Noise figure and Noise factor :

The noise factor (f) of an amplifier or any network is defined in terms of the signal to noise ratio at the i/p and o/p of the system. It is defined as:

$F = \frac{S/N at i/p}{S/N at o/p} $

$= \frac{Psi}{Pni} \times \frac{Pno}{Pso}$

Where, Psi and Pni = Signal and noise power at the i/p

Pso and Pno = Signal and noise power at the o/p

The S/N at the i/p will always be greater than that at the o/p, this is due to the noise added by the amplifier, therefore the noise factor is the means to measure the amount of noise added and it will be always greater than 1, it is sometimes frequency dependent.

Noise figure:

Noise figure is expressed in decibels when noise factor is expressed in decibels, it is called Noise figure.

$NF = FdB = 10 \ log \ F$

$\therefore$ $ = 10 \ log_{10} \ [\frac {S/N at i/p}{S/N at o/p}] $

$= 10 \ Log \ 10 \ (S/N)_i - 10 \ Log \ 10 \ (S/N)_0$

$\therefore$ Fdb = (S/N) idb - (S/N) 0 db

The ideal value of noise figure is 0 DB