Basic Amplifiers and Basic Amplifiers and

Differential Amplifier Differential Amplifier

CSE 577 Spring 2011

Insoo Kim, Kyusun Choi

Mixed Signal CHIP Design Lab.

Department of Computer Science & Engineering

The Penn State University

In today’s analog design, simulation of circuits is essential

because the behavior of short-channel MOSFETs cannot be

predicted accurately by hand calculations. Nonetheless, if the

designer avoids a simple and intuitive analysis of the circuits

and hence skips the task of gaining insight, then he/she

Don’t let the computer think for youDon’t let the computer think for you

Insoo Kim2/22/2011

and hence skips the task of gaining insight, then he/she

cannot interpret the simulation results intelligently. For this

reason, we say, “Don’t let the computer think for you.”“Don’t let the computer think for you.”

- Behzad Razavi

ContentsContents

� Fundamentals

� Basic Amplifiers: Low Frequency Analysis

� Basic Amplifiers: High Frequency Analysis

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� Differential Amplifier

� Feedback

FundamentalsFundamentals

� Definitions

� DC Operating Point & Load line

� Large Signal Analysis vs. Small Signal Analysis

� MOSFET intrinsic Capacitances

DefinitionsDefinitions

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mbmbmbmb

DC Operating Point & Load LineDC Operating Point & Load Line

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Large Signal Analysis vs. Small Signal AnalysisLarge Signal Analysis vs. Small Signal Analysis

� Large Signal Analysis

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Large Signal Analysis vs. Small Signal AnalysisLarge Signal Analysis vs. Small Signal Analysis

� Small Signal Analysis

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How convenient !! How convenient !! How convenient !! How convenient !! How convenient !! How convenient !! How convenient !! How convenient !!

MOSFET Intrinsic CapacitancesMOSFET Intrinsic Capacitances

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(cont’d) MOSFET Intrinsic Capacitances(cont’d) MOSFET Intrinsic Capacitances

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Basic Amplifiers: Low Frequency Basic Amplifiers: Low Frequency

AnalysisAnalysis

� Single Stage Amplifiers

� Multi Stage Amplifiers

Single Stage Amplifiers: CS, CD, and CG StageSingle Stage Amplifiers: CS, CD, and CG Stage

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Common Source Stage : Voltage GainCommon Source Stage : Voltage Gain

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Common Drain Stage: Output ResistanceCommon Drain Stage: Output Resistance

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Common Gate Stage : Input ResistanceCommon Gate Stage : Input Resistance

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SummarySummary

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QuizQuiz

� CD stage amplifier is suitable for output stage of OPAmp

due to its low output impedance and large bandwidth.

However, in CMOS analog IC, CS stage is more widely

used for output stage OPAmp than CD stage. Why?

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Loads for basic amplifiersLoads for basic amplifiers

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(cont’d) Loads for basic amplifiers(cont’d) Loads for basic amplifiers

� Diode Connected Load

mmbm

o

mbmombmX

X

ggg

rggrggI

V

11

||11

1

≈+

≈

+=

++=

−

1

1

ggA mv −=

XR

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2

1

2

1

)/(

)/(

LW

LW

gmmv

−≈

(cont’d) Loads for basic amplifiers(cont’d) Loads for basic amplifiers

� Source degeneration

Sm

mm

Rg

gG

+≈1

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]1)([

]1)[(

22

22

++⋅⋅≈

+++⋅=

mbmSo

oombmSout

ggRr

rrggRR

Cascode StageCascode Stage

� Small Signal Analysis

)||(

)||( 1 inmDoutout

RRgA

VgRRV

⋅=

⋅=

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� Rout

)||(1 Doutmv RRgA ⋅=

]1)([

]1)[(

2212

22221

++⋅⋅≈

+++⋅=

mbmoo

oombmoout

ggrr

rrggrR

Folded Cascode StageFolded Cascode Stage

om1

R||RR

RgA

=

=

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( )[ ] [ ]

( )

L

Lm1Dt

oLD

o3o4Cm4Co7o2o2Cm2C

o4Co2Co

2I/CSR

/CgA

RC/1

rrg||r||rrg

R||RR

=

==

=

=

=

ωω

ω

(cont’d) Folded Cascode Stage(cont’d) Folded Cascode Stage

� What are the advantages of folded cascode amplifier?

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� Disadvantages:

� Limited Output swing

� Large Voltage Headroom

� Large Power Consumption

Basic Amplifiers: High Frequency Basic Amplifiers: High Frequency

AnalysisAnalysis

� Frequency Analysis

� Dominant Pole Approach

Frequency AnalysisFrequency Analysis

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(cont’d) Frequency Analysis(cont’d) Frequency Analysis

� Bode Plot

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Dominant Pole ApproachDominant Pole Approach

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BW Estimation by Dominant Pole ApproachBW Estimation by Dominant Pole Approach

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Bandwidth ComparisonBandwidth Comparison

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QuizQuiz

� Design an amplifier which satisfy following features using

basic single-stage amplifiers.

� High gain

� Large Bandwidth

� High input impedance

� Low output impedance

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Differential AmplifierDifferential Amplifier

� Single Stage Amplifiers

� Multi Stage Amplifiers

Why differential Amplifier?Why differential Amplifier?

� Single Ended Signal can be easily contaminated

� A Differential Signal can be cleaned up

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� Power Supply noise can be reduced

Differential Amplifier AnalysisDifferential Amplifier Analysis

� Classic Diff Amp

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(cont’d) Differential Amplifier Analysis(cont’d) Differential Amplifier Analysis

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Diff Amp with Current Mirror LoadDiff Amp with Current Mirror Load

)||(

||

424,2

42

4,2

oomv

ooout

mm

rrgA

rrR

gG

⋅=

≈

=

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Common Mode Input Voltage Range

VSS+VTN1+VDSAT5+VDSAT1 < VIC < VDD–|VDSAT3|–|VTP3|+| VTN1|

)||()(

)||()2(

313

31351

oom

oomom

rrgloadRCMRR

rrgrgCMRR

×=

⋅=

1. What is CM Input Voltage?1. What is CM Input Voltage?1. What is CM Input Voltage?1. What is CM Input Voltage?

2. How do we prove this equation?2. How do we prove this equation?2. How do we prove this equation?2. How do we prove this equation?

(Std. Library) Design Exercise(Std. Library) Design Exercise

� Design Flow

� Determine Specifications

− Power Consumption (ex. 1mW)

− Voltage Gain (ex. >30)

− Active Common Mode Input range (as large as possible)

− Others: slew rate, CMRR, PSRR, etc.

� Determine minimum channel length

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� Determine channel width

− Determine W1,2 from voltage gain spec.

− Determine W5 & Bias Voltage from power consumption & CM min.

− Determine W3,4 from CM max.

� Determine Bias Level of current source tr.

� Check other specifications

FeedbackFeedback

� Feedback & Stability

� Voltage Amplifier Model

� Common Mode Feedback

Feedback & StabilityFeedback & Stability

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Voltage Amplifier ModelVoltage Amplifier Model

� Models

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(cont’d) Voltage Amplifier Model(cont’d) Voltage Amplifier Model

� 1st Order Model

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(cont’d) Voltage Amplifier Model(cont’d) Voltage Amplifier Model

� 2nd Order Model

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(cont’d) Voltage Amplifier Model(cont’d) Voltage Amplifier Model

� Time Response of the 2nd Order Model

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(cont’d) Voltage Amplifier Model(cont’d) Voltage Amplifier Model

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Feedback CharacteristicsFeedback Characteristics

� Gain desensitization

Band width extension

A

A

A1

1

A

A

A)(1

AA

A1

A

x

xA

f

f

2f

s

of

dd

dd

β

β

β

+=

+=

+=≡

� Noise Reduction

2

n

s

21

1n

21

21so

AV

V

N

S

AA1

AV

AA1

AAVV

=

++

+=

ββ

n

s

V

V

N

S=

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� Band width extension

)A1(/s1

)A1/(A

A(s)1

A(s))s(A

s/1

AA(s)

MH

MM

f

H

M

βω

β

β

ω

++

+=

+=

+=

� Non-linearity Reduction

(a)(a)(a)(a) w/o feedbackw/o feedbackw/o feedbackw/o feedback

(b)(b)(b)(b) w feedbackw feedbackw feedbackw feedback

Common Mode FeedbackCommon Mode Feedback

� Why is CMFB circuit needed?

� Due to TR mismatch, TRs may not be in saturation region at

operating point. � DM Gain decreases and CM gain increases

� Since output CM level is sensitive to device properties and

mismatches, it cannot be stabilized by means of differential

feedback.

� General Topology of CMFB Circuit

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(cont’d) Common Mode Feedback(cont’d) Common Mode Feedback

� Examples of CMFB

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Useful for low gain applicationsUseful for low gain applicationsUseful for low gain applicationsUseful for low gain applicationsFolded cascode amplifier with CMFBFolded cascode amplifier with CMFBFolded cascode amplifier with CMFBFolded cascode amplifier with CMFB

)||||( 4,32,12,1 FOOmv RrrgA ⋅=

ReferencesReferences

� Joongho Choi, “CMOS analog IC Design,” IDEC Lecture

Note, Mar. 1999.

� B. Razavi, “Design of Analog CMOS Integrated Circuits,”

McGraw-Hill, 2001.

� Hongjun Park, “CMOS Analog Integrated Circuits

Design,” Sigma Press, 1999.

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