Circuit critic and transistor changes

[amdx]

Hi Guys,
I'm going put this circuit together in the the new year.
http://www.crystal-radio.eu/enfetamp.htm
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

 

[amdx]

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)?

Thanks for the info.
MikeK

 

[amdx]

Hi Guys,
I'm going put this circuit together in the the new
year.http://www.crystal-radio.eu/enfetamp.htm
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

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

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

IMHO: To get a good thread on measuring Q do not make the subject line
be "circuit critic and transistor changes"

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.

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.

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

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

http://w7zoi.net/2faces/twofaces.html

Tim.

Thanks for the link.
Here's another one of his I have.
http://w7zoi.net/coilq.pdf

I'm building a coil similar to L 20 on this page.
http://www.crystal-radio.eu/enlctest8.htm
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