Wideband and Energy

Efficient Power Amplifiers for

Wireless Communications

Mustafa Özen

on behalf of Christian Fager

{chrisitan.fager,mustafa.ozen}@chalmers.se

Microwave Electronics Laboratory | Chalmers University of Technology

www.chalmers.se/ghz

Located by the west cost of Sweden

… Founded 1829 by William Chalmers

…11000 students (1150 doctoral students)

…Long tradition in microwaves

Sweden

Göteborg

Microwave Technologies at Chalmers

III-V MMIC designMultifunctional THz > 300 GHz

Communication > 100 GHz

GaN HEMT VCOs

Mixed signal (>100 Gbps)

GaN HEMT technologyGaN HEMT MMICs

Robust transceivers, high RF power

InP HEMT technologyInP and InAs HEMT MMICs

Cryogenic low-noise amplifiers

THz devices & instrumentationMixers:

Schottky diode, varactors

Hot-electron bolometer SIS

Heterodyne receivers beyond 1 THz

Transmitters for telecomPower amplifiers

High efficiency and linearization

Emerging MW componentsGraphene HF electronics

Ferroelectric tunable devices

Full Professors: Victor Belitsky, Spartak Gevorgian, Jan Grahn, Jan Stake, Herbert Zirath

4

Outline

• Background

• Energy efficient wideband transmitter architectures

– Varactor based dynamic load modulation

– Doherty power amplifiers (PA)

– Outphasing PAs

– Mixed Doherty-outphasing techniques

• Summary

5

Transmitter Demands

• A radio transmitter generates high power information carrying

electromagnetic signals.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

• Higher transmitter efficiency for

–Lower operational costs

–Smaller environmental footprint

• Most power hungry unit in a radio base station.

5

Message

6

Transmitter Demands

• Strong demand for higher data rates

• Wireless providers allocate more spectrum

– 44 different bands are utilized in LTE-A

• Wideband transmitters enable covering multiple bands with a

single unit

6

0.7 GHz 0.9 GHz 1.8 GHz 2.1 GHz 2.65 GHz2.3 GHz

7

Transmitter Demands

• Carrier aggregation in LTE-A for higher data rates

• In summary: Energy efficient, large RF and signal bandwidth

transmitters

f

f

f

Band I

Band I

Band I Band II

Intra-band contiguous

Intra-band non-contiguous

Inter-band non-contiguous

8

Traditional linear PA operation

• The peak output power is determined by PA saturation

– PA efficiency is maximum close to saturation

– Operating it into compression results in severe distortion

• The total PA efficiency is weighted by the signal input power

probability density function

– For this case: Peak PAE = 55%, total average PAE = 22%

9

Efficiency enhancement via Supply

modulation

• Provides large RF bandwidth

• Difficult to power scale at large

instantaneous signal bandwidths

• More suitable for handsets

Tran

sist

or

curr

en

t

Transistor voltage

High power load line

Imax

VDD1

Low power load line

VDD2

Dynamic reduction of VDD

Opt. supply

Fixed supply

10

Efficiency enhancement via load

modulation

• High power realization at large

signal bandwidths

• Challenging to achieve large

RF bandwidth

Tran

sist

or

curr

en

t

Transistor voltage

High power load line

Imax

VDD1

Low power load line

Power

AmplifierOutput

Load

Optimal Load

50 Ω

11

Outline

• Background

• Energy efficient wideband transmitter architectures

– Varactor based dynamic load modulation

– Doherty power amplifiers (PA)

– Outphasing PAs

– Mixed Doherty-outphasing techniques

• Summary

12

Varactor based DLM

• Variation of output power by dynamically tuning the PA load

network

• Varactors typically used as tuneable elements

– Breakdown voltage > 100V

– Low series resistance, large tuning range

• Simple and efficient control electronics

– No need for high power dc converters etc.

– Potentially wideband modulation

Chalmers SiC varactors

13

High power demonstrator

GaN HEMT

SiC Varactor

Microchip capacitor

Power scalable load network topology

Packaged (40x20mm) 100W GaN demo

[C. M. Andersson, et. al, “A Packaged 86 W GaN Transmitter with SiC Varactor-based Dynamic Load Modulation”, EuMC 2013]

Varactor-based DLM

14

Results @ 2.14 GHz

Vds = 20 V

Vds = 30 V

Vds = 40 V

• Peak power = 86W

• 6.7 dB PAPR WCDMA signal

– ACLR < -46 dBc

– 34% average efficiency

• Losses in load network limits efficiency enhancement

Reactive Class J DLM

15

Dual-band Varactor-based DLM

Dual band DLM PA prototype

Dual band tunable load networkOptimal load trajectories

• Dual band operation

– 700 MHz & 1900 MHz

• Double stub tuner

16

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

-50

-40

-30

-20

-10

0

Frequency [MHz]

No

rma

lize

d P

SD

[d

B]

w/o DPD

w DPD

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

-50

-40

-30

-20

-10

0

Frequency [MHz]

No

rma

lize

d P

SD

[d

B]

w/o DPD

w DPD

Dual-band Varactor-based DLM

Dual band DLM PA prototype

• Dual band operation

– 700 MHz & 1900 MHz

• Double stub tuner

28 30 32 34 36 38 40 420

10

20

30

40

50

60

70

80

Pout [dBm]

Dra

in e

ffic

ien

cy [%

]

Freq 685 MHzLower band Upper band

28 30 32 34 36 38 40 420

10

20

30

40

50

60

70

80

Pout [dBm]

Dra

in e

ffic

ien

cy [%

]

17

Outline

• Background

• Energy efficient wideband transmitter architectures

– Varactor based dynamic load modulation

– Doherty power amplifiers (PA)

– Outphasing PAs

– Mixed Doherty-outphasing techniques

• Summary

RL

Zo

Main

Aux

λ/4

Analog Power Divider

vin

18

Conventional Doherty PA Concept

Conventional Doherty PA

-15 -10 -5 00

20

40

60

80

100

back-off (dB)

Effic

iency (

%)

RL

Zo

Main

Aux

λ/4

Analog Power Divider

vin

Class-B

Class-C

Ideal efficiency

0 0.2 0.4 0.6 0.8 10

0.2

0.4

0.6

0.8

1

Input voltage

Vm

Im

Ia

Transistor voltages and currents

+Vm-

Im

+Va-

Ia

19

RL

Zo

Main

Aux

λ/4

Analog Power Divider

vin

Conventional Doherty PA Concept

• Higher PAPRLarger class-C

– Lower gain and PAE

– Uneven power division

• Increased manufacturing cost

Conventional Doherty PA

Class-B

Class-C

-15 -10 -5 00

20

40

60

80

100

back-off (dB)

Effic

iency (

%)

0 0.2 0.4 0.6 0.8 10

0.2

0.4

0.6

0.8

1

Input voltage

Vm

Im

Ia

Ideal efficiency

Transistor voltages and currents

+Va-

Ia

+Vm-

Im

20

Hypothesis

• Large efficiency range (>6 dB) with identical devices?

• Devices should be fully utilized

– Both devices are biased with nominal VDD

– Use all available current

RL

Zo

Main

Aux

λ/4

Analog Power Divider

vin

Modify?

21

Novel Symmetrical Doherty PA

• Calculate the combiner network parameters assuming identical

devices

– Efficiency range (arbitrary)

– Class-B and class-C impedances at peak power & back-off

-16 -14 -12 -10 -8 -6 -4 -2 020

40

60

80

100

Dra

in e

ff. (%

)

Normalized output power (dB)

Boundary Conditions:

Efficiency of symmetrical Doherty PASchematic used for the

derivations

22

3.5 GHz Hardware Demonstrator

• Combiner S-parameters:

– S11 = -0.81 + j0.24

– S21 = -0.022 - j0.38

– S22 = -0.27 + j0.24

MainPeak

Load

Empirical

combiner

topology

load pull data

30 32 34 36 38 40 42 44 460

20

40

60

80

100

Output power (dBm)

Effic

iency,P

AE

(%

)

PAE

Cut-ready simulation results

Novel Symmetrical Doherty PA

23

Experimental verification

• A 3.5 GHz 30 watt GaN HEMT symmetical Doherty PA

prototype

• A record high PAE of 55% at 8 dB back-off

– Symmetrical devices & novel load-pull based combiner

design approach

Fabricated prototype

31 33 35 37 39 41 43 450

20

40

60

80

100

Output power (dBm)

Effic

ien

cy,P

AE

(%

)

PAE

Static efficiency resultsCross-verified at NXP

(Credits Reza Abdoelgafoer)

Novel Symmetrical Doherty PA

24

Experimental verification

• Tested with carrier aggregated 100 MHz (5x20) OFDM signals

• -50 dBc ACLR with 100 MHz signals.

– 5 dB margin to spectral mask.

– High efficiency with excellent linearity

Fabricated prototype

3.3 3.35 3.4 3.45 3.5 3.55 3.6 3.65 3.7-60

-50

-40

-30

-20

-10

0

Frequency (GHz)

Po

w. sp

ec. d

en

sity (

dB

/Hz)

Output spectrum with

100 MHz OFDM signals

w/o DPD

w/ DPD [1]

[1] S. Afsardoost, T. Eriksson, and C. Fager, "Digital Predistortion Using a Vector-Switched Model," IEEE T-

MTT, 2012

Novel Symmetrical Doherty PA

25

)2

tan(

)2

tan(

1

of

fL

jRT

Z

of

fT

jZL

R

TZZ

Frequency response at back-off

A Novel Wideband Doherty

[D. Gustafsson et al., "A Modified Doherty Power Amplifier With Extended Bandwidth and Reconfigurable Efficiency," IEEE T-MTT, Jan. 2013]

Doherty PA

Doherty PA topology

)2

tan(

)2

tan(

1

of

fL

jZT

Z

of

fT

jZL

Z

TZZ

TZZ

1

Frequency response at the peak power

Back-off efficiency is

strongly

frequency dependent!

ZL

26

A Novel Wideband Doherty

• Doherty PA

– Backoff efficiency bandwidth limited by

l/4 impedance inverter

– ZT ≠RL

• Proposed PA

– ZT ≡ RL

– New drive scheme and biasing

[D. Gustafsson et al., "A Modified Doherty Power Amplifier With Extended Bandwidth and Reconfigurable Efficiency," IEEE T-MTT, Jan. 2013]

Doherty PA topology

27

Bandwidth performance

• Frequency independent backoff efficiency

• Extended average efficiency bandwidth

Proposed PADoherty PA

A Novel Wideband Doherty

28

GaN MMIC Demonstrator

• TriQuint 0.25µm GaN process

• 5.7-8.8 GHz (42% bandwidth)

• PAE: 30-39% @ 9 dB BO

• Reconfigurable PAE shape by

Vdd/Vgg adjustments only

Large PAE bandwidth Reconfigurable PAE @ 6.4 GHz

2.9 mm

2.9 mm

A Novel Wideband Doherty

29

Outline

• Background

• Energy efficient wideband transmitter architectures

– Varactor based dynamic load modulation

– Doherty power amplifiers (PA)

– Outphasing PAs

– Mixed Doherty-outphasing techniques

• Summary

Sout(Δt)

Δt

SMPA

SMPA

CMOSRL

Zo

Main

Aux

λ/4

Analog Power Divider

vin

30

Outphasing Transmitter Architecture

• Two constant envelope signals are summed to achieve

amplitude modulation

• Possibility for high efficiency switch mode operation

• Combiner determines the interaction between the PAs

𝜃

S1

S2

Sin

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

CombinerSignal

splitter

Sout

PowerAmplifier

0 1 2 3 4 5 6 7 8-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

0 1 2 3 4 5 6 7 8-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1 𝜃

S1

S2

Sin

31

Chireix Outphasing Combiner

• Chireix outphasing combiner enables proper load modulation

and thus high efficiency.

• Combiner is inherently narrowband (~5% efficiency bandwidth).

– Mainly due to quarter wave transformers.

Chireix combiner

Branch 1

input

Branch 2

input

0 0.2 0.4 0.6 0.8 10

0.2

0.4

0.6

0.8

0 0.2 0.4 0.6 0.8 10

0.5

1

1.5

2

Dra

in e

ffic

iency

Pro

ba

bili

ty

Output amplitude

Efficiency of Chireix System

32

• Combiner network parameters are derived from the boundary

conditions

– The transistors experience optimal class-E impedances at

peak and average power levels

Novel Outphasing Combiner Design

Approach

2 PortVie -jθ Vi

+

V1

-

+

V2

-

I2I1

P1 P2

A

nZ B

nZ

Branch 1

switch

Branch 2

switch

Combiner network,

including load

0.5 1 1.50

1

2

3

Voltage

0.5 1 1.50

2

t/T

Curr

ent

Class-E PA Waveforms

33

Wideband Outphasing Transmitter

Realization

• A 25 W 750-1050 MHz CMOS-GaN HEMT transmitter prototype

– Combiner S-parameter continuum is mapped to the

frequency response of practical network

VDD=6 V

50 Ω

VDD1 = 28 V

VDD1 = 28 V

VDD

0.2

0.5

1.0

2.0

5.0

+j0.2

-j0.2

+j0.5

-j0.5

+j1.0

-j1.0

+j2.0

-j2.0

+j5.0

-j5.0

0.0

Synthesized

Calculated

Combiner S-parameters

Theory

Circuit

S12

S11

S22

CMOS GaN

Combiner

f

34

Experimental Results

• Efficiency improvement is 20 to 40 percentage units

– Efficiency enhancement, large RF bandwidth (33%) and

possibility for high level of integration

Measured Drain efficiencyFabricated prototype

CMOS GaN

21 23 25 27 29 31 33 35 37 39 41 43 450

20

40

60

80

100

Output power (dBm)

Dra

in e

ffic

ien

cy (

%)

0.75 GHz

0.80 GHz

0.90 GHz

1.00 GHz

1.05 GHz

Class-B

Branch 1

input

Branch 2

input

Antenna

Wideband Outphasing Transmitter

35

Outline

• Background

• Energy efficient wideband transmitter architectures

– Varactor based dynamic load modulation

– Doherty power amplifiers (PA)

– Outphasing PAs

– Mixed Doherty-outphasing techniques

• Summary

Sout(Δt)

Δt

SMPA

SMPA

CMOS

36

Outphasing/Doherty continuum

Average efficiency

1

2

0 20 40 60 80 100 120 140 160 1800

20

40

60

80

100

120

140

160

180

40

45

50

55

60

65

70

Doherty

Doherty

Doherty

Doherty

Outphasing

Outphasing

[C. Andersson et al., "A 1–3-GHz Digitally Controlled Dual-RF Input Power-Amplifier Design Based on a Doherty-Outphasing Continuum Analysis," IEEE T-MTT, 2013]

Doherty (θ1 = 90°, θ2 = 0°) Outphasing (θ1 = 114°, θ2 = 57°)

General dual-input PA

37

Outphasing/Doherty continuum

Average efficiency

1

2

0 20 40 60 80 100 120 140 160 1800

20

40

60

80

100

120

140

160

180

40

45

50

55

60

65

70

Doherty

Doherty

Doherty

Doherty

Outphasing

Outphasing

[C. Andersson et al., "A 1–3-GHz Digitally Controlled Dual-RF Input Power-Amplifier Design Based on a Doherty-Outphasing Continuum Analysis," IEEE T-MTT, 2013]

General dual-input PA

• Continuum between Doherty and outphasing operation

• Potential for >octave bandwidth and efficient operation

– Class B (short circuited harmonics) assumed

38

Demonstrator results

ADS simulations Measurements

Outphasing/Doherty continuum

39

Excellent 1-3 GHz performance

Outphasing/Doherty continuum

• CW measurements

– Pmax = 44 ± 0.9 dBm

– >45 % PAE at 6 dB OPBO

from 1.0 – 3.0 GHz

• DPD linearized measurements

– 5 MHz WCDMA

– ACPR < -57 dBc

– PAE > 40/50%

Class-B @ 6dB

40

Summary

• Dynamic load modulation architectures

– Varactor-based dynamic load modulation

– Doherty PA

– Outphasing PA

– Mixed Doherty and outphasing techniques

• New circuits and design techniques

– Enabling large RF bandwidhts (1-3 GHz)

– Excellent linearity with 100 MHz carrier agg. OFDM signals

– Reduced cost solutions (Symmetrical Doherty)

41

Acknowledgments

• …past and present power amplifier research collaborators

• Companies and research funding agencies

T. Eriksson

(Prof.)

Adj. Prof. R. Jos

(NXP)

C. Fager

(Assoc. Prof.)

Dr. U. Gustavsson

(Ericsson)

Dr. M. Özen

(post-doc)

J. Chani

(PhD stud)

K. Hausmair

(PhD stud)

Dr. P. Landin

(post-doc)

Dr. C. Andersson

(Mitsubishi)

D. Gustafsson

(PhD stud)

Dr. A. Soltani (Qamcom)

& Dr. H. Cao (Ericsson)

Dr. P. Saad

(Ericsson)

Dr. H. Nemati

(Ericsson)

S. Afsardoost

(Ericsson)

X. Bland

(SATIMO)

S. Gustafsson

(PhD stud)F. Johansson

(MSc stud)

Dr. C. Sanchez

(post-doc)

W. Hallberg

(PhD stud)