stabilityandcompensation

stabilityandcompensation - Adaptive Integrated Microsystems...

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Unformatted text preview: Adaptive Integrated Microsystems Stability and Frequency compensation (Chapter 5) ECE 412 Introduction to mixed-signal circuits (Fall 2007) Class website: http://www.egr.msu.edu/classes/ece412/shantanu Adaptive Integrated Microsystems Feedback Even though we have applied small signal model for calculating gain of an amplifier, CMOS amplifier is inherently non-linear. Small signal gain (slope) varies with input voltage in V out V in V 1 Q 2 Q ref V b I 2 s V dd V dd V 3 Q 4 Q out V L C X V ref V Variation in gain could be more than 100% Adaptive Integrated Microsystems Feedback Open loop amplifiers are seldom used for precise amplification. - 1 R + 2 R out V ref V in V X V 2 1 R V V R V V out X X in- =- ) ( X ref out V V A V- = )] 1 ( 1 1 [ 1 2 1 2 R R A V R R V in out + + ∆- = ∆ ) 1 ( 1 2 R R A + in out V R R V ∆- = ∆ 1 2 Easy to design amplifiers with high gain (precision not required) Linearity and precision determined by ratio of resistors. The amplifier tries to fix the voltage Vx WITHOUT DRAWING ANY CURRENT. Adaptive Integrated Microsystems Feedback Drawbacks: Even though open loop amplifiers can be stable, feedback can introduce instability and induce oscillations. in out V R R V ∆- = ∆ 1 2 No oscillations No oscillations Sustained oscillations Why ? (Phase delay between input and output) Phase response important ! Adaptive Integrated Microsystems Analysis of feedback network Amplifier Feedback For oscillations/instability a 180 degrees phase shift is required at the negative feedback terminal. The feedback signal adds in phase with the input signal (could be noise) and leads to oscillation build up. - + out V in V Adaptive Integrated Microsystems Barkhausen’s Criteria The negative feedback system may oscillate at a frequency if 1. Phase shift around the loop at this frequency becomes less than -180 degrees....
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This note was uploaded on 03/01/2010 for the course ECE 412 taught by Professor - during the Fall '08 term at Michigan State University.

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stabilityandcompensation - Adaptive Integrated Microsystems...

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