The role of counter reaction to operational amplifiers.

Thanks to the feedback (also known as the negative reaction) applied to the amplifiers, results are obtained which are of great practical importance. The most representative results are:

The stabilization of the original gain, the value of which is generally very variable from one component to another and sensitive to temperature or to aging;

Reduction of the non-linear distortions due essentially to the characteristic of the diodes and transistors;

- extending the bandwidth of the amplifiers;

- the reduction of the electronic noise that is inevitably present in the components;

- control of input or output impedances.

When a negative feedback is applied, a portion of the output signal is subtracted from the input signal. This change results in a stabilization of the output signal in relation to possible variations in the value of the components forming the amplifier.

When a positive feedback is applied, a portion of the output signal is added to the input signal. This modification leads to a destabilization of the output signal by avalanche effect. This effect is sought in the case of comparator circuits or oscillators.

Note: It is not uncommon, if the design is not neat, that an amplifier transforms into an oscillator or, as each of us has experienced it once, "Hooking" occurs between a microphone and a loudspeaker; The sound amplification system turns into a deafening oscillator (feedback effect).

In these two cases, the system has changed from a negative reaction (stable system) to a positive reaction (unstable system, then oscillating due to non-linearities inherent in the system).