize the amp especially if the output is to high.
The top diagram is very close to the one Carl posted earlier this year. Think I'll experiment and put another resistor in series with the receiving coil.