Chapter_8_-_Feedback_2011

Chapter_8_-_Feedback_2011 - Faculty of Science and...

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Unformatted text preview: Faculty of Science and Technology Department of Electrical and Computer Engineering Chapter 8 | Feedback Chapter 8 | Feedback Chapter 8 | Feedback ELEC321 Applied Electronics III by Prof. K. W. Tam | kentam@umac.mo hapter Outline s Why Feedback? s Objectives s Feedback Structure and Properties s The Four Basic Feedback Topologies s Two-Port Network s Feedback Amplifiers s Stability Problem hy Feedback? hy Feedback? hy Feedback? hy Feedback? asic Feedback Structure Σ +- χ s χ i χ f χ o f s i x x x- = i o Ax x = o f x x β = Source Source A A β β Load Load hy Feedback? s No control mechanism for different circuitry parameters like gain, input/output impedance and so forth! mplifier without Feedback hy Feedback? mplifier with Feedback i i R 2 R 2 R 1 R 1 1 2 v v- 1 2 R R in = ( ) 1 2 1 2 R R v v v o =- Trade off between Input Resistance and Gain s Electronic circuit is thus easily designed for different application! hy Feedback? mplifier with Feedback ( ) [ ] ( ) [ ] L C m in C m in s in sig o R R g R R g R R R v v // // 2 2 2 1 1 1 1-- + ≈ Complicated enough for design and analysis in practice Scenario 2 Scenario 1 hy Feedback? Automatically lowers the microphone preamplifier gain and prevents voice signal clipping Receiving end hears a clipped and distorted signal With AGC Without AGC s Compensation the significant variations of input level and maintains a fixed level during recording or playback for microphone pplication of Feedback Automatic Gain Control (AGC) s A user talks too softly or too far away from the microphone. s A user speaks too loud to the microphone Increase the gain of microphone preamplifier automatically to a level that receiving end can is heard clearly Low-level voice signal may not be heard clearly at the receiving end With AGC Without AGC hy Feedback? s Audio electronic system such as radios, tape players, and CD players often provide separate VOLUME CONTROLS labeled “TREBLE” and “BASS”. These controls permit the user to select the volume of high frequency audio signals (“treble”) independent of the volume of low frequency audio signals (“bass”). pplication of Feedback s The audio range consists of signals having frequencies from 20 Hz to 20 kHz. The bass range includes frequencies up to 300 Hz. s If the gain is positive a signal in the bass range is amplified or boosted. If the gain is negative the signal is attenuated or cut. bjectives bjectives bjectives bjectives s Objectives s Analysis – classify the type of feedback and its parameters etc. s Design – use feedback parameters to meet the design specification s Feedback is to trade off gain for other desirable properties eedback Structure and roperties eedback Structure and eedback Structure and roperties roperties eedback Structure and Properties Σ +- χ s χ i χ f χ o f s i x x x- = i o Ax x = o f x x β = Source Source A A β β Load Load β A A x x A s o f + = ≡ 1 b Closed-loop gain A f b β – feedback factor b A β – loop gain b...
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This note was uploaded on 02/27/2012 for the course ELEC 232 taught by Professor Terry during the Spring '12 term at University of Macau.

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Chapter_8_-_Feedback_2011 - Faculty of Science and...

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