Feedback is used to reduce the error between the reference input and the system output. Feedback also has effects on such system performance characteristics as stability, bandwidth, overall gain, impedance, and sensitivity. Feedback affects the gain G of a non-feedback system by a factor of 1 + GH.
The system of Fig. give below is said to have negative feedback, because a minus sign is assigned to the feedback signal. The quantity GH may itself include a minus sign, so the general effect of feedback is that it may increase or decrease the gain G. In a practical control system, G and H are functions of frequency, so the magnitude of 1 - GH may be greater than 1 in one frequency range but less than 1 in another. Therefore, feedback could increase the gain of system in one frequency range but decrease it in another. Feedback may increase the gain of a system in one frequency range but decrease it in another.
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| Figure: Effect of Feedback on Overall Gain |