When a portion of output signal is return to the input and combined with the external input signal, it is called feedback amplifier. The portion or output signal returned to the input is called feedback signal. When the feedback signal is combined with the external input signal in phase opposition at the input terminals, it is called degenerative, inverse or negative feedback.
When the feedback signal is combined in phase with the external input signal at the input port, it is called regenerative, direct or positive feedback.
Basic schematic block diagram, if a single loop negative feedback amplifier.
(i) Voltage feedback
(ii) Current feedback
(1) Voltage series feedback
(2) Current series feedback
(3) Current shunt feedback
(4) Voltage shunt feedback