Abstract: The feedback amplification circuit consists of a basic amplification circuit and a feedback network. The feedback bridge is part or all of the feedback network. The feedback amplification circuit type is determined by: one end of the feedback bridge is connected to the "upper end" (or "lower end") of the output voltage to form a voltage (or current) feedback, and the other end of the feedback bridge is connected to the "front end" of the input signal (or The "back end") forms a parallel (or series) feedback, and the instantaneous polarity method is used to determine that the feedback that reduces (or increases) the net input is negative feedback (or positive feedback). This method can quickly determine the type of feedback amplifier circuit.
Key words: feedback bridge; feedback type; judgment method; current and voltage feedback; series parallel feedback; positive and negative feedback
The feedback amplifier circuit is an important part of electronic circuit technology. Introducing different types of feedback into the amplifying circuit can achieve the function of improving circuit performance or establishing oscillation, and correctly selecting the feedback type is the key. Together with the members of the course group, I have summarized a method for determining the type of feedback amplifying circuit. It is based on the structure of the circuit to determine the type of feedback amplifying circuit from a macroscopic point of view. We have used this method for many years in teaching, and the judgment of the type of feedback amplification circuit is simple and accurate.
1 Basic concept of feedback amplifier circuit
In an electronic circuit, the so-called feedback means that part or all of the output (voltage or current) of the amplifying circuit is sent back to the input circuit through a certain circuit (called a feedback network), and the external input signal is combined to form an amplification. The process of inputting a signal (voltage or current) to affect the output (voltage or current). The feedback reflects the reaction of the output signal to the input signal.
Feedback has positive and negative points. Positive feedback can improve the gain, but it will deteriorate the performance of the amplifier circuit, such as the stability of operation, distortion, frequency characteristics; negative feedback reduces the gain of the amplifier circuit, but it improves the performance of many aspects of the amplifier circuit, using negative The feedback technique can also constitute various arithmetic circuits. Therefore, negative feedback is introduced in the actual amplifying circuit, and positive feedback is mainly used in the oscillating circuit.
Feedback also includes DC feedback and AC feedback. DC negative feedback improves the DC performance of the amplifier circuit. It is commonly used to stabilize the static operating point. AC negative feedback improves the dynamic performance of the amplifier circuit. In many circuits, it is often the combination of AC and DC feedback.
2 feedback amplification circuit structure model
In order to determine the type of the feedback amplifying circuit, a feedback circuit structure model as shown in FIG. 1 is established.
Model description: The feedback amplifying circuit is composed of a basic amplifying circuit and a feedback network. The two input terminals of the basic amplifying circuit are respectively defined as “the front end of the input signal†(referred to as “front endâ€) and “the back end of the input signal†(referred to as "back end"); the potential difference between the "front end" and the "back end" is the net input voltage μid sent to the basic amplifying circuit; the output end of the amplifying circuit is divided into "the upper end of the output voltage" (referred to as "upper end") and "Lower end of the output voltage" (referred to as "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 the right end of the feedback bridge is connected to the "upper end" of the output voltage, it constitutes voltage feedback. If it is connected with the "lower end" of the output voltage, it constitutes current feedback. (Note: When forming current feedback, The lower end can not be directly grounded, and should be connected to a resistor, otherwise there will be no feedback.) The left end of the feedback bridge is connected to the input loop. The connection method is series and parallel. If it is connected with the “back end†of the input signal, feedback The signal is added and subtracted from the net input voltage uid in the form of voltage to form a series feedback. If connected to the "front end" of the input signal, the feedback signal is shunted (added and subtracted) with the input current ii in the form of current. The net input current iid is fed into the basic amplifying circuit, which constitutes parallel feedback. Therefore, there are four basic types of feedback, namely voltage series feedback, voltage parallel feedback, current series feedback, and current parallel feedback.
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