My Geomagnetic Amplifier Circuit

Must be a relative of the jeroff guy last year who wanted to take ECGs from plants in his garden and could not get over the AC hum problem.

... Phil

Scratch scratch, OK I can't make heads or tails of your output stage. Someone suggested you get LT spice and run your circuits through that as a test. That sounds like a good idea.

George H.

Just one final point. Do you need the attenuator formed by R17 & R18? If not just leave them out and if necessary compensate by reducing the gain of the preceding stage(s).

Sorry, I was at the wrong end. Yes the input coil is about 10K Ohnms.

It is 3" diameter and 12" long. Rewinding to CT not an option ;-)

Robert Stevens

Thanks to everyone for their suggestions.

I have provided the DC path from U1 output to sense.

My attempt at a class AB amp has been replaced with my attempt at a constant current op amp via the trusty L165. Please see circuit revision 3 below.

In some examples I have seen the position of R22 and the coil reversed. Not sure which is best here.

BTW the input coil is shielded in foil.

Robert Stevens

Thank you. Please see the revised circut v3 here.

Robert Stevens

Oh, if you have a tuned coil (the C impedance is intended to dominate) then you have (1) phase shifts of the signal with respect to the measured output and (2) frequency selection rather than broadband detection of the magnetic field.

Was there a reason to want either of those? Or, was the output coil intended to be driven in-phase and proportional to the field at the input coil?

Instrument amps are a good choice when you want high gain (20,000 Vout/Vin) in a single stage. My concern was that a broad extremely low frequency (ELF) measuring device wouldn't be cutoff at the low frequency end (and you can get VERY low frequency performance if you do tricks, like with negative-resistance loads).

If this were a measurement device, the preamp would feed two or three gain stages, each of which would be digitized; whichever output is highest-reading but not saturated, would be stored, and one could compute a band-limited measurement with FFT on the streamed data. Three gain-of-100 stages gets to 1000000 Vout/Vin, and doesn't require any pricey instrument amp.

If this is intended as a field-fluctuation-nulling device, it doesn't benefit from similarity to field-fluctuation-amplitude metering, nor from bandpass filtering.

It isn't hard; wind a second (buck) coil and align it to the input coil. Then with an op amp and high current buffer, drive the output coil in series with the buck coil.

The idea is, to treat all voltage on the input coil terminals as an error signal, to amplify that signal, invert it, and generate (with the buck coil) the opposing field. Because the buck coil and output coil are in series, their fields are proportional (they can have different areas or turns).

Yes, it was specially wound for this application. Bought it surplus. Very thin wire.

Robert Stevens

I sort of understand what you are saying. But I wonder then why every circuit I have seen for this purpose uses an IA. They are very similar to EEG amps.

Gain is nearly 20,000. The CMR is primarily for 60Hz interferrence.

The coil is tuned with a bipolar cap at around 10Hz.

Robert Stevens

I understood it was standard procedure to tune the coil to the frequency band of interest. Maybe there is a better way?

The object is to produce a stronger field than the one detected, for use in equipment calibration.

I am aware of the potential feedback issue. The output coil would be at distance from the input and oriented at a right angle.

The cylindrical input coil is shielded in foil and has a soft iron core. The output is air core, wide area turns.

Robert Stevens

Thank you. I have made the changes you kindly sugested on the new version below.

Oops.

BTW here is the circuit mine is loosely based on.

I removed R15 and R16, and changed R17 and R18 to 1K. Although I am not sure R18 is now needed.

I have changed my circuit accordingly to your simpler design. The circuit below is what I was attempting to replicate before.

If you can give me a final OK, I will build the circuit this weekend.

Robert Stevens

OK, I will try it without the cap to see how it works.

Yes, that is the idea.

That sounds like a perfect solution. Unfortunately, designing it from scratch would (obviously) be beyond my present ability.

Can you possibly point me to an existing circuit?

Robert Stevens

I deleted them. That happens sometimes when I try to cut and paste circuit blocks from the net.

Thanks again for your assistance.

Robert Stevens