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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)
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