architectural enhancements to power amplifiers & transmitters · cw: “power amplifier...

Gareth LLOYD

Cambridge Wireless “Power Amplifier Techniques Workshop”, May 2018

Architectural Enhancements to Power Amplifiers &

Transmitters

Agenda

ı Linearization: Cleaning up the mess

▪ Types and the limits

▪ Measurement example

▪ Classification

ı Predictive Post-Correction

▪ Construction from efficiently generated components

ı Envelope, Outphasing, Doherty

ı Conclusions

CW: “Power Amplifier Techniques Workshop”, 05/18 2


Linearization

Linearization: Types & Limits


ı Linearization is heavily researched in the literature.

ı Examples include Cartesian-, Polar- feedback, Analog-,

Digital- predistortion, Feedforward, Envelope tracking,

Filtering, Automatic Gain Control (AGC)…

ı What they have in common is a physical limit… the

“hard limiter” response

▪ AM-PM eliminated

▪ AM-AM brick wall

ı Performance limited only by saturated power.

ı Example calculation shown for hard clipper and UMTS

Rel99 standard test signal.

Linearization: Measurement Example

ı In this specific example, digital predistortion (DPD,

memoryless, polynomial) is applied to a device used

a mobile device front-end, with standard test signal.

ı The raw device reaches the -40dBc ACLR (linearity)

limit with an average power -4,3dB PSat.

ı The theoretical -40dBc power limit is -1.3dB.

ı With this DPD and this device, -2.1dB is achieved.

ı The useful power increase is 66% with linearization.

ı With perfect linearization, the power increase would

only be 23% (cf. 66%)


Linearization: Classification

ı Makes sense to apply a classification scheme, to help

understand the salient features.

ı A scheme is hereby proposed based on two methods

1. Predictive- or Measured-

2. Pre- or Post- Correction

ı Salient features (not exhaustive!)

▪ “Predictive” can completely eliminate distortion.

▪ “Measured” offer designable correction.

▪ “Post-Measured” schemes can correct some noise.

ı Not a “one-size-fits-all” scenario. There is not a single,

best solution


Pre- Post-

MeasuredCartesian-, Polar-

Feedback

Feedforward

Band Pass Filter

PredictedDigital-, Analog-

PredistortionDis

tort

ion

So

urc

e

Linearization

LocationLinearization

Classification


Predictive Post-Correction


ı It transpires that the relatively empty class is more

than full (the graphic is just a small selection).

ı Classically, these types marketed as “Efficiency

Enhancement” techniques.

ı Construct the signal from 2 (or more) efficiently

generated components.

ı Three known basic types, plus their hybrids:

▪ Envelope = Multiplication

▪ Outphasing = Summation

▪ Doherty = Reference

ı There may be other types, awaiting discovery.


Pre- Post-

MeasuredCartesian-, Polar-

Feedback

Feedforward

Band Pass Filter

PredictedDigital-, Analog-

PredistortionDis

tort

ion

So

urc

e

Linearization

LocationLinearization

Classification



Envelope

Envelope (=Multiplying)

ı The Envelope transmitter constructs the

signal by MULTIPLYING two signals

ı Decomposition into:

▪ PM: constant envelope, phase modulated

▪ AM: scalar quantity, with DC term

ı Envelope Restoration is the purest form,

multiplying AM and PM components

ı Can synthesize any waveform.


AM

PM

smultiply

X



ı In the Envelope Restoration example,

multiplication is perfomed using an off-the-

shelf mixer.

ı QAM-64 is created from a scalar and a phase

modulated signal.

ı Despite the severe decomposition and

multiplication operations, the end-to-end

result is quasi-linear.



ı Synthesising a transmitted RF signal using

ER, instead of conventional IF has some

interesting features.

ı The output spectrum is image-, LO-leakage-

and higher-order-product, free.

ı Without energy content in the distortions, the

output power/linearity is increased.

ı Reduced filtering requirements, decreased

cost



Outphasing

Outphasing (= Summing)

ı The Outphasing transmitter constructs the signal

by SUMMING two vectors that are (in base

form):

▪ Constant envelope

▪ Equal amplitude

ı Can synthesize any waveform DESPITE the

constant envelope constituents.

ı With the demonstrated “LINC” variant, the

difference signal is dumped wastefully in the

isolated port

ı In other variants, the difference is used to drive

efficient Load Modulation in the other amplifier.



ı Illustration of the LINC variant

ı Starting with a reference, 64-QAM signal

ı Decompose into two outphasing elements

(top)

ı Note the increased spectrum occupation

ı In the combining operation, those sidebands

destructively interfere at the output

(summing) port, and constructively combine

at the isolated (difference) port




ı In the LINC measurement example,

combining is performed using an off-

the-shelf power splitter

ı Constant envelope inputs generate a

64-QAM output

ı Note the quasi-linear overall transfer

function from the AM-AM and AM-PM

characteristic.



Doherty

Doherty

ı Comprises two amplifiers, commonly referred

to as “Main” and “Auxiliary”

ı Ideally…

▪ the output signal is a replica of “Main”

▪ “Auxiliary” is the efficiency channel

… in reality, the channels are not orthogonal.


Main

Auxiliary

sDoherty

sDoherty = sMain

2(vMain)-1 < sAuxiliary < vMain

Doherty

ı Measurement shown for a dual-

input Doherty GaN PA, intended

for 5G 3,5GHz operation.

ı Non-ideal behaviours blur the

orthogonality between linearity

and efficiency.

ı An additional linearization

scheme required for most

applications.



Conclusions

Conclusions

ı Energy efficiency is the key differentiator in PAs and Transmitters.

▪ Generally speaking, to achieve highest energy efficiency one should generate a crude signal efficiently, then

clean-up with Linearization.

ı Perfect linearization, even if it could be achieved, is not worth having.

▪ Leveraging the allowed distortion optimizes cost, energy efficiency.

ı Architectural concepts (including dual-path concepts like Doherty, ET, Outphasing) may be used to augment

the foundation techniques (e.g. Class AB) to improve performance.

ı There is no universal “best solution”, either for Linearization or Efficiency Enhancement.

▪ Dependent on: available interfaces, design competencies, functionality, semiconductor processes, etc.

ı Methods may be, and often are, hybridized to complement each other, enhancing performance further.



The End

architectural enhancements to power amplifiers & transmitters · cw: “power amplifier...

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