# Will hooking up a HF coil to a HF opamp nullify the induction in the coil?

I want to measure an oscillating magnetic field using a coil. Using a small self-spun coil, I can already measure very decent 100 KHz signals by hooking each end of the coil up to an oscilloscope.

I want to eventually pump the frequency up to 10 MHz. Using my extremely basic coil+oscilloscope setup I can already measure this for fairly strong oscillating magnetic fields but the fields that I'll be measuring will be much smaller. I can therefore not just hook my coil up directly to the oscilloscope anymore, but will need to amplify the signal.

My problem is as follows: if I want to push my signal through some filters, then those filters will invariably have some capacitance and resistance. Hooking up my coil to non-neglectable capacitances turns the whole thing into an electric oscillator, whose resonance frequency lies (for decent coils) far below 10 Mhz!

The solution I came up with was first passing the signal through an opamp, to "restart" the circuit:

simulate this circuit – Schematic created using CircuitLab

I do have some opamps that can handle 10 MHz signals. The datasheet for the OPA6999 shows that it works just fine for these frequencies (page 7).

But won't I run into the same problem using this circuit? Won't the opamp introduce capacitance and inductance on the left side of the circuit that could significantly decrease my resonance frequency, unless I use a tiny coil?

• Do you truly mean to have positive feedback as in your schematic? – Marla Apr 11 at 23:19
• Yes, this is the circuit for a voltage follower, right? What I mean to do is process the signal on the right hand side (i.e. by adding filters and/or active components) without interfering with the resonance frequency of the LHS coil. – Heatherfield Apr 11 at 23:34
• @Heatherfield, a voltage follower has negative feedback, not positive feedback. – The Photon Apr 11 at 23:35
• Also, on the OPA699 datasheet, notice the next bullet after it tells you the GBW product is 1000 MHz, it tells you it's stable for gain greater than 4 V/V. This means it isn't stable in follower configuration. – The Photon Apr 12 at 0:10
• Shame... I should have noticed that. What about turning the thing into a V/V converter using the right side schematic in this image? electronics-tutorials.ws/opamp/opamp48.gif That could give me the right gain to make the circuit stable, without compromising on the input impedance I wanted. Do you think I could use such a circuit like that to keep the coil resonating, and still filter the signal that comes through? – Heatherfield Apr 12 at 0:16