Lect2UP220_(100327)

Lect2UP220_(100327) - Lecture 220 Compensation of Op Amps...

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Lecture 220 – Compensation of Op Amps (3/27/10) Page 220-1 CMOS Analog Circuit Design © P.E. Allen - 2010 LECTURE 220 – INTRODUCTION TO OP AMPS LECTURE OUTLINE Outline • Op Amps • Categorization of Op Amps • Compensation of Op Amps • Miller Compensation • Other Forms of Compensation • Op Amp Slew Rate • Summary CMOS Analog Circuit Design, 2 nd Edition Reference Pages 243-269 Lecture 220 – Compensation of Op Amps (3/27/10) Page 220-2 CMOS Analog Circuit Design © P.E. Allen - 2010 OP AMPS What is an Op Amp? The op amp (operational amplifier) is a high gain, dc coupled amplifier designed to be used with negative feedback to precisely define a closed loop transfer function. The basic requirements for an op amp: • Sufficiently large gain (the accuracy of the signal processing determines this) • Differential inputs • Frequency characteristics that permit stable operation when negative feedback is applied Other requirements: • High input impedance • Low output impedance • High speed/frequency
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Lecture 220 – Compensation of Op Amps (3/27/10) Page 220-3 CMOS Analog Circuit Design © P.E. Allen - 2010 Why Op Amps? The op amp is designed to be used with single-loop, negative feedback to accomplish precision signal processing as illustrated below. 060625-01 + Σ F ( s ) A ( s ) Op Amp Feedback Network V in ( s ) V out ( s ) A v ( s ) V out ( s ) V in ( s ) + F ( s ) V f ( s ) V f ( s ) Single-Loop Negative Feedback Network Op Amp Implementation of a Single-Loop Negative Feedback Network The voltage gain, V out ( s ) V in ( s ) , can be shown to be equal to, V out ( s ) V in ( s ) = A v ( s ) 1+ A v ( s ) F ( s ) If the product of A v ( s ) F ( s ) is much greater than 1, then the voltage gain becomes, V out ( s ) V in ( s ) ± 1 F ( s ) ± The precision of the voltage gain is defined by F ( s ). Lecture 220 – Compensation of Op Amps (3/27/10) Page 220-4 CMOS Analog Circuit Design © P.E. Allen - 2010 OP AMP CHARACTERIZATION Linear and Static Characterization of the CMOS Op Amp A model for a nonideal op amp that includes some of the linear, static nonidealities: 060625-03 + - v 2 v 1 v 1 CMRR V OS R icm R icm e n 2 C id R id R out v out Ideal Op Amp * C icm C icm where R id = differential input resistance C id = differential input capacitance R icm = common mode input resistance R icm = common mode input capacitance V OS = input-offset voltage CMRR = common-mode rejection ratio (when v 1 = v 2 an output results) e 2 n = voltage-noise spectral density (mean-square volts/Hertz)
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Lecture 220 – Compensation of Op Amps (3/27/10) Page 220-5 CMOS Analog Circuit Design © P.E. Allen - 2010 Linear and Dynamic Characteristics of the Op Amp Differential and common-mode frequency response: V out ( s ) = A v ( s )[ V 1 ( s ) - V 2 ( s )] ± A c ( s ) ± ² ² ³ ´ µ µ V 1 ( s )+ V 2 ( s ) 2 Differential-frequency response: A v ( s ) = A v 0 ± ² ² ³ ´ µ µ s p 1 - 1 ± ² ² ³ ´ µ µ s p 2 - 1 ± ² ² ³ ´ µ µ s p 3 - 1 ··· = A v 0 p 1 p 2 p 3 ··· ( s - p 1 )( s - p 2 )( s - p 3 )··· where p 1 , p 2 , p 3 ,··· are the poles of the differential-frequency response (ignoring zeros).
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Lect2UP220_(100327) - Lecture 220 Compensation of Op Amps...

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