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EFFICIENCY ENHANCEMENTS FOR RF POWER AMPLIFIERS
Markus Lörner, Market Segment Manager RF &
Microwave Components
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Rohde & Schwarz
AGENDA
► Linearization:
Types and the limits
Measurement examples
Classification
► Predictive Post-Correction
Construction from efficiently generated components
► MISO Transmitters: Architectural Enhancements to Power Amplifiers & Transmitters (RFFEs)
Envelope Restauration
Outphasing
Doherty
► Conclusions
Efficiency enhancements for RF Power Amplifiers2
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Rohde & Schwarz
ENERGY EFFICIENCY
► Radio Frontend (RFFE) performance is
defined by 4 headline parameters:
Output Power, Bandwidth/Frequency,
Linearity, Efficiency
► The first three of those are governed or
dictated by specification or regulation
► Efficiency is the market differentiator.
► In battery powered applications, it drives time-
between-charges, in prime- or higher powered
applications, it drives cooling requirements;
size and weight.
Single Carrier per Tx (early 1990s)
• 4 Carrier GSM
• Passive Combining
• 5% Efficiency
• 19 W/W Wasted
Multicarrier Tx (late 1990s)
• 4 Carrier EDGE
• Feedfoward Linearization
• 10% Efficiency
• 9 W/W wasted
Multicarrier Tx(early 2000s)
• 2-4 Carrier UMTS
• DPD Linearization
• 20% Efficiency
• 4 W/W wasted
Remote Radio Head (late 2000s)
• n Carriers LTE
• DPD + Doherty
• 35% Efficiency
• 1,9 W/W wasted
Efficiency enhancements for RF Power Amplifiers3
Evolution of Energy Efficiency in Cellular
Communications Infrastructure
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Rohde & Schwarz
ENERGY EFFICIENCY
► In the early 20th Century, power consumption
and energy bills drove the demand for
improved efficiency for AM transmitters.
► Doherty and Outphasing were early
developments, later Envelope Restoration.
► The advent of mobile communications, using
non-constant envelope modulations, reignited
widespread research and development.
► Most of those developments were rehashes,
hybrids, industrialisations or otherwise of
those 3 building blocks.
Efficiency enhancements for RF Power Amplifiers4
Efficiency Enhancement techniques
leveraging Linearization
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Rohde & Schwarz
UNDERSTANDING DISTORTIONS
► Distortion limits RFFE performance
► Distortions = variations in complex gain (amplitude and
phase) in three domains:
Amplitude (e.g. non-linear distortion)
Frequency (e.g. linear distortions)
Time (e.g. memory effects)
► RFFE components demonstrate all the distortions, in
varying proportions:
Mixers and Amplifiers often contribute most to
non-linear and memory effect distortions
Filters often contribute the most linear distortion
► Distortion reduction is called Linearization
Efficiency enhancements for RF Power Amplifiers
Gain
Phase
Amplitude
Time
Frequency
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LINEARIZATION: TYPES & LIMITS
Efficiency enhancements for RF Power Amplifiers6
► Linearization: heavily researched in the literature
► Examples include Cartesian-, Polar- feedback,
Analog-, Digital- predistortion, Feedforward, Envelope
tracking, Filtering, Automatic Gain Control (AGC)…
► What they have in common is a physical limit:
the “hard limiter” response
AM-PM eliminated
AM-AM brick wall
► Performance limited only by saturated power.
► Example calculation for hard clipper and
UMTS Rel99 standard test signal
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MISO TRANSMITTERS: ARCHITECTURAL ENHANCEMENTS TO POWER AMPLIFIERS & TRANSMITTERS (RFFES)
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Rohde & Schwarz
ENVELOPE (=MULTIPLYING)
► The Envelope transmitter constructs the
signal by MULTIPLYING two signals
► Decomposition into:
PM: constant envelope, phase modulated
AM: scalar quantity, with DC term
► Envelope Restoration is the purest form,
multiplying AM and PM components
► Works for any waveform
Efficiency enhancements for RF Power Amplifiers8
AM
PM
smultiply
X
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Rohde & Schwarz
ENVELOPE (=MULTIPLYING)
Efficiency enhancements for RF Power Amplifiers9
Why
► Output spectrum: free of image, LO
leakage, and higher-order-product
► No energy went to distortion
► Output power / linearity improved
► Reduced filtering requirements,
decreased cost
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Rohde & Schwarz
ENVELOPE (=MULTIPLYING)
Efficiency enhancements for RF Power Amplifiers10
► In example, multiplication done using off-the-
shelf mixer
► QAM64 from scalar and phase component
signal
► Despite the severe decomposition and
multiplication operations, the end-to-end
result is quasi-linear.
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Rohde & Schwarz
OUTPHASING (= SUMMING)
► Outphasing transmitter: SUMMING the two
vectors
Constant envelope &
Equal amplitude
Called Linear Components = LINC
► Works for any waveform
DESPITE constant envelope vectors
► With LINC, the difference signal is sent
wastefully to isolated port
► In other variants, difference to drive efficient
Load Modulation in the other amplifier
Efficiency enhancements for RF Power Amplifiers11
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OUTPHASING (= SUMMING)
► Illustration of the LINC variant
► Starting with a reference, 64QAM signal
► Decompose into two outphasing elements
(top)
► Note the increased spectrum occupation
► In the combining operation, those sidebands
destructively interfere at the output
(summing) port, and constructively combine
at the isolated (difference) port
Efficiency enhancements for RF Power Amplifiers12
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OUTPHASING (= SUMMING)
Efficiency enhancements for RF Power Amplifiers13
► Example uses off-the-shelf power
splitter
► Constant envelope inputs generates
64QAM output
► Note the quasi-linear overall transfer
function from the AM-AM and AM-PM
characteristic.
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MEASUREMENT AIDED DOHERTY DESIGNS
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Rohde & Schwarz
WHAT’S ON OFFER?
► According to Darraji et.al, the difference between two solutions
Analog Doherty
Digital Doherty)
is as much as:
60% output power
20% efficiency
50% bandwidth
no degradation in DPD efficacy.
► But, how can the difference be identified on a case-by-case basis?
Efficiency enhancements for RF Power Amplifiers15
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BACKGROUND ON DOHERTY ARCHITECTURE
Efficiency enhancements for RF Power Amplifiers
Facts
Invented almost 100 years ago
Efficiency enhancement method
Linearity-preserving
Two (or more) amplifiers that interact
through a special combining network
Applications
Mostly for below 3 GHz until now
Dominates on base station infrastructure
New Frontier
Higher carrier frequencies, wider BW
5G in mmW, SatCom (Ku-, Ka-bands)
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DOHERTY CHALLENGES
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CHALLENGE 1
► Misalignment of signals at the output cause:
loss of power
loss of energy efficiency
destructive voltages/currents
► Input signals need to be matched for amplitude
and phase
Time-, frequency-, amplitude- domains
► The performance of the combining needs to be
considered in many scenarios, e.g.
Manufacturing (e.g. part-part variations)
Field operation (e.g. temperature changes)
Efficiency enhancements for RF Power Amplifiers
Main
PA
Doherty
Combiner
Auxiliary
PA
Two paths of the
Doherty Amplifier
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CHALLENGE 2
► Ideal performance by auxiliary characteristic is “dog leg”
Never perfect
Often approximated by “Class C” amplifier
► Performance driven by difference between the main and
auxiliary curves.
► The two extremes
Main, the Doherty ‘effect’ tends to 0 (or like
‘Balanced’)
Ideal, the Doherty ‘effect’ is maximized
Efficiency enhancements for RF Power Amplifiers
vin imain iaux
Zmain Zaux
ZL
Simplified
Doherty Schematic
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CHALLENGE 3
► Different classes of amplifier to drive the Doherty
difference engine can be disadvantageous
► The Fourier Analysis of conduction angle shows how,
power and efficiency might be compromised.
Power is lost from the auxiliary
Efficiency is lost from the main
► The quiescent bias power demands of the main can
prove costly, especially in TDMA operation.
Efficiency enhancements for RF Power Amplifiers
mainaux.
Power and Efficiency impacts
of conduction angle [Cripps].
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CHALLENGE 4
► How to design the input splitter?
► After design and alignment of the output section, designers
often use cut-and-try techniques on the input side.
► Salient features of this method:
Labour intensive
Non-exhaustive, sparse characterization
Global maxima unconfirmed
Cannot easily adjust amplitude balance
Poorly defined structures
Lossy components
Matching variations
Efficiency enhancements for RF Power Amplifiers
Main
PA
Doherty
Combiner
Auxiliary
PA
3dB 90deg.
Split
‘Arts & Crafts Movement’
Phase Shifters, printed on
PCB
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DOHERTYIN THE EXTREMES
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OPPOSITE ENDS OF THE SPECTRUM
► Extremes of Doherty implementations:
The default setup
Single gain stage inside a Split-Doherty Combine.
Differentially biased devices
Digital Doherty
Independent paths all the way from digital domain
Common biased devices
► In between lies a whole range of implementation solutions, with differing
features and trade-offs.
Efficiency enhancements for RF Power Amplifiers23
Auxiliary
PA
Driver
DriverPre-
Driver
Pre-
DriverMain
PA
Main
PA
Doherty
Combiner
Auxiliary
PA
SplitterDriverPre-
Driver
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DOHERTYMEASUREMENT AIDED DEVELOPMENT
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Rohde & Schwarz
MEASUREMENT AIDED DEVELOPMENT
► Idea
Additional measurement-based step in the
traditional Doherty development process
Remove the input split and phase shift networks
Drive two Doherty input ports directly from a signal
generator
► Benefits
Better view of performance tradeoffs
In-depth understanding of sensitivities
Benchmark maximum performance
Select best input split and specify performance with
confidence
Applicable to all input split architectures
Efficiency enhancements for RF Power Amplifiers
Sim
plifie
d B
lock D
iagra
mH
ard
wa
re T
est S
etu
p
Classic
Doherty
Dual-input
Doherty
Main
PA
Doherty
Combiner
Auxiliary
PA
DSP
Unit
DAC Up-
ConverterAnalog
Splitter
Doherty
DUT
Main
PA
Doherty
Combiner
Auxiliary
PA
DSP
Unit
DAC Up-
Converter
The test and measurement concept25
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THE EXAMPLE DUT
► QORVO reference Doherty design, based on TQP0103
► GaN power amplifier building block
► Single stage unmatched power amplifier transistor
► RF input splitter removed from layout
► Developed for 3.4 to 3.6GHz band
Efficiency enhancements for RF Power Amplifiers26
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LINEAR MEASUREMENT RESULTS & ANALYSIS
Main
PA
Doherty
Combiner
Auxiliary
PA
SplitterDriverPre-
Driver
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Rohde & Schwarz
DUAL-PATH MEASUREMENT (LINEAR)
► Same signal to both RF paths
► Sweep input power, amplitude and
phase difference (optionally bias, etc.)
► Measure what is of interest like
saturated power, RMS, PEP, Efficiency,
ACLR or PAPRo
Result:
► Dispersion of amplitude/phase between
parameter optima and frequency
► This is already far ahead of the usual
characterization dataset.
Efficiency enhancements for RF Power Amplifiers28
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DEFAULT ANALOG DOHERTY CONFIGURATION
► Focus on
saturated power and
modulated efficiency
► From linear data, extract the worst case value
for each amplitude-phase pair
frequency dependency is removed
► Similar to manual tuning the input phase offset –
except now, there is much more information,
gathered faster, and amplitude delta is added
Efficiency enhancements for RF Power Amplifiers29
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NON-LINEAR MEASUREMENT RESULTS & ANALYSIS
Auxiliary
PA
Driver
DriverPre-
Driver
Pre-
DriverMain
PA
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Rohde & Schwarz
DUAL-PATH MEASUREMENT (NON-LINEAR)
► Apply different, but related signals to the two RF paths
and common-mode biasing. Simple case:
Auxiliary signal derived from square of the Main signal
Biased at threshold
► Driven by the increased saturated power (representing
the limit of linearization)
47% higher output power (43,8dBm -> 45,5dBm)
11% higher efficiency (44% -> 49%)
94% reduction in “stand-by” power consumption
(100mA->6mA)
► Compare with the reported 60% output power, 20%
efficiency, 50% bandwidth and no degradation in
linearizability.
Efficiency enhancements for RF Power Amplifiers31
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RESULTS
Conventional Mode Operation Dual-Input Mode Operation
Efficiency enhancements for RF Power Amplifiers32
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R&S SOLUTION FOR DOHERTY WORK
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Rohde & Schwarz
HARDWARE
Efficiency enhancements for RF Power Amplifiers
► R&S®SMW200A Vector Signal Generator
Dual-path with precise signal alignment
Relative phase, amplitude, timing adjustment
No elaborate calibration routines to establish
and maintain signal alignment
Realtime digital pre-distortion
Large touch screen display
► R&S®FSW Signal and Spectrum Analyzer
Wide analysis bandwidth
Large intuitive touchscreen display
Dedicated amplifier test capabilities with all
interesting parameters from EVM to Gain
compression
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DEDICATED SOFTWARE APPLICATIONS
Efficiency enhancements for RF Power Amplifiers
► R&S®SMW-K546 Digital Doherty
Couple RF paths with precise power and phase
alignment
Power split and input-power dependent phase
delta in real time
Shaping
Realtime digital pre-distortion
► R&S®FSW-K18 Amplifier Measurements
Capture scalar quantities like spectral regrowth
and EVM
Vectors like AM-AM and AM-PM
Realtime digital pre-distortion together with
R&S®SMW-K541
R&S®SMW-K546 Digital Doherty Software Option
(available with SMW-Release-FW 4.50.100.xx)
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CONCLUSIONS FOR DOHERTY DESIGNS
► Perfect Doherty operation cannot be achieved. But, performance can be strongly differentiated by the input side architecture.
► Various input side designs for the Doherty amplifier, including:
Fixed constant, or fixed dispersive, RF split
Programmable RF split
Dispersive RF split
Digital domain split
… and so on, each correcting frequency, time or amplitude domain effects.
► The proposed measurement set-up enables a comprehensive, rapid and accurate characterization of the Doherty Prototype.
► Measuring as a Dual-Input:
Provides unprecedented insight.
Enables the best engineering decision to be made, supported by the most information, in the shortest time.
Efficiency enhancements for RF Power Amplifiers36
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SUMMARY
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SUMMARY
► Energy efficiency = differentiator in PAs and Transmitters
To get highest energy efficiency:
1. Generate a raw signal efficiently
2. then clean-up with Linearization
► Perfect linearization, even if possible, is often too expensive
Leveraging the allowed distortion optimizes cost, energy efficiency.
► Architectural concepts (including dual-path concepts like ET, Outphasing, Doherty) may be used to augment the foundation techniques (e.g. Class AB) to improve performance.
► There is no universal “best solution”, either for Linearization or Efficiency Enhancement.
Dependent on: available interfaces, design competencies, functionality, semiconductor processes, etc.
► Methods may be, and often are, hybridized to complement each other, enhancing performance further.
Efficiency enhancements for RF Power Amplifiers38