Most physical systems incorporate some form of feedback. It is interesting to note, though,that the theory of negative feedback has been developed by electronics engineers. In hissearch for methods for the design of amplifiers with stable gain for use in telephonerepeaters, Harold Black, an electronics engineer with the Western Electric Company,invented the feedback amplifier in 1928. Since then the technique has been so widely usedthat it is almost impossible to think of electronic circuits without some form of feedback,either implicit or explicit. Furthermore, the concept of feedback and its associated theoryare currently used in areas other than engineering, such as in the modeling of biological systems.
Feedback can be either negative (degenerative) or positive (regenerative). In amplifierdesign, negative feedback is applied to effect one or more of the following properties:1. Desensitize the gain: that is, make the value of the gain less sensitive to variations in the values of circuit components, such as might be caused by changes in temper-ature.2. Reduce nonlinear distortion: that is, make the output proportional to the input (in other words, make the gain constant, independent of signal level).3. Reduce the effect of noise: that is, minimize the contribution to the output of unwanted electric signals generated, either by the circuit components themselves, or by
extraneous interference.4. Control the input and output resistances: that is, raise or lower the input and output resistances by the selection of an appropriate feedback topology.5. Extend the bandwidth of the amplifier.