+ All Categories
Home > Documents > Power Amplifier Linearization Techniques: An Overviewsmirc.stanford.edu/papers/isscc01s-joel.pdf ·...

Power Amplifier Linearization Techniques: An Overviewsmirc.stanford.edu/papers/isscc01s-joel.pdf ·...

Date post: 05-Apr-2018
Category:
Upload: dinhmien
View: 219 times
Download: 1 times
Share this document with a friend
27
Power Amplifier Linearization Techniques: An Overview Workshop on RF Circuits for 2.5G and 3G Wireless Systems February 4, 2001 Joel L. Dawson Ph.D. Candidate, Stanford University
Transcript
Page 1: Power Amplifier Linearization Techniques: An Overviewsmirc.stanford.edu/papers/isscc01s-joel.pdf · Power Amplifier Linearization Techniques: An Overview ... • Depends on having

Power AmplifierLinearization Techniques:

An Overview

Workshop on RF Circuits for 2.5Gand 3G Wireless Systems

February 4, 2001

Joel L. Dawson

Ph.D. Candidate, Stanford University

Page 2: Power Amplifier Linearization Techniques: An Overviewsmirc.stanford.edu/papers/isscc01s-joel.pdf · Power Amplifier Linearization Techniques: An Overview ... • Depends on having

List of Topics

• Motivation for using linearization

• Linearization as a theoretical problem

• Survey of commonly employed techniques

Page 3: Power Amplifier Linearization Techniques: An Overviewsmirc.stanford.edu/papers/isscc01s-joel.pdf · Power Amplifier Linearization Techniques: An Overview ... • Depends on having

Linearity vs. Power Efficiency

Power efficiencyBattery lifetimeThermal management

LinearitySophisticatedModulationTechniques

Spectral efficiency

Page 4: Power Amplifier Linearization Techniques: An Overviewsmirc.stanford.edu/papers/isscc01s-joel.pdf · Power Amplifier Linearization Techniques: An Overview ... • Depends on having

PA Tradeoffs

• Power Efficiency => Switching PA’s(class D, E, F)

• Linearity => Class A, AB, B, C.

Page 5: Power Amplifier Linearization Techniques: An Overviewsmirc.stanford.edu/papers/isscc01s-joel.pdf · Power Amplifier Linearization Techniques: An Overview ... • Depends on having

Can nonlinear system theoryhelp?

Theory of time-invariant (or, “stationary”) nonlinearsystems is well-developed.

Volterra series, Weiner systems,Hammerstein systems...

Problem: Intensely formal, and usually lacks the conceptual clarity that leads to design insight.

Page 6: Power Amplifier Linearization Techniques: An Overviewsmirc.stanford.edu/papers/isscc01s-joel.pdf · Power Amplifier Linearization Techniques: An Overview ... • Depends on having

Example: 3rd order nonlinearityin a two-tone experiment

The input tones:tjtjtjtj eeee 2211 ,,, ωωωω ββαα −−

tt 21 cos2cos2 ωβωα +

Output sinusoids at: 3ω1, 3ω2, 2ω1-ω2, 2ω2-ω1, 2ω1+ω2, 2ω2+ω1

H3(s1,s2,s3)

An output product: ( )tjejjjH 2122113

2 ),,( ωωωωωβα −−

Page 7: Power Amplifier Linearization Techniques: An Overviewsmirc.stanford.edu/papers/isscc01s-joel.pdf · Power Amplifier Linearization Techniques: An Overview ... • Depends on having

So where does the theory leaveus?

• Odd order nonlinearities cause distortion productsthat are in-band.

• Outrageous behavior in the lab may imply that thedevice under test is not time-invariant.

Page 8: Power Amplifier Linearization Techniques: An Overviewsmirc.stanford.edu/papers/isscc01s-joel.pdf · Power Amplifier Linearization Techniques: An Overview ... • Depends on having

Design implications:

• An important measure of the strength of agiven linearization method is its robustnessto poor characterization of the PA.

• Design, if possible, to “force” PA to behaveas a benign nonlinear system (e.g., look forways to preserve time-invariance).

Page 9: Power Amplifier Linearization Techniques: An Overviewsmirc.stanford.edu/papers/isscc01s-joel.pdf · Power Amplifier Linearization Techniques: An Overview ... • Depends on having

Technique I: Power Backoff

Basic principle: ∑∞

==+++=

0

2210)(

n

nno xaxaxaaxV L

For “small” inputs,this term is dominant.

RFC

CB RL

IBIAS

iS

|IBIAS| >> |iS|

Low efficiency

Page 10: Power Amplifier Linearization Techniques: An Overviewsmirc.stanford.edu/papers/isscc01s-joel.pdf · Power Amplifier Linearization Techniques: An Overview ... • Depends on having

Power Backoff, cont.

Outputpower(dB)

Input Power(dB)

First-orderoutput

Higher-orderIM term

Page 11: Power Amplifier Linearization Techniques: An Overviewsmirc.stanford.edu/papers/isscc01s-joel.pdf · Power Amplifier Linearization Techniques: An Overview ... • Depends on having

Corrective distortion:predistortion and/or postdistortion

( )•1F ( )•2F( )•H

Most general problem: choose F1 and F2 such thatF2(H(F1(x))) is a linear function of the input variable x.

x

(Power amplifier)

Page 12: Power Amplifier Linearization Techniques: An Overviewsmirc.stanford.edu/papers/isscc01s-joel.pdf · Power Amplifier Linearization Techniques: An Overview ... • Depends on having

Technique II: Predistortion

PA( )•AF

LO

BasebandData

PA( )•DF

LO

BasebandData

Page 13: Power Amplifier Linearization Techniques: An Overviewsmirc.stanford.edu/papers/isscc01s-joel.pdf · Power Amplifier Linearization Techniques: An Overview ... • Depends on having

Key design issues for a predistorter:

• In analog case, how to realize thepredistortion function.

• Initial calibration or training.

• Sensitive to drift.

Page 14: Power Amplifier Linearization Techniques: An Overviewsmirc.stanford.edu/papers/isscc01s-joel.pdf · Power Amplifier Linearization Techniques: An Overview ... • Depends on having

Technique III: AdaptivePredistortion

PAPredistorter

System Estimator

Σ

Σ

x(t)

esys(t)

esym(t)

-

-

esys(t)

esym(t)esys(t)

Page 15: Power Amplifier Linearization Techniques: An Overviewsmirc.stanford.edu/papers/isscc01s-joel.pdf · Power Amplifier Linearization Techniques: An Overview ... • Depends on having

Adaptive Predistortion:advantages

• Does not suffer the bandwidth limitationincurred by continuous-feedbacktechniques.

• Solves the problem of drift sensitivity.

Page 16: Power Amplifier Linearization Techniques: An Overviewsmirc.stanford.edu/papers/isscc01s-joel.pdf · Power Amplifier Linearization Techniques: An Overview ... • Depends on having

Adaptive Predistortion: designissues

• New, discrete-time feedback stabilityproblem associated with model estimation.

• Depends on having a good power amplifiermodel.

• Complexity: incurs power overhead of aDSP chip.

Page 17: Power Amplifier Linearization Techniques: An Overviewsmirc.stanford.edu/papers/isscc01s-joel.pdf · Power Amplifier Linearization Techniques: An Overview ... • Depends on having

Technique IV: FeedforwardLinearization

PA

Gain = A0

delay #1

delay #2

Σ

Σ

1/A0

A0

-

Vin

Perfectlylinear

Vout

Page 18: Power Amplifier Linearization Techniques: An Overviewsmirc.stanford.edu/papers/isscc01s-joel.pdf · Power Amplifier Linearization Techniques: An Overview ... • Depends on having

Feedforward: stable, but...

• Matching delay lines, amplifier gains nottrivial.

• Susceptible to drift and aging.

• Low-loss delay lines, summations critical.

Page 19: Power Amplifier Linearization Techniques: An Overviewsmirc.stanford.edu/papers/isscc01s-joel.pdf · Power Amplifier Linearization Techniques: An Overview ... • Depends on having

Technique V: EnvelopeElimination and Restoration

EnvelopeDetector

Limiter PAVin

SwitchingPA

Page 20: Power Amplifier Linearization Techniques: An Overviewsmirc.stanford.edu/papers/isscc01s-joel.pdf · Power Amplifier Linearization Techniques: An Overview ... • Depends on having

EER design issues

• Phase matching between the two signalpaths critical.

• Polar feedback a possibility.

• Restoring the envelope in a power-efficientway is very challenging.

Page 21: Power Amplifier Linearization Techniques: An Overviewsmirc.stanford.edu/papers/isscc01s-joel.pdf · Power Amplifier Linearization Techniques: An Overview ... • Depends on having

Technique VI: LINC

LInear amplification with Nonlinear Components

SignalSeparator

PA1

PA2

ΣSwitchingPA’s

)]()(sin[5.0)]()(sin[5.0)](cos[)( 00 tttVtttVttta ccc θφωθφωφω ++−++=+

= −

0

1 )(sin)( Vtatθ

Constant-amplitude terms

Page 22: Power Amplifier Linearization Techniques: An Overviewsmirc.stanford.edu/papers/isscc01s-joel.pdf · Power Amplifier Linearization Techniques: An Overview ... • Depends on having

LINC design issues

• Signal separation complicated, but possible.

• Good power combining with low loss andhigh isolation is the key barrier.

Page 23: Power Amplifier Linearization Techniques: An Overviewsmirc.stanford.edu/papers/isscc01s-joel.pdf · Power Amplifier Linearization Techniques: An Overview ... • Depends on having

Technique VII: CartesianFeedback

Σ H(s)

Σ H(s)PA

sinωt

cosωt

sinωt

cosωt

-

-

I

Q

Page 24: Power Amplifier Linearization Techniques: An Overviewsmirc.stanford.edu/papers/isscc01s-joel.pdf · Power Amplifier Linearization Techniques: An Overview ... • Depends on having

Major design consideration in CFBsystems: phase alignment

PA

sinωt

cosωt

sin(ωt+φ)

cos(ωt+φ)

Leff = H(s)cosφ +[H(s)sinφ]2 1+H(s)cosφ

Page 25: Power Amplifier Linearization Techniques: An Overviewsmirc.stanford.edu/papers/isscc01s-joel.pdf · Power Amplifier Linearization Techniques: An Overview ... • Depends on having

CFB Strengths andWeaknesses

• W: Bandwidth limitation

• W: Stability concerns

• S: Low-complexity

• S: Highly resistant to drift and aging

• SS: Robust to poor characterization of PA

Page 26: Power Amplifier Linearization Techniques: An Overviewsmirc.stanford.edu/papers/isscc01s-joel.pdf · Power Amplifier Linearization Techniques: An Overview ... • Depends on having

Summary

• Power Backoff: simplicity; low power efficiency.

• Predistortion: conceptually clear; requires good PA model.

• Adaptive Predistortion: No drift problem; introduces complexity.

• Feedforward: No stability worries; matching and drift concerns.

• EER: Possibly high-efficiency; adds another “power amp” problem.

• LINC: Conceptually appealing; fundamental implementation issues.

• CFB: simplicity, robust to poor PA model; stability concerns.

Page 27: Power Amplifier Linearization Techniques: An Overviewsmirc.stanford.edu/papers/isscc01s-joel.pdf · Power Amplifier Linearization Techniques: An Overview ... • Depends on having

Copies of these slides...

http://smirc.stanford.edu

Available at the conclusion of ISSCC 2001.


Recommended