How to bootstrap

Left me worried, mikek. Was my explanation too poor to get the idea across?

Jon

I guess so. Is there anything you may want to ask about, then?

Jon

Are you talking about the FET amplifier fro measuring LC circuits? (I'm sorry it's not clear to me which page to look at.)

By the way, this page:

Shows some coil forms that would be trivial to make with my

3D printer. In fact, I could probably make quite a variety of coil formers of various kinds. That's a service I never even considered before, but would be very easy to offer and deliver on. Hmm.

Jon

Here's the amp that he uses.

As a test, he adds a second amp to see how it affects the measurement of the first amp.

Can you print polypropylene? That is one of the better low loss materials. An entrepreneur is born.

Here's a 3D coil I built with polystyrene. This was first iteration, then I lost access to my mill. The next was planned to be 600/46 litz, Now I've lost interest in the 3D unit. This was based on Kleijers design.

Mikek

Dang, keep forgetting my links.

I'd looked at it. Were you looking for an analysis of the BJT stage? (In effect, the 1k2 looks like a very high impedance to AC. But not to DC. So start there.)

Probably could. Supposedly, according to wiki, a "perfectly isotactic PP" has a melting point of 171C. Which is well within range of my 3D printer. All the rest of the types melt even lower still, so even easier. What I'd need is filament wire of it at 1.75mm diameter, though.

Yeah, I could build that thing.

Jon

No, the FET input stage, want to connect it to a high Q circuit and have it look as if nothing is connected.

Oh. Well, I'm weak at JFET design -- especially 1st stage. But I'll think about it. The main reason that jumps out at me for why the JFET is used here is about noise. The only current noise in a JFET comes from the shot noise due to leakage currents, which are very low. The voltage noise is Johnson noise due to the channel resistance. Only at very high source impedances (which is the case on that page) does a good JFET beat out a good BJT.

But as I said I'm not experienced with first stage JFET design issues and I need to think a bit about that part of the circuit before responding.

Jon

It's possible to position a small probe coil (with a low-Z preamplifier) so that it intercepts some very small flux from an RF coil. If the coupling is kept small, it looks like nothing is connected. Because, nothing is connected. Gain is cheap.