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
DI Holger Arthaber
+43-1-58801-35420