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ANovelPowerAmplifierLinearizationMethod forHandsetApplications U.Eswaran,H.Ramiah,J.Kanesan DepartmentofElectricalEngineering,FacultyofEngineering UniversityofMalaya,50603KualaLumpur,Malaysia

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Abstract Ǧ A new circuit concept to improve the maximum linear output power of a Universal Mobile TelecommunicationSystem(UMTS)poweramplifier(PA)isintroducedinthispaper.Theproposedcircuit consistsofananalogpreǦdistorter(APD)integratedattheinputofaclassBamplifier.TheproposedAPD extendsthemaximumlinearoutputpowerofthePAto28dBmwithcorrespondingpoweraddedefficiency (PAE)of52.3%.Simulationresultshowsthatat1.95GHz,PAhasaworstcaseadjacentchannelleakageratio (ACLR)ofǦ36dBcatoutputpowerof28dBm.WitharespectiveinputandoutputreturnlossesofǦ27.6dB and Ǧ13.2dB, the PA’s power gain is simulated to be 33.3dB while exhibiting an unconditional stability characteristicfromDCto3GHz.Themonolithicmicrowaveintegratedcircuit(MMIC)poweramplifier(PA) is designed in 2ɑm InGaP/GaAs process. The proposed APD technique serves to be a good solution to improvethemaximumlinearoutputpoweroftheUMTSPAwithoutsacrificingothercriticalperformance metrics.

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Keywords: Power amplifier, Analog Predistorter, Power added efficiency, Hetero junction bipolar transistor,Universalmobiletelecommunicationsystem,Adjacentchannelleakageratio

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Introduction

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Recently,thereisanexplosivegrowthinthenumberofUMTSstandardusers.Thisisduetotheexcitement in the high data rate usage around the globe. Therefore, handset designers, particularly power amplifier (PA) designers confront stiff challenges in improving the maximum linear output power so that linear transmission still occurs in remote areas. This transmission also needs to be efficient in order to prevent rapidenergydrainageofthebattery.

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SeveralmethodshavebeenreportedrecentlytoimprovethemaximumlinearoutputpowerofthePA.The mostprominenttechniqueamongthemisthepreǦdistortiontechnique.Inthismethod,theinputRFsignal to the PA is distorted prior amplification by changing its nonǦlinear transfer function’s magnitude and phasetohaveanoppositeresponsestothephaseandmagnitudeofthenonǦlinearcomponentsgenerated by the PA. The Digital Predistortion Technique (DPD) is the upcoming preǦdistortion method used to linearize the PA [1]Ǧ[3]. In this method, these nonǦlinear responses are generated with the aid of a DSP processor. The complexity in integration, resulting in larger chip size and dual fabrication process are among the visible disadvantages of DPD. On the other hand, APD offers a simple solution of integrating additional active devices, usually within the same process at the input of the power amplifier [4]Ǧ[9]. Alternatively,passiveelementsareutilizedasapredistorter,tolinearizeanonǦlinearclassǦEPA[10]. Inthispaper,anewAPDtechniqueisintroducedtoimprovethemaximumlinearoutputpowerofaUMTS PA.InordertomeetthestringentACLRspecificationforUMTS,ananalogpreǦdistorterblockisintegrated at the input of the main amplifier, realizing a single chip solution which reduces the cost and space requiredonthephoneboard.

DesignMethodology InordertoachieveanIMD3cancellation,thethirdordercomponentsgeneratedattheoutputoftheAPD needtohaveanoppositeresponserespectivetothethirdordercomponentsgeneratedbytheclassǦBmain

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amplifier. In practice, this can be achieved if an opposite AMÇŚAM and AMÇŚPM responses are generated betweentheAPDandmainamplifier[11]. Figure1illustratestheschematicoftheproposedPA.AdualstageoutputmatchingnetworkconsistsofL3, C8,TL3andC9isproposedtotransformthe50ohmloadtotheoutputimpedanceofthemainamplifier.L3 representsbondwires,whereasTL3denotesatransmissionline.

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In HBT, spectral reÇŚgrowth is mainly contributed by its baseÇŚcollector parasitic capacitance Cbc [12]. Therefore, in this work we propose to utilize the base collector diode at the input of the driver, as an integral part of the circuit which generates an opposite phase (AMÇŚPM) response. The reverse bias capacitanceCbcÇŚreversebiasandforwardbiascapacitanceCbcÇŚforwardbiasareexpressedasfollows[13]:



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WhereCbc0isthecollectornjbasecapacitancewhenVCB=0,ɛ0isthecollectorbasejunctionbuiltinvoltage and nc is the grading coefficient of the collector base junction. In order to generate an opposite output phaseresponse,thecollectornjbasejunctionisforwardbiased.Theforwardbiasedcollectorbasecapacitance isexpressedby:

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Basedon(1)and(2)thepositiveandnegativephaseinsightineffecttoVCBcancelsofftheCbcnjreversebias with a single forward biased base collector diode integration, Cbcnjforwardbias. However, with the aid of two base collector diodes (Cbcnjforwardbias > Cbcnjreversebias) and L3, an opposite phase response (AM nj PM)isobservedattheoutputoftheAPD.ThesimulatedAMnjPMresponsesofthedevicesatlocationAand CisillustratedinFigure2.TheAPD’sphaseexpansionandmainamplifier’sphasecompressioncancelsout eachothercontributingtotheimprovementoftheIMD3performance. On the other hand, generation of an opposite AMnjAM response is accomplished through the T section intermediate matching network, consists of C4, L2 and C5. The Smith plot of Figure 3 illustrates the locationoftheoutputimpedanceoftheAPDdenotedatpointA.PointBdescribestheinputimpedanceof the main amplifier. Based on the profile of Figure 4, matching towards point A which observes a gain expansioncompensatesthegaincompressionofthemainamplifier.

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Figure1.SchematicdiagramoftheUMTSPAwithbuiltinAPD.“A”denotestheoutputof theAPD,“C”istheoutputoftheclassǦBmainamplifier.“B”istheinputofthemainamplifier.



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Figure2.SimulatedAMǦPMresponseatlocationAandCrespectively.

 Figure3.LocationoftheimpedancepointoftheAPD(A)andmainamplifier(B).

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Figure4.AMǦAMprofileoftheAPDandthemainamplifier.

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SimulationResults

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TheSǦparametersoftheproposedUMTSPAwithsupplyheadroomof3.3VareshowninFigure5.TheS11 andS22arewellmatchedat1.95GHz,withacorrespondinggainS21of33.3dB.AlowS11indicatesthatthe APDdoesnotgenerateasevereinputmismatchlossatthefundamentalfrequency. 

 Figure5.MeasuredandsimulatedparametersofthePAwithasupplyvoltageof3.3V. Withmorethan30dBofpowergain,thePAisstillunconditionallystable.TheKǦfactorplotisillustratedin Figure6.FromDCto3GHz,KǦFactorisobservedtobemorethan1.

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Figure6.PAhasKǦFactor>1fromDCupto3GHz.

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ThepowergainacrossoutputpowerplotofthePAisshowninFigure7,whichmeasuresuptomaximum outputpowerof30.5dBm,or1.1W.The1dBcompressionpointofthePAisobservedtobe29.5dBm.

Figure7.GainvsoutputpowerplotofthePA.



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The resulting ACLR and PAE plot is shown in Figure 8. With a supply voltage of 3.3V, PA is capable to deliver PAE of 52.3% at output power of 28dBm, with a corresponding ACLR of Ǧ36dBc. The regulated specificationforACLRisǦ33dBc,asperstatedinthe3GPPspecifications. 

 Figure8.ACLRandPAEperformancesofthelinearizedPAat1.95GHz.

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InFigure9,thecontributionoftheproposedlinearizationtechniqueisapparent.TheACLRimproves14dB atoutputpowerof28dBm. 

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Figure9.ACLRplotbeforeandafterlinearization.

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In Figure 10, the ACLR spectrum at the maximum linear output power of 28dBm before and after linearizationisillustrated. 

 Figure10.Spectrumplotat28dBmbeforeandafterlinearization.

Conclusion AnovellinearizertopologytoimprovethemaximumlinearoutputpowerofaUMTSPAisintroduced.The integrated analog predistorter (APD) which provides a significant improvement in the ACLR does not jeopardize the input return loss, gain and stability of the PA, which are critical in constructing a reliable transmitter.Atoutputpowerof28dBm,PAdelivers52.3%PAE,meetingtheACLRspecificationsforUMTS. Thesinglechipsolutionsavesthedesigncosttoo.Theseresultshighlightthepotentialapplicationofthe proposed PA in a handset transmitter system, which favors a low voltage operation, thus prolonging the batterylife.

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Acknowledgement ThisresearchissupportedbytheeScienceFund03Ǧ01Ǧ03ǦSF0783fromtheMinistryofScience,Technology andInnovations.

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