The use and selection of negative feedback in amplifier circuits

1. Basic Views on Feedback

Feedback refers to a connection method that sends part or all of the circuit output (voltage or current) back to the input loop in a necessary way to affect the circuit performance. Feedback is divided into two categories: positive feedback and negative feedback. Almost all practical amplifier circuits are circuits with negative feedback; as for positive feedback, it is mostly used in oscillation circuits.

2. Basic forms of negative feedback
According to the different feedback sampling methods, it is divided into current feedback and voltage feedback; according to the different connection methods of the feedback signal and the input signal at the input end of the amplifier circuit, it is divided into series feedback and parallel feedback. Their combination forms four feedback methods.

3. Basic types and identification of negative feedback
1. Feedback branch
The so-called feedback branch refers to the path between the output loop and the input loop. Because negative feedback must return part (or all) of the output to the input end through a certain path, the branch that forms this path is the feedback branch.

2. DC and AC feedback
The feedback signal is DC electric quantity, which is DC feedback. Its circuit feature is that a signal filter is connected in the feedback branch (or between the two ends). DC feedback is mainly used to stabilize the static working point of the amplifier circuit; the feedback signal is AC electric quantity, which is AC feedback. Its circuit feature is that a capacitor is connected in series in the feedback branch, which is mainly used to improve the performance of the AC amplifier (such as stabilizing the circuit's amplification factor, widening the bandwidth, reducing distortion, etc.); if there is only a resistor element in the feedback branch, there are both AC and DC negative feedback, which has the above dual functions.

3. Identification of feedback polarity
The feedback polarity can be determined by the "instantaneous polarity method". It can be seen from the working characteristics of the transistor that when the amplifier circuit works normally, the phases of the b and c pole signals are opposite, and the phases of the b and e pole signals are similar. Therefore, the polarity of the input signal at a certain moment can be assumed and indicated by "+" and "-" signs in the circuit diagram. Then, according to the signal transmission route, the instantaneous polarity of each point in the circuit is deduced step by step. Finally, it is determined whether it is positive feedback or negative feedback based on whether the instantaneous polarity of the feedback signal is similar or opposite to that of the input signal.

4. The difference between voltage feedback and current feedback
Assume that the output load resistor RL is short-circuited to see if there is still a feedback signal. If the feedback signal disappears, it is voltage feedback; if the feedback signal still exists, it is current feedback.

5. The difference between series feedback and parallel feedback Series
feedback refers to the feedback signal and the input signal appearing in the form of voltage addition and subtraction at the input end, which can increase the input impedance of the circuit;

The feedback signal and the input signal appear at the input end in the form of current addition and subtraction, which is parallel feedback and can reduce the input impedance of the circuit.

IV. Summary
Voltage negative feedback can stabilize the output voltage of the amplifier circuit, so the output impedance is lower than when there is no negative feedback; current negative feedback can stabilize the output current of the amplifier circuit, so the output impedance is higher than when there is no feedback; series negative feedback increases the input impedance because the feedback branch is connected in series at the input end; parallel negative feedback reduces the input impedance because the feedback branch is connected in parallel at the input end.
After the introduction of negative feedback to the amplifier circuit, the amplification factor is reduced, but it plays a major role in improving the performance of the amplifier circuit. Therefore, fully understanding and mastering the feedback knowledge of the amplifier circuit will undoubtedly improve the learning content of beginners.