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.Advanced
designmethodstooperateacrosswidebandwidthandimprovethelinearityarealsodescribed.Forverification,theDohertyamplifierisimplementedusinglaterallydiffusedmetaloxidesemiconductor(LDMOS)transistorsandmeasuredusingaWCDMA4FAsignal.TheseresultsshowthattheDohertyamplifierisapromisingcandidateforbasestationpoweramplifierswithwidebandwidth,highefficiency,andlinearity.
DohertyAmplifierOperation
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Figure1.(a)OperationaldiagramoftheDohertyamplifier.(b)
Fundamentalcurrents.(c)Loadimpedances.
TheDohertyamplifierwasfirstproposedbyW.H.Dohertyin1936.TheoriginalDohertyamplifierconsistedoftwotubeamplifiersandanimpedanceinvertingnetwork.TheefficiencyofanRFpoweramplifierisincreasedusingtheRFDohertyamplifiertechnique,asdescribedindetail
in.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^V\n,max/2),thepeakingamplifierremainsinthecut-offstate,andtheloadimpedanceofthecarrieramplifieristwotimeslargerthanthatoftheconventionalamplifier.Thus,thecarrieramplifierreachesthesaturationstateattheinputvoltage(Vin,max)/2sincethemaximumfundamentalcurrentswingishalfandthemaximumvoltageswingreachesVdc.Asaresult,themaximumpowerlevelishalfofthecarrieramplifiera'lloswablepowerlevel(aquarterofthetotalmaximumpoweror6dBdownfromthetotalmaximumpower),andtheefficiencyoftheamplifierisequaltothemaximumefficiencyofthecarrieramplifierasshowninFigure2.
Inthehigh-powerregion(Vin,max/27/in,max),wherethepeakingamplifierisconducting,thecurrentlevelofthepeakingamplifierplaysanimportantroleindeterminingtheloadmodulationoftheDohertyamplifier[see
(1)and
(2)].Assumingthatgmofthepeakingamplifieristwiceaslargeasthatofthecarrieramplifier,thecurrentandvoltageswingsofthepeakingamplifierincreaseinproportiontotheinputvoltagelevelandthevoltageswingreachesthemaximumvoltageswingofVdconlyatthemaximuminputvoltage.Theloadimpedanceofthecarrieramplifiervariesfrom2ZopttoZopt,andthepeakingamplifiervariesfromtoZoptaccordingtothe
inputvoltagelevelasshowninFigure1(c).TheefficiencyoftheDohertyamplifieratthemaximuminputvoltageisequaltothemaximumefficiencyoftheamplifiers.Whenthepeakingamplifieristhesamesizeasthecarrieramplifier,whichisnormallythecase,gmofthetwoamplifiersareidenticalandthepeakingamplifiercannotbefullyturnedon,sothepowerperformanceisdegraded[4].Fromthebasicoperationprinciple,wehaveexploredtheDohertyamplifier,whichprovideshigherefficiencyoverwholepowerrangescomparedtotheconventionalclassABamplifiers.TheresultingDohertyamplifiercansolvetheproblemofmaintainingahighefficiencyforalargePARsignal.
LinearityoftheDohertyAmplifier
ThelinearityoftheDohertyamplifierismorecomplicatedthanthatofaclassABamplifier.TheclassABbiasedcarrieramplifierhasaloadimpedanceatthelowpowerlevelthatistwiceaslargeandthehighimpedanceofthecarrieramplifiercompensatesthelowgaincharacteristic
duetotheinputpowerdivision.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-waveimpedanceconv