text: Negative feedback Negative feedback is a technique used in most modern amplifiers to increase bandwidth, reduce distortion, and control gain.  In a negative feedback amplifier part of the output is fed back and added to the input in the opposite phase, subtracting from the input.   The main effect is to reduce the overall gain of the system. However, any unwanted signals introduced by the amplifier, such as distortion are also fed back.  Since they are not part of the original input, they are added to the input in opposite phase, subtracting them from the input. In this way, negative feedback also reduces nonlinearity, distortion and other errors introduced by the amplifier. Large amounts of negative feedback can reduce errors to the point that the response of the amplifier itself becomes almost irrelevant as long as it has a large gain, and the output performance of the system (the "closed loop performance") is defined entirely by the components in the feedback loop.  This technique is used particularly with operational amplifiers (op-amps).
  Non-feedback amplifiers can achieve only about 1% distortion for audio-frequency signals. With negative feedback, distortion can typically be reduced to 0.001%.  Noise, even crossover distortion, can be practically eliminated.  Negative feedback also compensates for changing temperatures, and degrading or nonlinear components in the gain stage, but any change or nonlinearity in the components in the feedback loop will affect the output. Indeed, the ability of the feedback loop to define the output is used to make active filter circuits.
  Another advantage of negative feedback is that it extends the bandwidth of the amplifier.    The concept of feedback is used in operational amplifiers to precisely define gain, bandwidth, and other parameters entirely based on the components in the feedback loop.