ch12 - Chapter 12 Feedback 12.1 General Considerations 12.2...

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1 Chapter 12 Feedback 12.1 General Considerations 12.2 Types of Amplifiers 12.3 Sense and Return Techniques 12.4 Polarity of Feedback 12.5 Feedback Topologies 12.6 Effect of Finite I/O Impedances 12.7 Stability in Feedback Systems
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CH 12 Feedback 2 Negative Feedback System A negative feedback system consists of four components: 1) feedforward system, 2) sense mechanism, 3) feedback network, and 4) comparison mechanism.
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CH 12 Feedback 3 Close-loop Transfer Function 1 1 1 KA A X Y
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CH 12 Feedback 4 Feedback Example A 1 is the feedforward network, R 1 and R 2 provide the sensing and feedback capabilities, and comparison is provided by differential input of A 1 . 1 2 1 2 1 1 A R R R A X Y
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CH 12 Feedback 5 Comparison Error As A 1 K increases, the error between the input and fed back signal decreases. Or the fed back signal approaches a good replica of the input. K A X E 1 1 E
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CH 12 Feedback 6 Comparison Error 2 1 1 R R X Y
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CH 12 Feedback 7 Loop Gain When the input is grounded, and the loop is broken at an arbitrary location, the loop gain is measured to be KA 1 . test N V V KA 1 0 X
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CH 12 Feedback 8 Example: Alternative Loop Gain Measurement test N V KA V 1
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CH 12 Feedback 9 Incorrect Calculation of Loop Gain Signal naturally flows from the input to the output of a feedforward/feedback system. If we apply the input the other way around, the “output” signal we get is not a result of the loop gain, but due to poor isolation.
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CH 12 Feedback 10 Gain Desensitization A large loop gain is needed to create a precise gain, one that does not depend on A 1 , which can vary by ±20% . 1 1  K A K X Y 1
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CH 12 Feedback 11 Ratio of Resistors When two resistors are composed of the same unit resistor, their ratio is very accurate. Since when they vary, they will vary together and maintain a constant ratio.
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CH 12 Feedback 12 Merits of Negative Feedback 1) Bandwidth enhancement 2) Modification of I/O Impedances 3) Linearization
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CH 12 Feedback 13 Bandwidth Enhancement Although negative feedback lowers the gain by (1+KA 0 ), it also extends the bandwidth by the same amount.   0 0 1 s A s A     0 0 0 0 1 1 1 KA s KA A s X Y Open Loop Closed Loop Negative Feedback
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CH 12 Feedback 14 Bandwidth Extension Example As the loop gain increases, we can see the decrease of the overall gain and the extension of the bandwidth.
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CH 12 Feedback 15 Example: Open Loop Parameters D out m in D m R R g R R g A 1 0
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CH 12 Feedback 16 Example: Closed Loop Voltage Gain D m D m in out R g R R R R g v v 2 1 2 1
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CH 12 Feedback 17 Example: Closed Loop I/O Impedance D m m in R g R R R g R 2 1 2 1 1 D m D out R g R R R R R 2 1 2 1
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18 Example: Load Desensitization 3 / D m D m R g R g D m D m D m D m R g R R R R g R g R R R R g 2 1 2 2 1
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ch12 - Chapter 12 Feedback 12.1 General Considerations 12.2...

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