# Lect_16 - ECE442 SolidStateDevices&Circuits 16.Feedback...

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ECE 342–Jose Schutt Aine 1 ECE 442 Solid State Devices & Circuits 16. Feedback Jose E. Schutt-Aine Electrical & Computer Engineering University of Illinois [email protected]

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ECE 342–Jose Schutt Aine 2 Why Use Feedback? 1. To desensitize the gain 2. To reduce nonlinear distortion 3. To reduce the effect of noise 4. To control terminal impedances 5. To increase the bandwidth Feedback
ECE 342–Jose Schutt Aine 3 Feedback – Basic Concepts

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ECE 342–Jose Schutt Aine 4 out ia v A v = out in v G v = out out in ia vv GA === 0 ia in in v =− = The ideal amplifier gain is defined by The overall gain of the feedback circuit is defined by If input 2 is grounded We then get This is the open-loop gain Feedback – Analysis
ECE 342–Jose Schutt Aine 5 () out in out in vA v v G v β =− = 1 == + out in v A G ia in out vv v When the switch is closed, then Feedback – Analysis so that from which we get The closed-loop gain is always less than the open- loop gain for negative feedback which is the closed-loop gain

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ECE 342–Jose Schutt Aine 6 () 1/ ω = + M H A As s The high-frequency response of an amplifier (single-pole) is given by: Feedback – Bandwidth Extension A M is the midband gain and H is the upper 3-dB frequency. With negative feedback, we get 1( ) β = + f /(1 ) 1/( 1 ) + = ++ MM f HM AA sA After substitution,
ECE 342–Jose Schutt Aine 7 The feedback amplifier will have a midband gain of Feedback – Bandwidth Extension and an upper 3-dB frequency of (1 ) β + M M A A ) ω + HM A Bandwidth is increased by factor equal to amount of feedback. It can also be shown that the lower 3dB frequency is ) + L M A The gain-bandwidth product is constant

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ECE 342–Jose Schutt Aine 8 Series-Shunt Feedback Voltage mixing-voltage sampling feedback
ECE 342–Jose Schutt Aine 9 Shunt- Series Feedback Current mixing-current sampling feedback

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ECE 342–Jose Schutt Aine 10 Series-Series Feedback Voltage mixing-current sampling feedback
ECE 342–Jose Schutt Aine 11 Shunt-Shunt Feedback Current mixing-voltage sampling feedback

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ECE 342–Jose Schutt Aine 12 Transfer Function Representation Use a two-terminal representation of system for input and output
ECE 342–Jose Schutt Aine 13 Y-parameter Representation 11 1 1 1 2 2 22 1 1 2 2 2 I yV yV I yV = + =+

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ECE 342–Jose Schutt Aine 14 Y Parameter Calculations 22 12 11 21 11 00 VV II yy == To make V 2 = 0 , place a short at port 2
ECE 342–Jose Schutt Aine 15 Z Parameters 11 1 1 1 2 2 22 1 1 2 2 2 Vz Iz I I z I = + =+

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ECE 342–Jose Schutt Aine 16 Z-parameter Calculations 22 12 11 21 11 00 II VV zz == To make I 2 = 0 , place an open at port 2
ECE 342–Jose Schutt Aine 17 H Parameters 11 1 1 1 2 2 22 1 1 2 2 2 Vh Ih V I hI h V = + =+

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ECE 342–Jose Schutt Aine 18 H Parameter Calculations To make V 2 = 0 , place a short at port 2 22 12 11 21 11 00 VV VI hh II ==
ECE 342–Jose Schutt Aine 19 G Parameters 11 1 1 1 2 2 22 1 1 2 2 2 I gV g I Vg V g I = + =+

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ECE 342–Jose Schutt Aine 20 G-Parameter Calculations 22 12 11 21 11 00 II IV gg VV == To make I 2 = 0 , place an open at port 2
ECE 342–Jose Schutt Aine 21 Series-Shunt Feedback - Ideal 1 o f s V A A VA β ≡= + Negative feedback decreases the gain o i V A V

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Lect_16 - ECE442 SolidStateDevices&Circuits 16.Feedback...

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