the noise figure
/ noyz FIG-yer /
Every amplifier adds a little hiss of its own to whatever passes through it. The noise figure is a single number that says how much an amplifier degrades the signal-to-noise ratio (SNR), in other words, how much dirtier the signal looks coming out than it did going in. A perfect, noiseless amplifier would have a noise figure of 0 dB; every real one is worse, and the smaller the number, the quieter and better the part. It is the headline spec for any receiver front-end.
Precisely, the noise factor F is the ratio of input SNR to output SNR: F = (SNR in) / (SNR out). The noise figure is just that ratio expressed in decibels, NF = 10 times log10(F). An ideal device has F = 1, so NF = 0 dB. If an amplifier has a noise figure of 3 dB, it has roughly halved the signal-to-noise ratio, the signal comes out twice as noisy relative to the noise floor as it went in. A good LNA might manage under 1 dB; a 10 dB noise figure means the SNR was cut by a factor of ten. Because the noise figure is referenced to a standard temperature, it lets you compare parts fairly and predict how a whole chain will perform.
Noise figure matters most at the front of a receiver, where the signal is weakest, because of the cascade rule from Friis: the first stage's noise figure dominates the whole receiver, and later stages matter less since the first stage has already provided gain. That is exactly why the LNA is chosen for the lowest possible noise figure. Honest caveats: noise figure only describes added noise, not gain or distortion, a part can have a wonderful noise figure and still be a poor amplifier in other ways. And a lossy component before the LNA (a long cable, a filter, a switch) adds its loss directly to the system noise figure, which is why anything ahead of the first amplifier is kept as low-loss as possible.
Two amplifiers each give 20 dB of gain, but one has a 1 dB noise figure and the other 5 dB. Put the 1 dB part first as the LNA, and by Friis the whole receiver's noise figure stays near 1 dB; swap the order and the 5 dB part out front drags the whole chain to about 5 dB.
By Friis's rule the first stage's noise figure dominates the whole receiver.
Noise figure measures only added noise, not gain or distortion. Any lossy part before the LNA (cable, filter, switch) adds its loss straight to the system noise figure, so keep the front end low-loss.