ch3-1

Feed Back concept

Feedback, whether intentional or parasitic, is pervasive in all electronic circuits and systems. Usually, a feedback network includes a subcircuit that allows a fraction of the output signal to modify the effective input signal, in such a way as to produce a circuit response that can differ substantially from the response produced in the absence of such feedback. If the magnitude and the relative phase angle of the fed back signal decreases the magnitude of the signal applied to the input, the feedback is said to be negative or degenerative.
Otherwise, the feedback is said to be positive or regenerative. Because negative feedback tends to produce stable1 circuit responses, it is used in most applications. Note, however, that the parasitic feedback incurred by the energy storage elements associated with circuit layout, circuit packaging, and second order high frequency device phenomena often degrades an otherwise negative feedback circuit into a potentially regenerative or severely underdumped network. In contrast, positive feedback often enhances an inclination towards unstable behavior.
This property proves useful when designing oscillators. Moreover, small amounts of positive feedback can be useful even in linear applications.