Re: Norton noiseless feedback amp calculation

Jake wrote...

Winfield Hill wrote:

There's also been some good discussion here on s.e.d. And check
Lepaisant in RSI 63-3 (1992). "Low-noise preamplifier with input
and feedback transformers for low source resistance sensors" by
J. Lepaisant, M. Lam Chok Sing, and D. Bloyet

Sorry this is Africa..... If it is not available on the net I am
not going to get it in a hurry or without traveling some distance
and begging to use a U library.

Well, that what the rest of us have to do. It always amazes
me how some folks who are asking for our help, and want us to
spend our time giving it, can be so demanding, with details
of what they are or are not willing to do on their part.

Anyway, I've added the article, Lepaisant_preamp-xfmrs_RSI.pdf
on my wesbsite, at http://www.picovolt.com/win/elec/articles/

As for the technique, it's simply substituting a transformer,
with its noiseless turns ratio, for resistors in a feedback
circuit. You can use the technique for all kinds of circuits,
but its best value as a low-noise technique comes when you're
working with very low impedances. That's because if resistors
are used, they must have absurdly-low values. For example, if
you're making a 100 pV/root-Hz amplifier, you may need to use
feedback and bias-setting resistors, etc, with values that are
under 0.6 ohms, to keep their Johnson noise from ruining your
amplifier. On the other hand, if you use a transformer, you
just have to keep the winding resistance well under 0.6 ohms.

OTOH, if you're making a high-impedance amplifier, then it's
likely you won't need the transformer's low noise-resistance.


--
Thanks,
- Win
.

Relevant Pages

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