反馈

本文主要介绍了反馈在电子学中的基本概念和应用介绍。主要学习内容涵盖了:一般反馈结构、负反馈的若干性质、四种基本反馈结构、串联并联反馈放大器、串联串联反馈放大器、并联分路并联反馈放大器、波德图稳定性研究、频率补偿等。
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1.Chapter 8 Feedback Introduction 8.1 The general feedback structure 8.2 Some properties of negative feedback 8.3 The four basic feedback topologies 8.4 The series-shunt feedback amplifier 8.5 The series-series feedback amplifier 8.6 The shunt-shunt and shunt-series feedback amplifier 8.10 Stability study using bode plot 8.11 Frequency compensation

2.Introduction It’s impossible to think of electronic circuits without some forms of feedback. Negative feedback Desensitize the gain Reduce nonlinear distortion Reduce the effect of noise Control the input and output impedance Extend the bandwidth of the amplifier The basic idea of negative feedback is to trade off between gain and other desirable properties. Positive feedback will cause the amplifier oscillation.

3.Three Parts: PartI : The basic concept and some Properties of negative feedback PartII : The four basic feedback and analysis PartIII : The loop gain, stability problem and frequency compensation

4.PartI The basic concept Judgment and Properties of feedback examples

5.PartI : The basic concept and some Properties of negative feedback This is a signal-flow diagram, and the quantities x represent either voltage or current signals. In electronic circuits, part of or all output signal is fed back to input, and affects the input signal value, which is called feedback. 8.1 The General Feedback Structure

6.The feedback judgment for amplifier circuits

7.Negative feedback and positive feedback : According to the effecting of feedback 1) positive feedback increases the signal that appears at the input of the basic amplifier 2) negative feedback reduces the signal that appears at the input of the basic amplifier DC feedback and AC feedback : Feedback quantity only contains DC quantity , is called DC feedback 2) Feedback quantity only contains AC quantity , is called AC feedback Usually AC feedback and DC feedback are concomitant

8.The feedback judgment : No feedback Feedback exists No feedback

9.Instantaneous polarity method : The judgment of feedback parity 1) Regulate the polarity of input signal relative to ground at sometime. 2) Decide all points’ parity step by step, at last get the parity of output signal. 3) According to the parity of output signal decides the parity of amount of feedback. 4) If amount of feedback increases the signal that appears at the input of the basic amplifier, the circuit inducts the positive feedback. Otherwise, it inducts the negative feedback.

10.To integrated operational amplifiers , the input quantity can be U D or i N ( i P )

11.To discrete components amplifiers , the input quantity can be Ube or i b

12.AC feedback , no DC feedback DC feedback , no AC feedback The judgment of DC feedback and AC feedback

13.Example : feedback ? Positive or negative ? DC or AC ? AC and DC negative feedback

14.The General Feedback Equation Open loop gain A Feedback factor β Loop gain Aβ Closed loop gain A f Amount of feedback (1+ Aβ )

15.The General Feedback Equation If Aβ >>1, The gain of the feedback amplifier is almost entirely determined by the feedback network. If Aβ >>1, which implies that the signal Xi at the input of the basic amplifier is reduced to almost zero.

16.8.2 Some Properties of Negative Feedback Gain desensitivity the percentage change in A f (due to variations in some circuit parameter) is smaller than the percentage change in A by the amount of feedback. For this reason the amount of feedback, 1 + Aβ , is also known as the desensitivity factor.

17.2. Bandwidth extension Some Properties of Negative Feedback Note that the amplifier bandwidth is increased by the same factor by which its midband gain is decreased, maintaining the gain-bandwidth product at a constant value .

18.Some Properties of Negative Feedback 3. Noise reduction

19.Some Properties of Negative Feedback 4. Reduction in nonlinear distortion

20.Some Properties of Negative Feedback 4. Reduction in nonlinear distortion

21.Homework: May 11th, 2008 8.1; 8.7;

22.PartII : The four basic feedback and analysis

23.8.3 The Four Basic Feedback Topologies Voltage amplifier---series-shunt feedback voltage mixing and voltage sampling

24.The Four Basic Feedback Topologies Current amplifier---shunt-series feedback Current mixing and current sampling

25.Example: Figure 8.5 A transistor amplifier with shunt–series feedback. (Biasing not shown.)

26.Transconductance amplifier---series-series feedback Voltage mixing and current sampling The Four Basic Feedback Topologies

27.Example: Figure 8.6 An example of the series–series feedback topology. (Biasing not shown.)

28.Transresistance amplifier---shunt-shunt feedback Current mixing and voltage sampling The Four Basic Feedback Topologies

29.Example: Figure 8.7 (a) The inverting op-amp configuration redrawn as (b) an example of shunt–shunt feedback.