Doherty功率放大器英文.docx

Doherty功率放大器英文.docx

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Doherty功率放大器英文

TheDohertyPowerAmplifier

Thecurrentwirelesscommunicationsystemshavemadesignificantprogresstowardincreasedbandwidthandnumberofcarriersforhigh-data-rateapplications.Memoryeffects,however,makeitverydifficulttodesignahigh-poweramplifierwithawideinstantaneousbandwidth.Inadditiontobandwidthconcerns,theinstantaneoustransmitpowersofthewirelesscommunicationsystems,suchasCDMA-2000,wide-bandcodedivisionmultipleaccess（WCDMA）,orthogonalfrequencydivisionmultiplexity（OFDM）andsoon,varywidelyandrapidly,carryinghighpeak-to-averageratio（PAR）signals.ThebasestationpoweramplifiersforthesystemsrequireahighlinearitytoamplifythehighPARsignalsourcewithoutdistortion.Tosatisfylinearityrequirements,thepoweramplifiersareusuallybiasedatclassAorABmodeandmustoperateatalargeamountofback-offfromthepeakoutputpower.Anotherrequirementofthebasestationpoweramplifierforthemodernwirelesscommunicationsystemsishighefficiency.Asthecommunicationsystemsarereducedinbothsizeandcost,thecoolingsystemshouldbesimpleandsmall,requiringapoweramplifierwithhighefficiency.Becausethebasestationpoweramplifiershavealowefficiencyduetotheback-offoperation,efficiencyenhancementtechniquesbecomeveryimportant.Thedesigntechniqueofthebasestationpoweramplifierswithhighefficiencyandlinearityacrossawideinstantaneousbandwidthhasbecomeahotissue.

Inthisarticle,weshowthattheDohertyamplifieriscapableofdeliveringthestringentrequirementsofthebasestationpoweramplifiers.Weexplaintheoperationprinciples,includingbothlinearityandefficiencyimprovements,andthebasiccircuitconfigurationoftheamplifier.Advanceddesignmethodstooperateacrosswidebandwidthandimprovethelinearityarealsodescribed.Forverification,theDohertyamplifierisimplementedusinglaterallydiffusedmetaloxidesemiconductor（LDMOS）transistorsandmeasuredusingaWCDMA4FAsignal.TheseresultsshowthattheDohertyamplifierisapromisingcandidateforbasestationpoweramplifierswithwidebandwidth,highefficiency,andlinearity.

DohertyAmplifierOperation

Figure1.（a）OperationaldiagramoftheDohertyamplifier.（b）Fundamentalcurrents.（c）Loadimpedances.

TheDohertyamplifierwasfirstproposedbyW.H.Dohertyin1936.TheoriginalDohertyamplifierconsistedoftwotubeamplifiersandanimpedanceinvertingnetwork.TheefficiencyofanRFpoweramplifierisincreasedusingtheRFDohertyamplifiertechnique,asdescribedindetailin.Thisamplifierconsistedofacarrieramplifierandapeakingamplifier.Theoutputloadisconnectedtothecarrieramplifierthroughanimpedanceinverter（aquarter-wavetransmissionline）anddirectlytothepeakingamplifier.Figure1（a）showsanoperationaldiagramtoanalyzetheDohertyamplifiercircuit.Twocurrentsourcesrepresenttheamplifiers.Itisassumedthateachcurrentsourceislinearlyproportionaltotheinputvoltagesignal,operatingasaclassABorclassBamplifierwithharmonicshortcircuitsafteritisturnedon,andtheefficiencyanalysiscanbecarriedoutusingthefundamentalanddccomponentsonly.AsshowninFigure1（b）,thepeakingamplifierturnsonatone-halfthemaximuminputvoltage.

TheDohertyamplifiertechniqueisbasedontheloadimpedancechangeofeachamplifier,referredtoasloadmodulation,accordingtotheinputpowerlevel.Figure1（b）showsthefundamentalcurrentsfromtheamplifiers.Theloadimpedancesoftwoamplifiersaregivenby

whereZListheloadimpedanceoftheDohertyamplifier;ICandIPrepresentthefundamentalcurrentsofthecarrierandpeakingamplifiers,respectively;andZCandZParetheoutputloadimpedancesofthecarrierandpeakingamplifiers,respectively,andaredepictedinFigure1（c）.

Inthelow-powerregion（0∼Vin,max/2）,thepeakingamplifierremainsinthecut-offstate,andtheloadimpedanceofthecarrieramplifieristwotimeslargerthanthatoftheconventionalamplifier.Thus,thecarrieramplifierreachesthesaturationstateattheinputvoltage（Vin,max）/2sincethemaximumfundamentalcurrentswingishalfandthemaximumvoltageswingreachesVdc.Asaresult,themaximumpowerlevelishalfofthecarrieramplifier’sallowablepowerlevel（aquarterofthetotalmaximumpoweror6dBdownfromthetotalmaximumpower）,andtheefficiencyoftheamplifierisequaltothemaximumefficiencyofthecarrieramplifierasshowninFigure2.

Inthehigh-powerregion（Vin,max/2∼Vin,max）,wherethepeakingamplifierisconducting,thecurrentlevelofthepeakingamplifierplaysanimportantroleindeterminingtheloadmodulationoftheDohertyamplifier[see

（1）and

（2）].Assumingthatgmofthepeakingamplifieristwiceaslargeasthatofthecarrieramplifier,thecurrentandvoltageswingsofthepeakingamplifierincreaseinproportiontotheinputvoltagelevelandthevoltageswingreachesthemaximumvoltageswingofVdconlyatthemaximuminputvoltage.Theloadimpedanceofthecarrieramplifiervariesfrom2ZopttoZopt,andthepeakingamplifiervariesfrom∞toZoptaccordingtotheinputvoltagelevelasshowninFigure1（c）.TheefficiencyoftheDohertyamplifieratthemaximuminputvoltageisequaltothemaximumefficiencyoftheamplifiers.Whenthepeakingamplifieristhesamesizeasthecarrieramplifier,whichisnormallythecase,gmofthetwoamplifiersareidenticalandthepeakingamplifiercannotbefullyturnedon,sothepowerperformanceisdegraded[4].Fromthebasicoperationprinciple,wehaveexploredtheDohertyamplifier,whichprovideshigherefficiencyoverwholepowerrangescomparedtotheconventionalclassABamplifiers.TheresultingDohertyamplifiercansolvetheproblemofmaintainingahighefficiencyforalargePARsignal.

LinearityoftheDohertyAmplifier

ThelinearityoftheDohertyamplifierismorecomplicatedthanthatofaclassABamplifier.TheclassABbiasedcarrieramplifierhasaloadimpedanceatthelowpowerlevelthatistwiceaslargeandthehighimpedanceofthecarrieramplifiercompensatesthelowgaincharacteristicduetotheinputpowerdivision.Athighpowerlevels,thetwoamplifiersgeneratefullpowerusingnormalloadimpedances,equalizingthepowergain.Additionally,inthelow-powerregion,thelinearityoftheamplifierisentirelydeterminedbythecarrieramplifier.Therefore,thecarrieramplifiershouldbehighlylineareventhoughtheloadimpedanceishigh

Atahighpowerlevel,linearityoftheamplifierisimprovedbytheharmoniccancellationfromthetwoamplifiersusingappropriategatebiases.Figure3showsthethird-orderharmonicgenerationcoefficientgm3ofanLDMOStransistorandthebiaspointsofthetwoamplifiers.Intermsofgaincharacteristicsofeachamplifier,alategainexpansionoftheclassCbiasedpeakingamplifiercompensatesthegaincompressionoftheclassABcarrieramplifier.Thus,theDohertyamplifier,whichisbasedontheloadmodulationtechnique,iscapableofdeliveringmorelinearoutputpowerthanaconventionalclassABpoweramplifier.Thethird-orderintermodulation（IM3）levelfromthecarrieramplifierincreasesandthephaseoftheIM3decreasesbecausethegainofthecarrieramplifieriscompressed.Incontrast,whenthegainofthepeakingamplifierisexpanded,boththeIM3levelandphaseincrease.TocancelouttheIM3sfromthetwoamplifiers,thecomponentsmustbe180◦outofphasewiththesameamplitudes.Therefore,thepeakingamplifiershouldbedesignedappropriatelytocanceltheharmonicsofthecarrieramplifier..

TheCircuitConfigurationofDohertyAmplifier

Figure4showsaschematicdiagramofthefullymatchedmicrowaveDohertyamplifierwithoffsettransmissionlinesattheoutputcircuits[5].Thecarrierandpeakingamplifiershaveinput/outputmatchingcircuits,whichtransformfromtheinputimpedancesofthedevicesto50andfromtheoptimumloadimpedanceZoptofthedevicesto50,respectively.Theadditionaloffsettransmissionlineswithcharacteristicimpedanceof50areconnectedafterthematchingcircuitsofthecarrierandpeakingamplifiers.Inthelow-powerregion,thephaseadjustmentsoftheoffsetlinescausethepeakingamplifiertobeopen-circuitedandthecharacteristicloadimpedanceofthecarrieramplifierisdoubledto2Robyaquarterwaveimpedancetransformer.ThisisillustratedinFigure5（a）and（b）.TheoffsetlineofthecarrieramplifiervariesfromZoptto2ZoptfortheproperloadmodulationasshowninFigure5（a）.Figure5（b）illustratesthattheoffsetlineofthepeakingamplifieradjuststothehighimpedancesothatitpreventspowerleakage.Figure5（c）showstheappropriatetransformationsonaSmithcharttodeterminetheoffsetlinelengthofeachamplifier.Thelinesdonotaffecttheoverallmatchingconditionandloadmodulationbecausetheyarematchedtothecharacteristicimpedanceof50.TheDohertyoutputcombiningcircuitconsistsofaquarter-wavetransmissionlinewiththecharacteristicimpedanceof50andaquarter-wavetransmissionlinethattransformsfrom50to25todeterminetheloadimpedanceoftheoutputcombiningcircuit.Aphasedelaylineisneededattheinputofthepeakingamplifiertoadjustthesamedelaybetweenthecarrierandpeakingamplifiers[6].

TheDohertyamplifierconsistsofaclassABbiasedcarrieramplifierandaclassCbiasedpeakingamplifier.Duetothedifferentbiasing,theRFcurrentfromtheamplifiersaredifferentdependingontheinputdrivelevel.TheasymmetricpowersarecombinedbytheDohertyoperationthroughaquarter-waveimpedanceconverter.

AdvancedDesignMethodsfortheDohertyAmplifier

The