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Analog Design Essentials

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2012-05-08 0人阅读 0 0 0 暂无简介 举报

简介:本文档为《Analog Design Essentialspdf》,可适用于IT/计算机领域

ANALOGDESIGNESSENTIALSANALOGDESIGNESSENTIALSbyWillyMCSansenCatholicUniversity,Leuven,BelgiumACIPCataloguerecordisavailablefromtheLibraryofCongressPublishedbySpringer,POBox,AADordrecht,TheNetherlandswwwspringercomPrintedonacidfreepaperAllRightsReservedNopartofthispublicationmaybereproduced,storedinaretrievalsystem,ortransmittedinanyformorbyanymeans,electronic,mechanical,photocopying,microfilming,recordingorotherwise,withoutwrittenpermissionfromthePublisher,withtheexceptionofanymaterialsuppliedspecificallyforthepurposeofbeingenteredandexecutedonacomputersystem,forexclusiveusebythepurchaseroftheworkPrintedintheNetherlands©SpringerISBN(HB)ISBN(HB)ISBN(ebook)ISBN(ebook)DedicationThisbookisdedicatedtomywifeHadewychHammeneckerContentsChapter#ComparisonofMOSTandbipolartransistorsChapter#Amplifiers,sourcefollowersandcascodesChapter#DifferentialvoltageandcurrentamplifiersChapter#NoiseperformanceofelementarytransistorstagesChapter#StabilityofoperationalamplifiersChapter#SystematicdesignofoperationalamplifiersChapter#ImportantopampconfigurationsChapter#FullydifferentialamplifiersChapter#DesignofmultistageoperationalamplifiersChapter#CurrentinputoperationalamplifiersChapter#RailtorailinputandoutputamplifiersChapter#ClassABanddriveramplifiersChapter#FeedbackvoltageandtransconductanceamplifiersChapter#FeedbacktransimpedanceandcurrentamplifiersChapter#OffsetandCMRR:randomandsystematicChapter#BandgapandcurrentreferencecircuitsChapter#SwitchedcapacitorfiltersChapter#DistortioninelementarytransistorcircuitsChapter#ContinuoustimefiltersChapter#CMOSADCandDACprinciplesChapter#LowpowersigmadeltaADconvertersChapter#DesignofcrystaloscillatorsChapter#LownoiseamplifiersChapter#CouplingeffectsinmixedanalogdigitalICsIndexofsubjectsAnalogdesignisartandscienceatthesametimeItisartbecauseitrequirescreativitytostriketherightcompromisesbetweenthespecificationsimposedandtheonesforgottenItisalsosciencebecauseitrequiresacertainlevelofmethodologytocarryoutadesign,inevitablyleadingtomoreinsightinthecompromisestakenThisbookisaguidethroughthiswonderfulworldofartandscienceItclaimstoprovidethenovicedesignerswithallaspectsofanalogdesign,whichareessentialtothisunderstandingAsteachingisthebestwaytolearn,allslidesareaddedonaCDROM,withandwithoutthecommentsaddedasnotesinthepdffilesThereaderissuggestedtotrytoexplainpartsofthiscoursetohisfellowdesignersThisisthewaytoexperienceandtocultivatethecirclesofartandscienceembeddedinthisbookAlldesignisaboutcircuitsAllcircuitscontaintransistorsHandmodelsarerequiredofthesedevicesinordertobeabletopredictcircuitperformanceCADtoolssuchasSPICE,ELDO,SPECTRE,etcarethenusedtoverifythepredictedperformanceThisfeedbackloopisessentialtoconvergetoarealdesignThisloopwillbeusedcontinuouslyinthisbookForthedesignofanalogintegratedcircuits,weneedtobeabletopredicttheperformancebymeansofsimpleexpressionsAsaresult,simplemodelsarerequiredThismeansthatthesmallsignaloperationofeachtransistormustbedescribedbymeansofasfewequationsaspossibleClearlytheperformanceofthecircuitcanthenonlybedescribedinanapproximatewayThemainadvantagehowever,isthattransistorChapter#sizingandcurrentlevelscaneasilybederivedfromsuchsimpleexpressionsTheycanthenbeusedtosimulatethecircuitperformancebymeansofaconventionalcircuitsimulatorsuchasSPICEorELDOInthesesimulators,modelsareusedwhicharemuchmoreaccuratebutalsomuchmorecomplicatedThesesimulationsarerequiredafterwardstoverifythecircuitperformanceTheinitialdesignwithsimplemodelsisthefirststepinthedesignprocedureTheyareaimedindeedatthedeterminationofalltransistorcurrentsandsizes,accordingtothespecificationsimposedWestartwithMOSTdevices,althoughthebipolartransistorarehistoricallyfirstNowadaysthenumberofMOStransistorsintegratedonchips,vastlyoutnumberthebipolaronesIndeed,previouslyCMOSdeviceswerereservedforlogicastheyofferthehighestdensity(ingatesmm)MosthighfrequencycircuitrywascarriedoutinbipolartechnologyAsaresult,alotofanalogfunctionswererealizedinbipolartechnologyThehighestfrequencycircuitshavebeenrealizedinexotictechnologiessuchasGaAsandnowInPtechnologiesTheyarequiteexpensivehoweverandreallyreservedforthehighfrequencyendThechannellengthofCMOStransistorsshrinkscontinuouslyhoweverIn,achannellengthofmicrometerisstandardbutseveralcircuitsusingnmhavealreadybeenpublished(seeISSCC)ThiseverdecreasingchannellengthgivesrisetoeverincreasingspeedsAsaresult,CMOSdevicesarecapableofgainateverhigherfrequenciesTodayCMOSandbipolartechnologiesareincompetitionoverawidefrequencyregion,extendingallthewaytoandevenGHz,aspredictedinthisslideForthesefrequenciesthequestionisindeed,whichtechnologyfulfillsbestthesystemandcircuitrequirementsatareasonablecostBICMOSisalwaysmoreexpensivethanstandardCMOStechnologyThequestionis,whethertheincreaseincostcompensatestheincreaseinperformanceComparisonofMOSTandbipolartransistorsThiseverdecreasingchannellengthhasbeenpredictedbytheSIAroadmapIttriestopredictwhatthechannellengthwillbeinafewyears,byextrapolatingthepastevolutionItisclearhowever,thattheshrinkingofthechannellengthhasbeencarriedoutmuchfasterthanpredictedForexample,thenmtechnologywasoriginallyexpectedonlyin,butwasalreadyofferedinThistechnologywasexpectedtoallowmilliontransistorstobeintegratedononesinglechipPresentdayprocessorsandmemoriesofferdoublethatamountMoreover,thistechnologywasexpectedtogiverisetoclockspeedsaroundGHzHighendPC’salreadyclockspeedsbeyondGHz!ThiseverdecreasingchannellengthhasalsobeenpredictedbythecurveofMooreThisissimplyasketchofchannellengthversustimeItisagraphicrepresentationofthenumbersoftheSIAroadmapIndeednmisreachedin!TheslopeofthatcurvehasnotalwaysbeenthesameIndeed,theslopewashigherintheearlyeighties,buthasdeclinedabitasaresultofeconomicrecessionsAlso,thecostoftheproductionequipmentandthemaskmakinggrowsexponentially,delayingtheintroductionofevernewertechnologiessomewhatChapter#WhicharethemostusedchannellengthstodayToexplorethis,thenumberofpapersisshownofthelastIEEEISSCCconference(heldatSanFranciscoinFebruary)fortwocategories,thedigitalcircuitsandtheanalogorRFcircuitsItisclearthatthedigitalcircuitspeakatnmchannellength,whereastheanalogoneslagbehindbytwogenerationstheypeakataboutnmIndeedforsmallquantities,siliconfoundriesoffersiliconathighercostifthechannellengthsaresmallerThisisclearlyillustratedbythiscostofaMultiProjectWaferchipversuschannellengthInsuchaMPWrun,manydesignsareassembledandputtogetherinonesinglemaskandrunAsaresultthetotalcostisdividedoverallparticipantsofthisrunThishasbeenthesourceofcheapsiliconformanyuniversitiesandfablessdesigncentersThecostin$mmisreasonableuptoaboutmmFrommmonthecostincreasesdramatically,deprivingmanyuniversitiesfromcheapsiliconWhatthecostwillbeofnmandnmiseasilyfoundbyextrapolation!Thisshowsveryclearlythatacrisisisathand!LetushaveacloserlooknowataMOSTdeviceWhatarethemainparametersinvolved,andwhatarethesimplestpossiblemodelequationsthatstilldescribethetransistormodelsinanadequatewayforhandanalysisComparisonofMOSTandbipolartransistorsThecrosssectionofaMOStransistorisshownwithitslayoutOntheleft,theMOSTisshownwithoutbiasingOntheright,voltagesareappliedtoGateandDrainThemaindimensionsofaMOSTaretheLengthandWidthBotharedrawndimensionsonthemaskInpracticetheyareusuallyabitsmallerThisisaresultofunderdiffusionandsomemoretechnologicalstepsInthislayouttheWLonthemaskisaboutApplicationofapositivevoltageattheGateVGScausesanegativelychargedinversionlayer,whichconnectstheSourceandDrainnislandsItisaconductingchannelbetweenSourceandDrainandthusactsasaresistorbetweenSourceandDrainApplicationofapositivevoltageattheDrainVDS,withrespecttotheSource,allowssomecurrenttoflowfromDraintoSource(orelectronsfromSourcetoDrain)ThiscurrentisIDSAsaresult,thechannelbecomesnonhomogeneousItconductsbetterontheSourcesidethanontheDrainsideThechannelmayevendisappearontheDrainsideNevertheless,theelectronsalwaysmanagetomakeittotheotherside,becausetheyhaveacquiredsufficientspeedalongthechannelZoominginonthechannelregion,disappearsonceVDSistoohighThechannelregion,togetherwiththetwonislandsofSourceandDrain,areenvelopedbyanisolationlayerIndeed,inapnjunctionthepandnregionsarealwaysseparatedbyanisolationregion,whichiscalleddepletionregionThesiliconisdepletedofelectronsorholesitisnonconducting,itisanisolator,verymuchasoxideanisolatorisTheoxidehasathicknesstox,whereasthedepletionlayerhasathicknessoftsiBothgiverisetocapacitancesCoxandCD,respectivelyBothhavedimensionsFcm−NormallyCDisaboutonethirdofCoxaswewillcalculateindetailonthenextslideTheirratioisnTsividisChapter#Itismportant,however,tonotethatthechannelinversionlayeriscoupledtotheGatebymeansofCox,butasmuchcoupledtotheBulkbyCDChangingtheGatevoltagewillthuschangetheconductivityofthechannelandhencethecurrentIDSInasimilarway,changingtheBulkvoltagewillthusalsochangetheconductivityofthechannelandwillthuschangethecurrentIDSaswellThetopgategivestheMOSToperation,whereasthebulkgivesJFEToperationIndeed,aJunctionFETisbydefinitionaFETinwhichthecurrentiscontrolledbyajunctioncapacitanceAllMOSTdevicesarethusparallelcombinationsofMOSTsandJFETsWenormallyuseonlytheMOSTwhereastheJFETiscalledthebodyeffect,andistreatedasaparasiticeffectThewidthofthedepletionregiondependstoalargeextentondopinglevelsandthevoltageacrossitThelargerthedopinglevelsonbothsidesofthejunction,thenarrowerthedepletionregionisOntheotherhand,thelargerthevoltageisacrossthedepletionregion,thewiderthisregionbecomes,asshownbytheequationItincludesthesilicondielectricconstantesi,thejunctionbuiltinvoltageQ,thechargeofanelectronqandthebulkdopinglevelNBValuesaregiveninthisslideForexampleforammtechnology,adrainbulkvoltageVBDyieldsadepletionlayerthicknessofaboutnmItisabouttimesthickerthanthegateoxideThisisoffsetsomewhatbythefactthatthesilicondielectricconstantisthreetimeshigherthantheoxideoneSiliconisthreetimesmoreefficienttomakecapacitorswiththanoxideSiliconcapacitancesareverynonlinearbecausetheydependonthevoltage,whereasoxidescapacitancesdonotTheration−isthenaboutMostvaluesofnareindeedbetweenand,dependingonthevalueoftsiParameternisthusneverknownaccuratelyasitdependsonbiasingvoltagesNotethatallcapacitancesareinFcmForaGateareaofWLof××mmthetotalGateoxidecapacitanceisthusCoxWL#fF,whichisquiteasmallvalueindeed!ThebulkdopinglevelNBisnotthesameforanMOSTandapMOSTdeviceIndeednormallynMOSTdevicesareimplementeddirectlyonthepsubstrateThissubstrateisthuscommontoallnMOStransistorsonthatchipThepMOStransistorhasapchannelhoweverandhastobeimplementedinantubornwell,whichisalwayshigherdopedthanthecommonpsubstrateThedisadvantageisthatthebulkdopingforapMOSTisalwayshigherthanthebulkdopingofanMOSTThepMOSTComparisonofMOSTandbipolartransistorsCDwillbehigherandsoisitsnfactorTheadvantageofapMOSThoweveristhatitsbulkisisolatedfromthesubstrateandcanbeusedtocontrolthetransistorcurrentIDSSuchpMOSTdeviceshavetwogates,ieatopgateandabottomgateBothcanbedrivenindependentlyMosttechnologiesarenwellCMOStechnologiesalthoughsomepwellonesarestillaroundApplicationofapositiveGatevoltageVGScausesaninversionlayer(orchannel)whichconnectsSourcetoDrainApplicationofapositivevoltageVDScausessomecurrentIDStoflowfromDraintoSourceNowwewanttofindsimpleexpressionsforthiscurrent,sothatwecanusethemfordesignpurposesThecurveofIDSversusVGSissketchedontheleftThecurrentstartsflowingassoonasVGSexceedsVT,calledthethresholdvoltageForlargervaluesofVGS,thecurrentincreasesinanonlinearwayHowmuchweactuallyexceedVTisVGS−VTthiswillbethemostimportantdesignparameterlateron!ThecurveofIDSversusVDSissketchedontheleftForsmallvaluesofVDS,thecurrentincreaseslinearlyIndeed,thetransistorbehavesasaresistorThisiscalledthelinearregionForlargervaluesofVDSthecurrentstopsincreasingbutlevelsofftowardsnearlyconstantvalues:thecurrentissaidtosaturateThisiscalledthesaturationregionCurvesaregivenforfourdifferentvaluesofVGSThelinearandsaturationregionsareseparatedbyaparabola,whichisdescribedbyVDS=VGS−VTWewillconcentrateonthelinearregionfirstChapter#InmanyapplicationsaMOSTissimplyusedasaswitchItsvoltageVDSisthenverysmallTheMOSTisthenoperatinginthelinearregion(sometimescalledtheohmicregion)InthisregiontheMOSTtransistorisreallyasmallresistorItprovidesalinearvoltagecurrentcharacteristicThechannelhasthesameconductivityatbothsides–theSourcesideandtheDrainsideLetusinvestigatewhattheactualresistancethenisZoominginonthecorner,forverysmallvaluesofVDS,wefindthatindeedtheIDS−VDScurvesareverylinearTheMOSTbehavesasapureresistorTheresistancevalueRonisgiveninthisslideInadditiontothedimensionsWandL,atechnologicalparameterappears,calledKPThisparametercharacterizesacertainCMOStechnologyaswillbeexplainedonthenextslideItsdimensionisAVItisclearthatthetransistorturnsverynonlinearwhenweapplylargerVDSvoltagesThecrossovervaluetowardsthesaturationisreachedforVDS=VGS−VT,ormoreaccuratelyforVDS=(VGS−VT)nWewilldropthisfactornhowever,asakindofsafetyfactorWewill,fromnowon,assumethatatransistorisoperatinginthesaturationregionprovidedVDS>VGS−VTForsakeofillustrationletusahaveacloserlookatthisresistor‘‘inthecorner’’ForthispurposewehavetofindaneasyapproximationforKPItisgiveninthisslideFactorb(Greekbeta)containsboththeparameterKPandthedimensionsoftheresistorWandLActually,KPcontainstheoxidecapacitanceCox,andthemobilitym(Greekmu)ThisfactorComparisonofMOSTandbipolartransistorsshowswhatspeed(cms)anelectroncandevelop,subjecttoanelectricfield(Vcm)ItisgivenincmVsElectronstravelabouttwiceasfastasholesValuesforastandardmmCMOStechnologyaregiveninthisslideNotethattheoxidethicknessisaboutL,ashasbeencheckedonmoststandardCMOSprocessesofthelastyearsAsaruleofthumb,theresistorofasquaretransistor(WL=)foradrivevoltageVGS−VT=VisaboutkVinmmCMOSFordeepersubmicronCMOStechnologies,KPishigherbecauseofCoxThissquareresistornowdecreases!TohaveatimeconstantofnswithpFweneedaswitchresistanceofVThiswill,toalargeextent,dependonthevalueofVGS−VTusedIndeed,assoonastheswitchturnson,theoutputvoltageisstillatzeroVoltandVGS−VT=VAttheendoftheswitchingin,theoutputvoltagehasrisentoV:ithasbecomethesameastheinputvoltageTheVGS−VThasdecreasedbyVtowardsVGS−VT=VTheaveragevalueisnowVGS−VT=VForatransistorsizeWL=,theonresistanceisthuskV(usingKP=mAV)Thisis×largerthanwhatwecanallowWethushavetotakeaWLofNotethatwewillhavegreatdifficultiesinswitchinglargeinputvoltagesIndeed,forvOUT=vIN=V,theVGShasbecomezeroAsaresult,theresistorhasbecomeinfinity:theswitchcannotbeswitchedonanymore!!NotealsothatwehaveforgottentotakeintoaccountthebulkeffectIndeed,VBSisnotzero,itisVTheparasiticJFETwillplayaswellaswewillseelaterChapter#ForatransistorsizeWL=,theKP×WLproductis×−S(usingKP=mAV)TakingtheswitchasaresistorwithvalueRon,asshowninthisslideandsubstitutionofRonbyitsexpression,requiresaniteration,whichyieldsavalueofRonofVandanoutputvoltageofVNotethatwehaveforgottentotakeintoaccountthebulkeffectIndeed,VBSisnotzero,itisaboutVTheparasiticJFETwillbecomeactiveaswell,aswewillseenextThedrainsourcecurrentIDSandthechannelresistanceRonshowtheinfluenceofVGSinanexplicitway,butnotthatofthebulksourcevoltageVBSIndeed,theeffectofVBSisembeddedinthethresholdvoltageVTIncreasingtheVBSwillincreasethedepletionlayerwidthunderthechannelandwillincreaseVTMorereversebiasingthatjunctionwillincreaseVTinabsolutevalueanddecreasethecurrentForzeroVBS,VTevidentlyequalsVTParameterc(Greekgamma)hastodowiththejunctiondepletionregionandislinkedtoparameternActually,factorcdependsonthetechnologyused(suchasthebulkdopingNB)butisnotvoltagedependentThedenominatorofnnowshowsexplicitlythevoltagedependenceofnSomeapproximateparametervaluesaregivenaswellforammCMOSComparisonofMOSTandbipolartransistorsForatransistorsizeWL=,theKP×WLproductisagain×−S(usingKP=mAV)TakingtheswitchasaresistorwithvalueRon,asshowninthisslide,andsubstitutionofRonbyitsexpressionrequiresanotheriteration,asnowVTdependsontheoutputvoltageThisyieldsavalueofRonwhichisnowlargerItisV,insteadofVAlso,theoutputvoltageisalittlebitlowerItisVinsteadofVThetimeconstantisnowsimplytheproductofVandpF

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