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Unformatted text preview: 1 ECSE334 – Introduction to Microelectronics ECSE 334 Introduction to Microelectronics Roni Khazaka Lecture #15 ECSE334 – Introduction to Microelectronics 2 Khazaka  2008 Feedback • Feedback a very powerful mechanism for precise control. • Feedback exists in nature in many physical and biological systems. • The theory of negative feedback was developed by electronics engineers 2 ECSE334 – Introduction to Microelectronics 3 Khazaka  2008 Advantages of Feedback in Amplifiers • Desensitize the Gain to variations in circuit components. • Reduce nonlinear distortion. • Reduce the effect of noise. • Control the input and output impedances. • Extend the bandwidth of the amplifier Tradeoff: Reduction in Gain ECSE334 – Introduction to Microelectronics 4 Khazaka  2008 Signal Flow Diagram Σ + − Source Load A s x o x o x 2 x s o Ax x = 1 x 2 1 x x x o − = • Each variable is either a voltage or a current. • No loading effect. 3 ECSE334 – Introduction to Microelectronics 5 Khazaka  2008 Negative Feedback System Σ + − Source Load A β s x i x o x f x • Output is fed back into the input with a feedback factor of β. • The feedback is subtracted from the input signal Î negative feedback. Negative Feedback ECSE334 – Introduction to Microelectronics 6 Khazaka  2008 Negative Feedback System Σ + − Source Load A β s x i x o x f x i o Ax x = o f x x β = f s i x x x − = ( ) ( ) o s f s o x x A x x A x β − = − = s o o Ax x A x = + β s o x A A x β + = 1 4 ECSE334 – Introduction to Microelectronics 7 Khazaka  2008 Negative Feedback System Σ + − Source Load A β s x i x o x f x β A A x x A s o f + = ≡ 1 ( ) β A + 1 Amount of feedback β A Loop Gain / Open Loop Gain (should be positive) Closed loop gain ECSE334 – Introduction to Microelectronics 8 Khazaka  2008 Negative Feedback System Σ + − Source Load A β s x i x o x f x β A A x x A s o f + = ≡ 1 1 >> β A The gain is determined by the feed back network only! (as long as A is large) β 1 = f A 5 ECSE334 – Introduction to Microelectronics 9 Khazaka  2008 Negative Feedback System Σ + − Source Load A β s x i x o x f x s o x A A x β + = 1 s s o f x x A A x x ≅ + = = β β β 1 Tracking behaviour. x i often referred to as error signal x e ....
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This note was uploaded on 01/27/2011 for the course ECSE 334 taught by Professor Anashamoui during the Spring '10 term at McGill.
 Spring '10
 ANASHAMOUI

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