2.3 Negatively Feedback Amplifier Circuit
2.3.1 Working Principle of Negative Feedback Amplifier Circuit
Negative feedback has a very wide range of applications in electronic circuits. Although it reduces the gain of the amplifier, it can improve the dynamic indicators of the amplifier in many aspects, such as stabilizing the gain, changing the input and output resistance, reducing nonlinear distortion, and broadening the bandwidth. Therefore, almost all practical amplifiers are equipped with negative feedback.
There are four types of configurations for negative feedback amplifiers: voltage series, voltage parallel, current series, and current parallel. In this example, voltage series negative feedback is used to analyze the effect of feedback on various performance indicators of the amplifier.
Figure 2.3.1 shows a two-stage RC-coupled amplification circuit with negative feedback. The output voltage uo is fed back to the input terminal through RF to the emitter of transistor VT1, forming a feedback voltage uf on the emitter resistance RF1. According to the judgment method of feedback, it belongs to voltage series negative feedback.
The main performance indicators of the RC-coupled amplifier with voltage series negative feedback are as follows:
1. Closed-loop voltage gain Auf = Where:
Au is the voltage gain of the basic amplifier (without feedback), i.e., open-loop voltage gain.
1 + AuFu -- Feedback depth, which determines the extent of improvement in amplifier performance due to negative feedback.
2. Feedback coefficient
3. Input resistance Rif = (1 + AuFu)Ri
4. Output resistance
These performance indicators reflect the impact of negative feedback on the amplifier circuit, demonstrating the importance and benefits of incorporating negative feedback in amplifier designs. Negative feedback not only reduces gain but also significantly improves various dynamic indicators, making it an essential aspect of amplifier circuit design.
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