The quality of an amplifier with respect to noise is indicated by its noise factor. The noise factor (F) is determined by measuring the ratio of signal-to-noise power (Si/Ni) at the input of the amplifier and the ratio of signal-to-noise power (So/No) at the output of the amplifier. The noise factor is equal to the input signal-to-noise ratio divided by the output signal-to-noise ratio:
The smaller the value of this ratio (F) the better
the noise quality of the amplifier. The noise factor of an amplifier with
a given transistor is affected mainly by the operating point,
the resistance of the signal source, and the frequency of the signal being
Note: The logarithm of the ratio for noise factor gives the noise factor in dB.