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You can't use an agc circuit because it will make the output of both COILS the same no matter where they are in relationship to the wire.
D2 is a "clamp diode" prevents the input of the LM339 from going more negative than about -0.7V but the LM339 must not have its input go more negative than only -0.3V.
The single TL081 does not have enough bandwidth (3 Mhz) to run with the gain you have attempted to get from it.
In addition the low input impedance (1K) will lower the Q (and gain) of your resonant circuit.
Are you thinking of a new design?
I've not had problems with the clamp diode if the input current is low. But to make it sure you can use the circuit shown in figure 10 of this application note.
https://www.electro-tech-online.com/custompdfs/2011/05/AN-74-1.pdf
Lets say everything is working at nominal and you get a 2.5 volt output. Now it you move one COIL closer and one further away from the WIRE what would the lowest and highest voltage be.
I'm not sure this method will work for what you want to do. Is it your intent to adjust focus, F stop etc.?
See if you can fill in the question marks in the picture.
If it is possible to simply think of a circuit that can 'see' between 1mV and 10V Dc and output 0V to 5V Dc. Is this possible?
Is that really the right question? Don't you really want an input from1 mv to 10 volts and an output from say 1 volt to 5 volts?
Camerat,
I think this thread is getting to long. Maybe you should start a new one so you can get some different input.
I'm watching your new one and you are getting some good input!
The amplitudes from the long wire.