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Circuit critic and transistor changes

Hi Guys, I'm going put this circuit together in the the new year.

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The idea behind the circuit is to measure a high Q parallel LC without altering the measurement. Thus the high input resistance, low capacitance and low loss capacitance. He doesn't say it anywhere, but I assume he is using the 3db method to find Q. I would prefer to use part numbers more common in the US. I may use polystyrene vs polypropylene for it's lower loss. As you can see the designer went on to build a second circuit to improve on the first, his measurements confirm he did make improvement.

So, I'm interested in discussion of:

1) Circuit improvements 2) Suitable U.S. common transistors

Thanks, MikeK

Reply to
amdx
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You could use a jfet follower with gate guards and a bootstrapped drain (very Hobbsonian) and get fractional-pF loading and close to unity gain. A tiny bit of positive feedback could literally hit zero input capacitance. After the fet, just use opamps.

I have a problem just like that now, need a low-noise amp with zero Cin.

John

Reply to
John Larkin

For the front-end FET (a BF256C) - well, Mouser has that one in stock easily enough, as well as the 2N5486. DigiKey has the BF245 and the J310. These are all spec'ed as RF parts and I suspect that they'd work, although you might want to compare input capacitances of the various different versions.

Both companies have BFR92 parts.

The output-stage bipolars don't look terribly exotic... maybe

2N4401/2N4403 (respectably low noise and quite common)? 
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Reply to
Dave Platt

Thanks for the info. MikeK

Reply to
amdx

You probably want to get that 100 uf electrolytic off of the input of the 7812 and replace it with a nice low inductive 100 nf disk ceramic. The 78 series is fairly bulletproof, but it does like a nice low-z path to ground where the open loop bode plot goes through zero db gain.

Jim

Reply to
RST Engineering

keK

That circuit seems to be OK for the AM broadcast band but "measuring Q" is much broader.

IMHO: To get a good thread on measuring Q do not make the subject line be "circuit critic and transistor changes" but make it be something about measuring Q :-). Specmanship for parts in a circuit often is only tangentially related to measuring something accurately - when we're talking about Q's above 500 then the whole methodology becomes crucial.

At the intersection of circuits and methodology, check out W7ZOI's thoughts and latest experiments measuring in the high-Q regime:

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Tim.

Reply to
Tim Shoppa

I assume by broader you mean frequency range, ok. My interest now is the AM broadcast band.

Well, I was curious what the gurus on sed. would say about the circuit. Thought I might pickup another tidbit. So far no big negatives, and I liked the idea of paralleling the second circuit and seeing how it affected the measurement of the first or vise versa.. I assumed the transistors were European numbers and I'd have to find crosses, Dave set me right on that thought.

Measuring high Qs is difficult, getting repeatable measurements on high Q physically large air core inductors is even harder.

Tim.

Thanks for the link. Here's another one of his I have.

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I'm building a coil similar to L 20 on this page.

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He measured a Q of 1304 at 900 khz with 660/46 litz. I'm contemplating 3 strands of 660/46 litz to see if I can get a Q over 2000, just for fun and amusement. I'm on a search for a high quality air variable capacitor, 360 to 500pf. In the end I'll end up with a crystal radio. Thanks for your critic :-) and the information. MikeK

Reply to
amdx

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