WideBand CMOS LowNoise Amplifier CMOS宽频低噪声放大器的设计.docx
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WideBandCMOSLowNoiseAmplifierCMOS宽频低噪声放大器的设计
Wide-BandCMOSLow-NoiseAmplifier
Abstract
Knownelementarywide-bandamplifierssufferfromafundamentaltradeoffbetweennoisefigure(NF)andsourceimpedancematching,whichlimitstheNFtovaluestypicallyabove3dB.Globalnegativefeedbackcanbeusedtobreakthistradeoff,however,atthepriceofpotentialinstability.Incontrast,thispaperpresentsafeedforwardnoise-cancelingtechnique,whichallowsforsimultaneousnoiseandimpedancematching,whilecancelingthenoiseanddistortioncontributionsofthematchingdevice.Thisallowsfordesigningwide-bandimpedance-matchingamplifierswithNFwellbelow3dB,withoutsufferingfrominstabilityissues.Anamplifierrealizedin0.25-mstandardCMOSshowsNFvaluesbelow2.4dBovermorethanonedecadeofbandwidth(i.e.,150–2000MHz)andbelow2dBovermorethantwooctaves(i.e.,250–1100MHz).Furthermore,thetotalvoltagegainis13.7dB,the3-dBbandwidthisfrom2MHzto1.6GHz,theIIP2is+12dBm,andtheIIP3is0dBm.TheLNAdrains14mAfroma2.5-Vsupplyandthedieareais0.30.25mm2.
IndexTerms—Broadband,distortioncanceling,low-noiseamplifier(LNA),noisecanceling.
I.INTRODUCTION
WIDE-BANDlow-noiseamplifiers(LNAs)areusedinreceivingsystemswheretheratiobetweenbandwidth(BW)anditscenterfrequencycanbeaslargeastwo.Applicationexamplesareanalogcable(50–850MHz),satellite(950–2150MHz),andterrestrialdigital(450–850MHz)videobroadcasting.Moreover,awide-bandLNAcanreplaceseveralLC-tunedLNAstypicallyusedinmultibandandmultimodenarrow-bandreceivers.Awide-bandsolutionsaveschipareaandfitsbetterwiththetrendtowardsflexibleradioswithasmuchsignalprocessing(e.g.,channelselection,imagerejection,etc.)aspossibleinthedigitaldomain(toward“softwareradio”).
High-sensitivityintegratedreceiversrequireLNAswithsufficientlylargegain,noisefigure(NF)wellbelow3dB,adequatelinearity,andsourceimpedancematching.ThelatteristoavoidsignalreflectionsonacableoralterationsofthecharacteristicsoftheRFfilterprecedingtheLNA,suchaspass-bandrippleandstop-bandattenuation[1].Theserequirementsmustbeachievedoverawiderangeoffrequencieswhilealsoallowingsomevariablegaintohandleinterferencegeneratedbystrongadjacentchannels.
Traditionalwide-bandLNAsbuiltofMOSFETsandresistorshavedifficultiesinmeetingtheabove-mentionedrequirements.Knownelementaryamplifiers[2],[3]failtoachievelowNFupon.Ontheotherhand,amplifiersexploitingglobalnegativefeedbackmightachievelowNFwith,buttheyarepronetoinstability[4].Inthispaper,athermal-noisecancelingtechniqueispresentedthatallowsfordesigningLNAswithlowNFandsourceimpedancematchingoverawiderangeoffrequencieswithoutinstabilityproblems.Inearlierwork[2],[5],alimitedformofnoisecancellationwasalreadypresented.However,itdoesnotallowforlowNFdBupon.Incontrast,thetechniquepresentedinthispapercanreachmuchlowerNF,aswasvalidatedthroughthedesignofasub-2-dBnoisefigurewide-bandLNAina0.25-mCMOS[6].Thispaperanalyzesthenoise-cancelingtechniqueanditspropertiesindepth.
Thepaperisorganizedasfollows.SectionIIreviewsexistingwide-bandCMOSlow-noisetechniques.SectionIIIintroducesthenoise-cancelingtechnique.SectionIVanalyzespropertiesandlimitationsofnoisecanceling.SectionVdescribestheICdesignofawide-bandCMOSLNA.SectionVIdealswiththemeasurements.Finally,SectionVIIpresentstheconclusions.
II.REVIEWOFEXISTINGTECHNIQUES
Inthissection,commonwide-bandCMOSlow-noisetechniquesarereviewedinordertohighlighttheirNFlimitations.AMOSFETinsaturationismodeledasavoltage-controlledcurrentsourcewithtransconductance.
Fig.1(a)–(e)showsknownelementarywide-bandamplifierscapableofmatchingarealsourceimpedance(biasingnotshown).TheseamplifierssufferfromafundamentaltradeoffbetweentheirnoisefactorNFandimpedancematching.
Thetradeoffbetweenandsourceimpedancematchingcanbebroken,exploitingnegativefeedbackproperly.Fig.1(g)showsacommonlyusedwide-bandfeedbackamplifiercapableofalowupon.Inthiscase,thefeedbackresistordeterminestheminimumnoisefactor2).Thelattercanbewellbelow2(i.e.,3dB),providedadequategainisavailable.Despiteitsnoiseperformance,thisamplifiersuffersfromimportantdrawbacks,asfollows,motivatingthesearchforalternatives.
•Sufficientgainandgigahertzbandwidthoftenmandatetheuseofmultiplecascadedstageswithinthefeedbackloop[2inFig.1(g)],makingitsoperationpronetoinstability.
•For,theopen-loopgainislowerthan1.Thus,theclosed-looplinearityisnotmuchbetterthanthatoftheloopamplifierA.IfAconsistsofcascadedstagesandmostofthegainisinthefirstone(i.e.,tooptimizenoise),linearitycanbepoor[4].
•dependsonand,soitissensitivetoprocessvariations.Next,andaredirectlycoupledandvariablegainatisnotstraightforward.
III.NOISE-CANCELINGTECHNIQUE
Inthissection,awide-bandlow-noisetechniqueispresented,whichisabletodecouplefromwithoutneedingglobalnegativefeedbackorcompromisingthesourcematch.Thisisachievedbycancelingtheoutputnoiseofthematchingdevicewithoutdegradingthesignaltransfer.
A.NoiseCancelingPrinciple
Thisisdonebycreatinganewoutput,wherethevoltageatnodeYisaddedtoascalednegativereplicaofthevoltageatnodeX.Apropervalueforthisscalingfactorrendersnoisecancelingattheoutputnode,forthethermalnoiseoriginatingfromthematchingdevice.Fig.3(a)showsastraightforwardimplementationusinganidealfeedforwardvoltageamplifierAwithagain(with).Bycircuitinspection,thematchingdevicenoisevoltagesatnodeXandYare:
(1)
Theoutputnoisevoltageduetothenoiseofthematchingdevice:
(2)
Outputnoisecancellation,,isachievedforagain:
(3)
wheretheindexdenotesthecancellation.Ontheotherhand,signalcomponentsalongthetwopathsaddconstructively,leadingtoanoverallgain:
(4)
From(3),twocharacteristicsofnoisecancelingareevident.
•Noisecancelingdependsontheabsolutevalueoftherealimpedanceofthesource,(e.g.,theimpedanceseen“lookinginto”aproperlyterminatedcoaxcable).
•Thecancellationisindependentonandonthequalityofthesourceimpedancematch.Thisisbecauseanychangeofequallyaffectsthenoisevoltages
Fig.3(b)showsanelementaryimplementationofthenoisecancelingLNAinFig.3(a).AmplifierAandtheadderarereplaced
withthecommon-sourcestageM2–M3,renderinganoutputvoltageequaltothevoltageatnodeXtimesthegain.TransistorM3alsoactsasasourcefollower,copyingthevoltageatnodeYtotheoutput.Thesuperpositionprinciplerendersthefinaladditionofvoltageswithanoverallgain.
Notethatanysmallsignalthatcanbemodeledbyacurrentsourcebetweenthedrainandsourceofthematchingdeviceiscancelledaswell(e.g.,noise,thermalnoiseofthedistributedgateresistance,andthebiasnoisecurrentinjectedintonodeY).However,thenoiseofRisnotcancelled.
B.NoiseFactor
ThenoisefactorFoftheLNAinFig.3(a)canbewrittenas:
(5)
wheretheexcessnoisefactorEFisusedtoquantifythecontributionofdifferentdevicesto,whereindexreferstothematchingdevice,totheresistor,andtoamplifierA.FortheimplementationinFig.3(b),expressionsforEFforare(assumingequalNEF)
(6)
Uponcancellation,(6)becomes
(7)
Thenoisefactoratcancellation,,isthusonlydeterminedbyEFandEF,neitherofwhichareconstrainedbythematchingrequirement.EFcanbemadearbitrarilysmallerthan1byincreasingofitsinputstage,atthepriceofpowerdissipation.
TheLNAconceptinFig.3(a)wassimulatedusingMOSmodel9ina0.25-mCMOSprocessusinganidealnoiselessamplifierA(i.e.,avoltage-controlledvoltagesource).ThematchingstageprovideswithandavoltagegainofdB.Fig.3(c)showsthetransferfunctionfromtotheLNAoutput(rightaxis)versus.Itisevaluatedat1GHz,whichismore
thanafactoroftenbelowthe3-dBbandwidthofthematchingstage.Thisnoisetransferiszerofor,meaningthatthenoisefromthematchingdevicecancelsattheoutput.Ontheotherhand,thenoisetransferrisesforduetoimperfectcancellation.Fig.3(c)alsoshowsthesimulatedNFversusat1GHz(leftaxis).TheNFdropsfromamaximumof6dBfor,(i.e.,NFofthematchingstagestandalone)toNFdBfor(i.e.,thecontributionof),whichisveryclosetothevaluepredictedfrom(3)and(7).
C.Generalization
TheconceptofnoisecancelingcanbegeneralizedtoothercircuittopologiesaccordingtothemodelshowninFig.4(a).Itconsistsofthefollowingfunctionalblocks:
1)anamplifierstageprovidingthesourceimpedancematching,;2)anauxiliaryamplifiersensingthevoltage(signalandnoise)acrosstherealinputsource;and3)anetworkcombiningtheoutputofthetwoamplifiers,suchthatnoisefromthematchingdevicecancelswhilesignalcontributionsadd.
Fig.4(b)showsanotherimplementationexample(biasingnotshown)amongseveralalternatives[9].Noisecancellationoccursfor,whilelowrequireshigh.The2-MOSFETsinFig.4(b)isawell-knowntransconductor[10],alsousedforadouble-balancedactivemixer[11].However,inbothcases,noisecancelingwasapparentlynotrecognized.Asshownintheprevioussection,thenoise-cancelingtechniqueiscapableofNFwellbelow3dBupon.Moreover,itoffersadvantagescomparedtofeedbacktechniques.
•Itis