What Is a Feedback Circuit?

The feedback circuit is to recover part or all of the amplifier output signal (voltage or current) to the input end of the amplifier for comparison (addition or subtraction) with the input signal, and use the effective input signal obtained from the comparison to control the output. Feedback process.

In social psychology, feedback is an important link in the process of interpersonal communication. The interpersonal communication process is the process of information exchange, which is consistent with the process of information exchange. In the process of interpersonal communication, the information source sends information through a certain information channel and passes it to the receiver. In this process, the reaction between the sender and the receiver of the message is called feedback.

In the basic amplifier circuit,

Feedback circuits are widely used in various electronic circuits.

According to its circuit structure, it is divided into: current feedback circuit and voltage feedback circuit. Positive feedback circuits are mostly used in electronic oscillation circuits, while negative feedback circuits are mostly used in various high and low frequency amplifier circuits.

According to the circuit characteristics can be divided into: series feedback and parallel feedback.

Due to its wide application, negative feedback has five effects on amplifier performance:

1. Negative feedback can improve the stability of the amplifier gain.

2. Negative feedback can widen the passband of the amplifier.

3. Negative feedback can reduce amplifier distortion.

4. Negative feedback can improve the signal-to-noise ratio of the amplifier.

5. Negative feedback affects the output and input resistance of the amplifier.

The amplifier circuit containing the feedback network is called a feedback amplifier circuit. The feedback amplifier circuit is mainly composed of a basic amplifier circuit and a feedback circuit. Feedback can be divided into negative feedback and positive feedback.

The feedback amplifier circuit is composed of a basic amplifier circuit and a feedback network. The two input ends of the basic amplifier circuit are respectively defined as "front end of input signal" (referred to as "front end") and "back end of input signal" (referred to as "back end" "); The potential difference between" front end "and" back end "is the net input voltage id sent to the basic amplifier circuit; the output end of the amplifier circuit is divided into" the upper end of the output voltage "(referred to as" the upper end ") and" output voltage The lower end "(referred to as the" lower end "); the small rectangle in the figure represents the feedback bridge, which is part or all of the feedback network. The feedback bridge also has two terminals. If its right end is connected to the "upper end" of the output voltage, it constitutes voltage feedback. If it is connected to the "lower end" of the output voltage, it constitutes current feedback (Note: When forming the current feedback, The lower end cannot be directly grounded, and a resistor should be connected, otherwise there is no feedback.) The left end of the feedback bridge in the figure is connected to the input circuit. There are two types of connection methods: series and parallel. If it is connected to the "back end" of the input signal, the feedback The signal is in the form of voltage and the net input voltage

Addition and subtraction form a series feedback. If it is connected to the "front end" of the input signal, the feedback signal is in the form of current and the input current.

After shunting (addition and subtraction), the net input current

Feeding into the basic amplifier circuit constitutes parallel feedback. So there are four basic types of feedback, namely voltage series feedback, voltage parallel feedback, current series feedback and current parallel feedback ^[1]

(1) Find a feedback bridge and determine the feedback network. The feedback bridge is the shortest path that directly connects the output and input. It is collectively called the feedback network along with the other common parts of the output and input. The feedback bridge can be found from the input side to the output side.

(2) The basic form of judgment feedback. The feedback bridge is connected to the "upper end" (or "lower end") of the output voltage at the output end to form voltage feedback (or current feedback); Parallel feedback (or series feedback).

(3) Determine the feedback polarity. Use the instantaneous polarity method to determine whether it is positive feedback or negative feedback. The specific method is: first assume the input voltage signal

At a certain instant, the polarity is positive (relative to the reference ground), marked with , and then along with the signal transmission direction, the instantaneous polarity of the output signal and the feedback signal (combined with or marked) is gradually derived, and finally judged Whether the feedback signal strengthens or weakens the net input signal, if it is weakened, it is negative feedback, if it is enhanced, it is positive feedback.