# feedback-2p - ELE 311 Electronics II...

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1 ELE 311 Electronics II http://www.ee.hacettepe.edu.tr/~usezen/ele311/ • Millman and Halkias, Integrated Electronics , McGraw-Hill • Millman and Grabel, Microelectronics , McGraw- Hill • Boylestad and Nashelsky, Electronic Devices & Circuit Theory , Prentice Hall, 8th ed . • Schilling and Belove, Electronic Circuits: Discrete and Integrated , McGraw-Hill Textbooks

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2 Contents • Feedback concept and feedback amplifiers • Multistage amplifiers • Differential Amplifiers • Operational Amplifiers • Power amplifiers • Positive feedback and oscillators Feedback and Feedback Amplifiers
3 Feedback Concepts The effect of negative feedback on an amplifier: Disadvantage: •Lowergain Advantages: • Improved input impedance • More stable gain • Improved frequency response • Improved output impedance • Reduced distortion • More linear operation Type of Amplifiers used in negative feedback 1. Voltage Gain Amplifier voltage input voltage output 2. Transresistance Amplifier current input voltage output 3. Transconductance Amplifier voltage input current output 4. Current Gain Amplifier current input current output

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4 1. Voltage Gain Amplifier = = L R i o v v v A o L L v s i i s o V R R R A R R R v v A + + = = 2. Transresistance Amplifier = = L R i o m i v R o L L m i s s s o M R R R R R R R i v R + + = =
5 3. Transconductance Amplifier = = L R i o m v i G L o o m s i i s o M R R R G R R R v i G + + = = 2 i 4. Current Gain Amplifier = = L R s o i i i A L o o i i s s s o I R R R A R R R i i A + + = =

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6 Negative Feedback Gain with feedback i AX Y = Y X f β = f i X X X = A A X Y A f + = = 1 1 >> A 1 f A If then so is desired 1 Negative Feedback Improvements - Gain Stability A A A f + = 1 () 2 1 1 A dA dA f + = A A A A + + = 1 1 1 A A A f + = 1 1 A dA A A dA f f + = 1 1 Ex: Assume a system where open-loop gain equals A = 1000 and changes %20 due to the temperature changes. Consider a negative feedback with = 0.1 and calculate the change for A f . % . % . 2 0 20 1000 1 0 1 1 = × + = f f A dA A Y f X i X Y
7 D AX Y + = Y X X i β = A D X A A Y + + + = 1 1 D : distortion in the open-loop system D AX Y i + = D Y X A Y + = ) ( Negative Feedback Improvements - Reduced Distortion Negative Feedback Improvements - Bandwith Improvement f f j A f A L m L + = 1 ) ( Lowpass A L ( f ) : lowpass gain, A m : midband gain, f L : lower cut-off frequency Open-loop A fL ( f ) : lowpass gain, A fm : midband gain, f fL : lower cut-off frequency Closed-loop ) ( ) ( ) ( f A f

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feedback-2p - ELE 311 Electronics II...

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