ch12 - 1 Chapter 12 Feedback 12.1 General Considerations...

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Unformatted text preview: 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 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. CH 12 Feedback 3 Close-loop Transfer Function 1 1 1 KA A X Y + = 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 + + = 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 CH 12 Feedback 6 Comparison Error 2 1 1 R R X Y + 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 = X CH 12 Feedback 8 Example: Alternative Loop Gain Measurement test N V KA V 1- = 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. 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 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. CH 12 Feedback 12 Merits of Negative Feedback 1) Bandwidth enhancement 2) Modification of I/O Impedances 3) Linearization CH 12 Feedback 13 Bandwidth Enhancement Although negative feedback lowers the gain by (1+KA ), it also extends the bandwidth by the same amount. ( 29 1 s A s A + = ( 29 ( 29 1 1 1 KA s KA A s X Y + + + = Open Loop Closed Loop Negative Feedback 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. CH 12 Feedback 15 Example: Open Loop Parameters D out m in D m R R g R R g A = = 1 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 + + = 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 + + = CH 12 Feedback 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 2 3 1 + + + + W/O Feedback Large Difference With Feedback Small Difference CH 12 Feedback 19 Linearization Before feedback After feedback CH 12 Feedback 20 Four Types of Amplifiers CH 12 Feedback 21 Ideal Models of the Four Amplifier Types CH 12 Feedback 22 Realistic Models of the Four Amplifier Types CH 12 Feedback 23 Examples of the Four Amplifier Types CH 12 Feedback...
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This note was uploaded on 04/05/2010 for the course EE EE taught by Professor Ee during the Spring '10 term at Korea Advanced Institute of Science and Technology.

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ch12 - 1 Chapter 12 Feedback 12.1 General Considerations...

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