Practical Benefits:Negative Feedback

Most practical amplifier circuits use negative feedback for the following practical benefits:

Stabilization of Voltage Gain

One of the benefits of negative feedback is the stabilization of the voltage gain of an amplifier against changes in the components (e.g., with temperature, frequency, etc. ). If you represent the gain without feedback (the open loop gain) by A₀ , then the system gain with negative feedback is

where B is the fraction of the output which feeds back as a negative voltage at the input. The extent of this stabilizing influence can be illustrated as follows:

This stabilization increases the effective bandwidth.

Increasing Input Impedance

The input impedance is

which without negative feedback is Zin0 .

But with feedback, the current is reduced to

Decreasing Output Impedance

As in the approach to input impedance, the effect of negative feedback on output impedance can be obtained by analysis of the equivalent circuit.