Understanding of noise figure
From the equation of cascade noise figure, we can see that the noise figure of the chain is determined by the first several stages of LNA. But how to physically understand it? Why is cascade noise figure set up bu the first several stages?
F(total) = F1 + (F2 ? 1)/G1
where:
F(total) = Total noise factor
F = Noise factor of each stage
Noise Figure (dB) = 10*LOG(F)
G = Gain of each stage (linear not in dB)
From the two stage amplifier example below we can see that the first amplifier in the chain has the most contribution on the total noise figure. For this reason the lowest noise figure amplifier, always should be placed first in a line of amplifiers.
Also, from the equation of the noise factor we can see that the gain of the 2nd amplifier doesn?t have any contribution on the total noise figure, but the gain of the first stage (G1) does have an impact.
A rule of thumb is that the first stage to have a gain greater than 10dB.
Noise figure stands the ratio between the total noise above thermal noise. After LNA, both the signal and the noise level at the output has been significantly boosted, so the extra noise introduced by later stages becomes less significant.