Negative feedback is used in amplifiers for a variety of reasons. The term 'feedback' means using a fraction of the output voltage of the amplifier as input or as part of input. When the signals at input and output are of opposite phase, then the feedback signal is said to be negative.
Negative feedback signals are subtracted from the amplifier's input signals. In effect, they reduce the overall gain of the amplifier. If G is the gain of the amplifier with no feedback, and n is the feedback fraction such that Vout/n is fed back to the input of the amplifier, then the gain of the amplifier when negative feedback is applied is:
Note that if the open-loop gain G is very much larger than the loop gain n, then the closed loop gain becomes approximately G/(G/n), or simply equal to n.
Negative feedback, aside from reducing gain, also reduces noise signals generated by the components of the amplifier. Distortion that does not result in loss of open-loop gain will also be reduced by negative feedback.
The input resistance of an amplifier may also be affected by feedback. If the feedback signal is in shunt with the input signal, then the input resistance of the amplifier decreases. Using a feedback signal that is in series with the input signal, on the other hand, will increase the amplifier's input resistance.
The output resistance may also be affected by a feedback network, although to a lesser extent than the input resistance. Connecting the feedback circuit in series with the output load increases the amplifier's output resistance while connecting it in parallel with the output load will decrease the amplifier's output resistance.
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Negative feedback is used in amplifiers for a variety of reasons. The term 'feedback' means using a fraction of the output voltage of the amplifier as input or as part of input. When the signals at input and output are of opposite phase, then the feedback signal is said to be negative.
Negative feedback signals are subtracted from the amplifier's input signals. In effect, they reduce the overall gain of the amplifier. If G is the gain of the amplifier with no feedback, and n is the feedback fraction such that Vout/n is fed back to the input of the amplifier, then the gain of the amplifier when negative feedback is applied is:
Closed-Loop Gain = G / (1 + G/n).
Note that if the open-loop gain G is very much larger than the loop gain n, then the closed loop gain becomes approximately G/(G/n), or simply equal to n.
Negative feedback, aside from reducing gain, also reduces noise signals generated by the components of the amplifier. Distortion that does not result in loss of open-loop gain will also be reduced by negative feedback.
The input resistance of an amplifier may also be affected by feedback. If the feedback signal is in shunt with the input signal, then the input resistance of the amplifier decreases. Using a feedback signal that is in series with the input signal, on the other hand, will increase the amplifier's input resistance.
The output resistance may also be affected by a feedback network, although to a lesser extent than the input resistance. Connecting the feedback circuit in series with the output load increases the amplifier's output resistance while connecting it in parallel with the output load will decrease the amplifier's output resistance.
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