On Sat, 27 Jun 2015 14:02:49 +0100, Jeff Gave us:


Is that formula correct? If the input SNR is poor, an amplifier with a
high NF has very impact on the output SNR.

Also, are the units ratios, or are they in dB?



For a particular NF the effect on the output s/n ratio is always the
same regardless of the actual input s/n, until you get to the point
where the signal vanishes in the noise, but even then it still holds
true but you just can't see it.

The signal will go up by the gain of the amplifier, and the noise will
go up by the sum of *power* of the input noise times the gain and the
noise power of the calculated from the NF times the gain.

The noise powers being in watts calculated from the NF; in a 1Hz
Bandwidth by convention. So its dB above kTB converted to watts if you
are working with NF in dB.

So for a particular NF the added noise is always the same, therefore the
SNRin/SNRout holds, and is a standard definition of NF (not in dB).

Jeff

https://en.wikipedia.org/wiki/DBm

Look at the last four entries in the table.