# Lect_14 - ECE442 SolidStateDevices&Circuits 14.Feedback...

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1 Jose E. Schutt Aine ECE 442 ECE 442 Solid State Devices & Circuits 14. Feedback Jose E. Schutt-Aine Electrical & Computer Engineering University of Illinois [email protected]

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2 Jose E. Schutt Aine ECE 442 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
3 Jose E. Schutt Aine ECE 442 Feedback – Basic Concepts

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4 Jose E. Schutt Aine ECE 442 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
5 Jose E. Schutt Aine ECE 442 () 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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6 Jose E. Schutt Aine ECE 442 () 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,
7 Jose E. Schutt Aine ECE 442 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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8 Jose E. Schutt Aine ECE 442 Series-Shunt Feedback Voltage mixing-voltage sampling feedback
9 Jose E. Schutt Aine ECE 442 Shunt- Series Feedback Current mixing-current sampling feedback

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10 Jose E. Schutt Aine ECE 442 Series-Series Feedback Voltage mixing-current sampling feedback
11 Jose E. Schutt Aine ECE 442 Shunt-Shunt Feedback Current mixing-voltage sampling feedback

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12 Jose E. Schutt Aine ECE 442 Transfer Function Representation Use a two-terminal representation of system for input and output
13 Jose E. Schutt Aine ECE 442 Y-parameter Representation 11 1 1 1 2 2 22 1 1 2 2 2 I yV yV I yV = + =+

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14 Jose E. Schutt Aine ECE 442 Y Parameter Calculations 22 12 11 21 11 00 VV II yy == To make V 2 = 0 , place a short at port 2
15 Jose E. Schutt Aine ECE 442 Z Parameters 11 1 1 1 2 2 22 1 1 2 2 2 Vz Iz I I z I = + =+

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16 Jose E. Schutt Aine ECE 442 Z-parameter Calculations 22 12 11 21 11 00 II VV zz == To make I 2 = 0 , place an open at port 2
17 Jose E. Schutt Aine ECE 442 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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18 Jose E. Schutt Aine ECE 442 H Parameter Calculations To make V 2 = 0 , place a short at port 2 22 12 11 21 11 00 VV VI hh II ==
19 Jose E. Schutt Aine ECE 442 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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20 Jose E. Schutt Aine ECE 442 G-Parameter Calculations 22 12 11 21 11 00 II IV gg VV == To make I 2 = 0 , place an open at port 2
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Lect_14 - ECE442 SolidStateDevices&Circuits 14.Feedback...

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