stabilityandcompensation - (Chapter5 ECE412 (Fall2007...

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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
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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%
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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.
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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 !
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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
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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. 1. The loop gain is large enough to allow signal build up.
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