EFFICIENCY ENHANCEMENTS FOR RF POWER AMPLIFIERS · Rohde & Schwarz ENERGY EFFICIENCY In the early...

EFFICIENCY ENHANCEMENTS FOR RF POWER AMPLIFIERS

Markus Lörner, Market Segment Manager RF &

Microwave Components

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

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


Evolution of Energy Efficiency in Cellular

Communications Infrastructure

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 Enhancement techniques

leveraging Linearization

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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

5

Rohde & Schwarz

LINEARIZATION: TYPES & LIMITS


► 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

MISO TRANSMITTERS: ARCHITECTURAL ENHANCEMENTS TO POWER AMPLIFIERS & TRANSMITTERS (RFFES)

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


AM

PM

smultiply

X

Rohde & Schwarz



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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► 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.

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


Rohde & Schwarz


► 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


Rohde & Schwarz



► 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.

MEASUREMENT AIDED DOHERTY DESIGNS

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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?


Rohde & Schwarz

BACKGROUND ON DOHERTY ARCHITECTURE


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)

16

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)


Main

PA

Doherty

Combiner

Auxiliary

PA

Two paths of the

Doherty Amplifier

18

Rohde & Schwarz

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


vin imain iaux

Zmain Zaux

ZL

Simplified

Doherty Schematic

19

Rohde & Schwarz

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.


mainaux.

Power and Efficiency impacts

of conduction angle [Cripps].

20

Rohde & Schwarz

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


Main

PA

Doherty

Combiner

Auxiliary

PA

3dB 90deg.

Split

‘Arts & Crafts Movement’

Phase Shifters, printed on

PCB

21

DOHERTYIN THE EXTREMES

22

Rohde & Schwarz

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.


Auxiliary

PA

Driver

DriverPre-

Driver

Pre-

DriverMain

PA

Main

PA

Doherty

Combiner

Auxiliary

PA

SplitterDriverPre-

Driver

DOHERTYMEASUREMENT AIDED DEVELOPMENT

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


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

Rohde & Schwarz

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


LINEAR MEASUREMENT RESULTS & ANALYSIS

Main

PA

Doherty

Combiner

Auxiliary

PA

SplitterDriverPre-

Driver

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.


Rohde & Schwarz

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


NON-LINEAR MEASUREMENT RESULTS & ANALYSIS

Auxiliary

PA

Driver

DriverPre-

Driver

Pre-

DriverMain

PA

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.


Rohde & Schwarz

RESULTS

Conventional Mode Operation Dual-Input Mode Operation


R&S SOLUTION FOR DOHERTY WORK

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HARDWARE


► 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

34

Rohde & Schwarz

DEDICATED SOFTWARE APPLICATIONS


► 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)

35

Rohde & Schwarz

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.


SUMMARY

Rohde & Schwarz

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.


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EFFICIENCY ENHANCEMENTS FOR RF POWER AMPLIFIERS · Rohde & Schwarz ENERGY EFFICIENCY In the early...

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