Feedback in amplifiers and oscillators

In the amplifiers described in the four previous chapters, no attempt was made to connect any part of the output signal back again to the input terminals via an external circuit. Such amplifiers tend in practice, however, to exhibit a variable, and therefore unreliable, performance. The amplifiers used in modern communications systems, analogue computers and measuring equipment must be extremely stable and free from distortion, and these requirements can be satisfied only by the application of a controlled amount of negative feedback (nfb): that is, by taking a fraction of the output signal and feeding it back to the input terminals in opposition to the signal already there.