A feedback amplifier, also known as negative feedback amplifier is an amplifier which uses a feedback network, generally for improving performance (gain stability, linearity, frequency response etc.). Feedback can be negative or positive, but must be negative for overall stability of the system.
Negative feedback (NFB) was invented by Harold Stephen Black (US patent 2,102,671 http://eepatents.com/patents/2102671.pdf ). The idea of this invention is simple: if the loop gain is large, overall response of the whole system can be very close to inverted feedback network response. As feedback network can be simple a passive, high degree of stability and linearity can be obtained. The most widely-known realisation of this principle in electronics are circuits, based on operational amplifiers.
Feedback may be negative, which tends to reduce output, (but in amplifiers, stabilises and linearises operation), or positive, which tends to increase output.
Negative feedback is often deliberately introduced to increase the stability and accuracy of a system, as in the feedbackamplifier invented by Harold Stephen Black.
Positive feedbackamplifies possibilities of divergences (evolution, change of goals); it is the condition to change, evolution, growth; it gives the system the ability to access new points of equilibrium.
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