Negative feedback affects the input and output impedance of the amplifier. Voltage mixing will increase the input impedance, while current mixing will decrease the input impedance. Current sampling will increase the output impedance, while voltage sampling will decrease the output impedance. The use of negative feedback can also greatly reduce the distortion produced by the amplifier in a stable state and weaken various interference levels inside the amplifier.

If the feedback signal and the input signal have the same polarity or change direction, the net input signal of the amplifier will be greater than the output signal due to the mixing of the two signals. This kind of feedback is called positive feedback. Positive feedback is mainly used in signal generation circuit.

On the other hand, if the polarity of the feedback signal and the input signal are opposite or the change direction is opposite (opposite phase), the net input signal will be weakened due to superposition. This kind of feedback is called negative feedback amplifier circuit, and the automatic control system usually uses negative feedback technology to stabilize the working state of the system.

Extended data:

Negative feedback sampling adopts current sampling or voltage sampling. Because of its unique advantages, negative feedback has been widely used in practical amplifiers, which has changed the performance of amplifiers. By using negative feedback, the closed-loop gain of the amplifier tends to be stable, eliminating the influence of open-loop gain.

Impedance matching is an important problem in electronic circuits. Negative feedback will also affect the input and output impedance of the amplifier. Voltage mixing will increase the input impedance, while current mixing will decrease the input impedance. Current sampling will increase the output impedance, while voltage sampling will decrease the output impedance. The use of negative feedback can also greatly reduce the distortion produced by the amplifier in a stable state and weaken various interference levels inside the amplifier.

Negative feedback can also broaden the frequency band of the amplifier and make the amplitude-frequency characteristics of the amplifier relatively flat. Therefore, negative feedback can greatly improve the amplification quality of the amplifier and improve many performance indicators, and the deeper the feedback, the greater the improvement. However, too deep negative feedback may cause the amplifier to fail to work normally and lead to self-excitation, so a stable negative feedback amplifier usually does not exceed three levels.

Baidu Encyclopedia-Negative Feedback