Oscillating inductor/buffer

I suggested a JFET off the top of the coil when I didn't know what he was doing.

Now I'll suggest that he either tap down on the coil, or intentionally resonate it with both a fixed capacitor and a variable. Further, he can make the fixed capacitor into a nice voltage divider, and drive the base of a transistor with that. Yes, the voltage will be lower -- but with the right circuit, the overall noise will be lower than a circuit involving the misuse of an op-amp.

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com

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=A0 =A0 ...Jim Thompson

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Thanks Jim... I did that today. And thanks to Fred B. learned that I measured the opamp differential capacitance. No way to shield that.

George H.

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Thanks Tim. I was thinking today of trying a little jfet follower on the front. But I really don't have space for any extra stuff. The amp has other more generic tasks, beside the radio front end. It's for a 'lab' about Fourier methods. We should have a newsletter out in a few weeks and I can say more about the details.

It's funny we do all this neat physics, NMR, optical pumping, Doppler free spectroscopy, ...(I won't bore you with more, you can always troll the website.) And when I was testing the AM radio, the production people seemed more impressed by it than the other stuff. "Wow, you can make a radio?" I'll admit as a radio it must be the world's most expensive for the quality of reception and output.

The nice thing about opamps is it's all there in one package. Input protection, output current limit, known frequency response. Apply power, a few bypass caps, R's and away you go.

George H.

(Oh and I know 'jack ..' about how a 'real' AM radio works. I'm sure I could learn, but there's bigger fish to fry.)

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Grin... Lots of times I'm not sure I know what I'm doing.

But it's been my experience that sometimes you should just try something. The results may not be what you expect, but ... to paraphrase A.B. Pippard from the intro to "Vibrations Vol. I*" "I'm not as smart as these other guys and it helps to have some data to help guide my thinking."

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The goal is not to make a good radio, but to teach something about signal processing. We want to have the signal 'avalible' all the way along the chain.

Did you ever see our lockin amplifier.

Hardly the best lockin.... (maybe the worst?) But a student gets to see all the signals. I never thought we'd sell many of these since is priced about the same as the low end SRS. But we have sales of about

20/year.

George H.

*A good read in my opinion

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That's unlikely to get you much happiness... try instead just

=A0 =A0 =A0 =A0 |\ =A0opa2134 +--+-R2--|+\ | =A0| =A0 | =A0>-+out L =A0R =A0 +|-/ =A0| L =A0R =A0 ||/ =A0 | L =A0R =A0 +--R2-+ | =A0| +--C--+ +--+ GND

I'd try R2 =3D 1k ohm, C=3D 33 pF.

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OK I'll try it on Monday. If it works I'll be back for an explanation. I tried R2 (1k ohm) in the non-inverting input, but it didn't stop the problem.

I ordered some 220uH and 330 uH inductors I figure if I put a few in series the inductacne will add, but they shouldn't couple together too much.

I also ordered a few AD825 opamps. They'll be nice to have in a drawer anyways.

George H.

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Hi Whit3rd, I'm sorry to say I couldn't get this to work. The biggest R2 on the input I can live with is 1k ohm. I tried C's from

10pF to 1000pF. Bumping up the feed back R2 made the output signal look a little different. At 100k ohm the output was much more symmetric... but still singing away at ~1.5 MHz

George H.