Effects of Negative Feedback

EFFECTS OF NEGATIVE FEEDBACK

Stabilization of Gain

The gain of the amplifier with negative feedback is

Differentiating the above equation with respect to A, we get,

The sensitivity is defined as the ratio of percentage change in voltage gain with feedback to the percentage change in voltage gain without feedback.

The reciprocal of the term sensitivity is called desensitivity i.e., desensitivity 1 + Aβ.

Increase of bandwidth

The bandwidth of an amplifier is the difference between the upper cut-off 'frequency (f₂) and the lower cut-off frequency f₁.

The product of voltage gain and bandwidth of an amplifier without feedback and with feedback remains the same, ie A_f x B W_f = A x BW

As the voltage gain of the feedback amplifier reduces by the factor 1/(1 + Aβ), its bandwidth would be increased by (1 + Aβ).

Due to the negative feedback in the amplifier, the upper cut-off frequency is increased by the factor (1 + Aβ) and the lower cutoff frequency is decreased by the same factor (1 + Aβ).

Decrease in Output Impedance

An amplifier with low output impedance is capable of delivering power (or voltage) to the load without much loss. This characteristic is achieved by employing negative series voltage feedback in an amplifier.

The output impedance with feedback is expressed by Z_of = Z_o / (1 + Aβ)

The output impedance is reduced by a factor (1 + Aβ).