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RF Power Amplifier Design

Markus Mayer & Holger ArthaberDepartment of Electrical Measurements and Circuit Design

Vienna University of Technology

June 11, 2001

2

Contents

Basic Amplifier ConceptsClass A, B, C, F, hHCALinearity AspectsAmplifier Example

Enhanced Amplifier ConceptsFeedback, Feedforward, ...PredistortionLINC, Doherty, EER, ...

3

Efficiency Definitions

Drain Efficiency:

Power Added Efficiency:

DC

OUTD P

P=η

−⋅=−=

GPPP

DDC

INOUTPA

11ηη

4

Ideal FET Input and Output Characteristics

0 VGS

IDS

Im

2VP VP VDSmaxVDDVKVDS0

V =VGS P

V =0GS

Ohmic Saturation Breakdown

gm

DD

KDD

VVV −

=κ

5

Maximum Output Power Match

0 VGS

IDS

Im

2VP VP VDSmaxVDDVKVDS0

V =VGS P

V =0GS

Ohmic Saturation Breakdown

gm

m

KDSOPT I

VVR −= max

6

Class A

0 VGS

IDS

Im

2VP VP VDSmaxVDDVKVDS0

VGS VDS

2p

p

Q

IDS

Im

0 2pp Q

7

Class A – Circuit

VDD

RLDGS

48%

dB) 14 (e.g.

50%

PA ⋅=

=

⋅=

κη

κη

A

D

GG

8

Class B

0 VGS

IDS

Im

2VP VP VDSmaxVDDVKVDS0

VGS VDS

2p

p

Q

IDS

Im

0 2pp Q

9

Class C

0 VGS

IDS

Im

2VP VP VDSmaxVDDVKVDS0

VGS VDS

2p

p

Q

IDS

Im

0 2pp Q

10

Class B and C – Circuit

VDD

RLDGS

f0

Class B Class C

%65

dB) (8 6dB-

%78

PA ⋅=

=

⋅=

κη

κη

A

D

GG

%0

1

%100

PA →

→

→

η

η

G

D

11

Influence of Conduction Angle

12

Class F (HCA ... harmonic controlled amplifier)

0 VGS

IDS

Im

2VP VP VDSmaxVDDVKVDS0

VGS VDS

2p

p

Q

IDS

Im

0 2pp Q

13

hHCA (half sinusoidally driven HCA)

0 VGS

IDS

Im

2VP VP VDSmaxVDDVKVDS0

VGS VDS

2p

p

Q

IDS

Im

0 2pp Q

14

Class F and hHCA – Circuit

VDD

RLVDSID Ze(n)

0, n=eveninf, n=even

Zo(n)

0, n=1inf, n=odd

Class F hHCA

%87

dB) (9 5dB-

0%10

PA ⋅=

=

⋅=

κη

κη

A

D

GG

%96

dB) (15 1dB

0%10

PA ⋅=

+=

⋅=

κη

κη

A

D

GG

15

hHCA – Third Harmonic Peaking

0 VGS

IDS

Im

2VP VP VDSmaxVDDVKVDS0

VGS VDS

2p

p

Q

IDS

Im

0 2pp Q

16

Third Harmonic Peaking – Circuit

VDD

RLDGS

f03f0

%87

dB) (14.6 0.6dB

91%

PA ⋅=

+=

⋅=

κη

κη

A

D

GG

17

Linearity Aspects

18

Linearity Aspects

Class AB

Class C

Class A

Class B

19

Linearity Aspects

Ideal strongly nonlinear model Strong-weak nonlinear model

20

Amplifier Design – An Example Balanced Amplifier Configuration

Port 1Z=50 Ohm Port 2

Z=50 Ohm

21

Amplifier Design – Simulation Gate & Drain Waveforms

0 500 1000 1300Time (ps)

Drain waveforms

-5

0

5

10

15

20

25

-1000

0

1000

2000

3000

4000

5000Inner Drain Voltage (L, V)Amp

Inner Drain Current (R, mA)Amp

0 500 1000 1300Time (ps)

Gate waveforms

-3

-2

-1

0

1

-1000

-500

0

500

1000

Inner Gate Voltage (L, V)Amp

Inner Gate Current (R, mA)Amp

22

Amplifier Design – Simulation Dynamic Load Line & Power Sweep

0 3 6 9 12 15Voltage (V)

Dynamic load line

-2000

0

2000

4000

6000

8000IVCurve (mA)IV_Curve

Dynamic Load Line (mA)Amp

0 5 10 15 20 24Power (dBm)

Power Sweep 1 Tone

0

10

20

30

40

0

10

20

30

40

50

60

70

80Output Power (L, dBm)Amp

PAE (R)Amp

23

Amplifier Design – MeasurementsSingle Tone & Two Tone

0

5

10

15

20

25

30

35

40

0 5 10 15 20 25 30 35

P in [dBm]

P ou

t [dB

m],

Gai

n [d

B]

0

10

20

30

40

50

60

70

80 PAE [%]

P outGainGamma InPAE

1dB CP

0

10

20

30

40

50

60

0 5 10 15 20 25 30 35

P in [dBm]

P ou

t [dB

m],

IMD

D [d

Bc]

, Gai

n [d

B]

0

10

20

30

40

50

60 PAE [%]

P outIMDDGainPAE

24

Amplifier NonlinearityGain and Phase depends on Input Signal

3rd Order Gain-Nonlinearities:

25

Amplifier NonlinearityHigher Output Level (close to Saturation) resultsin more Distortion/Nonlinearity

26

Nonlinearity leads to?Generation of Harmonics

Intermodulation Distortion / Spectral Regrowth

SNR (NPR) Degradation

Constellation Deformation

27

Intermodulation and Harmonics

28

Spectral Regrowth

-15 -10 -5 0 5 10 15-60

-50

-40

-30

-20

-10

0

10

rela

tive

pow

er /

dB

relative frequency / MHz

ACPR1>60dBACPR2>60dB

ACPR1=16dBACPR2=43dB

Energy in adjacent ChannelsACPR (Adjacent Channel Leakage Power Ratio) increases

29

Reduced NPR (Noise Power Ratio)

Output Signal of Nonlinear Amplifier

Input Signal

Degradation of Inband SNR„Noisy“ Constellation

30

Constellation DeformationInput Signal Output Signal of

Nonlinear Amplifier(with Gain- and Phase-Distortion)

31

Modeling of Nonlinearitieswith Memory-Effects

Volterra Series (=„Taylor Series with Memory“)

without Memory-EffectsSaleh ModelTaylor SeriesBlum and Jeruchim ModelAM/AM- and AM/PM-conversion

2

2

2 1)(

1)(

rrrg

rrrfa

a

Θ

Θ

+=

+=

βα

βα

bette

rpe

rfor

man

ce

32

AM/AM- and AM/PM-ConversionGaAs-PA

33

AM/AM- and AM/PM-ConversionLDMOS-PA

34

How to preserve Linearity?Backed-Off Operation of PA

Simplest Way to achieve Linearity

Linearity improving ConceptsPredistortionFeedforward...

35

How to preserve Efficiency?Efficiency improving Concepts

DohertyEnvelope Elimination and Restoration...

Linearity improving ConceptsHigher Linearity at constant Efficiency

Higher Efficiency at constant Linearity

36

Direct (RF) Feedback

Classical MethodDecrease of Gain Low EfficiencyFeedback needs more Bandwidth than SignalStability Problems at high Bandwidths

37

Distortion Feedback

Feedback of outband Products onlyHigher Gain than RF feedbackStability Problems due to Reverse Loop

38

Feedforward

Overcomes Stability Problem by forward-only LoopsCritical to Gain/Phase-Imbalances0.5dB Gain Error -31dB Cancellation2.5° Phase Error -27dB CancellationWell suited for narrowband application

39

Cartesian Feedback

-30 -20 -10 0 10 20 30-60

-50

-40

-30

-20

-10

0

10

rela

tive

pow

er /

dB

relative frequency / MHz

original signal predistorted signal

AM/AM- and AM/PM-correctionHigh Feedback-BandwidthStability Problems

I

Q

IQ

IQ

modulator

demodulator

OPAs

main amp.

localoscillator

RF-output

base

band

inpu

t

UMTS example:

40

Digital PredistortionDigital Implementation of „Cartesian Feedback“Additional ADCs, DSP Power, Oversampling neededLoop can be opened no Stability Problems

41

Analog Predistortion

Predistorter has inverse Function of AmplifierLeads to infinite Bandwidth (!)Hard to realize (accuracy)

42

Analog PredistortionPossible Realizations:

43

LINC (Linear Amplification by Nonlinear Components)

AM/AM- and AM/PM-correctionDigital separation required(accuracy!)High Bandwidth,oversampling necessaryStability guaranteed

signalseparation

s(t)

s (t)1 K

Ks (t)2

K(s (t)+1 s (t))=Ks(t)

2

Ks (t)1

Ks (t)2

-30 -20 -10 0 10 20 30-60

-50

-40

-30

-20

-10

0

10

rela

tive

pow

er /

dB

relative frequency / MHz

ACPR1>60dBACPR2>60dB

ACPR1=18dBACPR2=29dB

s(t) s1(t)

UMTS example:

44

Doherty AmplifierAuxiliary amplifier supports main amplifier during saturationPAE can be kept high over a 6dB range

45

Doherty AmplifierGain vs. Input Power

No improvement of AM/AM- and AM/PM-distortionBehavior of auxiliary amplifier very hard (impossible) to realizeStability guaranteed

Efficiency vs. Input Power

main amp. (A1)

aux. amp. (A2)

PIN

POUT

dohe

rty co

nfigu

ration

(A1+

A2)

46

EER (Envelope Elimination and Restoration)

Separating phase and magnitude informationElimination of AM/AM-distortionApplication of high-efficient amplifiers(independent of amplitude distortion)Stability guaranteed

signalseparation

amplitude information

phase information

RF input

RF output

high efficiencypower amplifier

47

EER (Envelope Elimination and Restoration)

Analog realizationLimiter hard to buildAccuracy problemsFeedback necessary

Digital realizationOversampling + high D/A-conversion rates requiredHigh power consumptionof DSP and D/A-convertersPossible feedbackeliminationCompensation of AM/PM-distortion possible

peak detectorsupply voltage

amplifier

limiter

high efficiencypower amplifier

RF output

peak detector

RF input

DA

DA

DA

amplitude information

phase information

modulatorRF output

high efficiencypower amplifier

digitalsignal

processor

local oscillator

supply voltage amplifier

I

Q IQ

digi

tal b

aseb

and

inpu

t

48

EER (Envelope Elimination and Restoration)

Five times (!) oversamplingnecessary to achieve standard requirements

Bandwidth of Magnitude- and phase-signal have higher than transmit signal

-30 -20 -10 0 10 20 30-60

-50

-40

-30

-20

-10

0

10

rela

tive

pow

er /

dB

re lative frequency / MHz

MagnitudePhase

-30 -20 -10 0 10 20 30-60

-50

-40

-30

-20

-10

0

10

rela

tive

pow

er /

dB

relative frequency / MHz

ACPR1>60dBACPR2>60dB

ACPR1=33dBACPR2=40dB

ACPR1=51dBACPR2=36dB

ACPR1=53dBACPR2=49dB

full bandwidth 3⋅B0 bandwidth5⋅B0 bandwidth7⋅B0 bandwidth

UMTS example:UMTS example:

49

Adaptive BiasVarying/Switching of Bias-Voltage depending on Input Power LevelSelection of Operating Point with high PAEApplicably for nearly each type of Amplifier

RF input

peak detector

biascontrol

RF output

high efficiencypower amplifier

50

Adaptive Bias

32 33 34 35 36 37 38 39 4020

30

40

50

60

70

80

90

output power / dBm

pow

er a

dded

effi

cien

cy /

%

VD=3.5VVD=4.5VVD=6.5V

Single tone PAE for switched VDD with VG kept constant

Simply to implement ConceptStability guaranteedPossible problems:

DC-DC converter with high efficiency necessaryPossible Linearity Change (can increase and decrease)especially for HCAs

51

SummaryDigital Realization required to achieve Accuracy

Problem of Stability for high Bandwidth Application

Higher Bandwidths (Oversampling) necessary,depending on Order of IMD cancellation

Predistortion gives best Results while keeping Efficiency high (valid for high Output Levels > 40dBm)

52

Figure ReferencesF. Zavosh et al,“Digital Predistortion Techniques for RF Power Amplifiers with CDMA Applications”,Microwave Journal, Oct. 1999

Peter B. Kenington, “High-Linearity RF Amplifier Design”,Artech House, 2000

Steve C. Cripps,“RF Power Amplifiers for Wireless Communications”,Artech House, 1999

53

Contact Information

DI Markus Mayer

+43-1-58801-35425

markus.mayer@tuwien.ac.at

DI Holger Arthaber

+43-1-58801-35420

holger.arthaber@tuwien.ac.at