AW: Re: Couple EE questions...

Haible_Juergen#Tel2743 HJ2743 at denbgm3xm.scnn1.msmgate.m30x.nbg.scn.de
Thu Oct 24 00:15:00 CEST 1996


> Vnoise(RMS) =3D VnR =3D (4kTRB)^(1/2)
[...]
> The term "Noise Figure" is a dB scale value saying how much _more_ noise =
> the
> system has than the "perfect" system.

Thanks for looking it up - yes that's the same as "Rauschzahl" then.

> A small detail, it's the SNR and not the NF that should be optimised...
> You can optimize the NF but destroy the SNR.

Which brings me back to the original question: So choosing a higher
impedance (and selecting a fitting opamp ... fet input etc) might
improove NF, but not SNR.
SNR should *always* be best at lowest impedance, regardless what
active dvice you choose. So impedance should be always as low
as possible (from a noise point of view). Then, if you have chosen
an impedance, the second step is choosing an opamp (etc) that
won't destroy SNR too much (i.e. which has a good NF for the
selected impedance).
As far as I see, the whole NF concept won't work the other way round.
It's a method to select an active device to make the best of a given
impedance, but *not* to select the impedance.

The only way I can think of to *worsen* SNR by lowering impedance,
is when you exceed the current drive capabillity of your active devices,
i.e maximal signal voltage (the "S" in SNR) is reduced, but "N" doesn't
change much anymore. (But that's a rather extreme case, IMO.)

JH.



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