The SNR and the Negative FeedBack Amplifier

    The Negative Feed-Back technique applied to amplifiers cannot  improve the Signal to Noise Ratio (SNR), since both the useful signal and the noise are divided by the same amount 1+Ab.

Fig. 1a depicts an "open-loop" amplifier with an input-output relationship given by:

V_out=AV_in+e           [1]

and a SNR given by

SNR=AV_in/e            [2]

The FeedBack amplifier in Fig.1b yields the same result of expression  [2] as can be easily observed by applying the "superimposition" method (Fig. 1c-1d).