Neg. feedback amp
A negative feedback amplifier (or more commonly simply a feedback amplifier) is an amplifier which combines a fraction of the output with the input so that a negative feedback opposes the original signal. The applied negative feedback improves performance (gain stability, linearity, frequency response, step response) and reduces sensitivity to parameter variations due to manufacturing or environment. Because of these advantages, negative feedback is used in this way in many amplifiers and control systems.
A negative feedback amplifier is a system of three elements (see Figure 1): an amplifier with gain AOL, an attenuating feedback network with a constant β < 1 and a summing circuit acting as a subtractor (the circle in the figure). The amplifier is the only obligatory; the other elements may be omitted in some cases. For example, in a voltage (emitter, source, op-amp) follower the feedback network and the summing circuit are not necessary.