EE240 project_A Gain-Boosted 90-dB Dynamic Range F

AGain-Boosted90-dBDynamicRangeFastSettlingOTAwith7.8-mWPowerConsumptionYunChiu,KenWojciechowskiElectricalEngineeringandComputerScienceDepartmentUniversityofCalifornia,BerkeleyBerkeley,CA94720Abstract-Afullydifferentialhigh-dynamicrangeoperationaltransconductanceamplifier(OTA)tobeusedinswitched-capacitorfiltersand/orover-sampledA/Dconvertersispresented.Thearchitecturechosenforthisdesignisfoldedcascodewithfullydifferentialgainboosting.ItisdemonstratedthroughthedesignanalysisandHSPICEsimulationthatsuchastruc-turerealizesthebesttrade-offbetweenpowerconsumption,speed,anddynamicrangeperformancesforthisdesign.TheOTAachievesaconstantlargesignalDCgain(ADC)of84dBoverprocessandtemperaturevariations.Itisdesignedina0.35-μmCMOSprocessanddrawsaDCpowerof7.8mWfroma3-Vsupply.Theachievedrmsoutputnoisevoltage()is50.1μVfortheworstcasescenario.Withapeak-to-peakoutputvoltageswing(Vo,pp)of4.5V,theachievedoutputdynamicrange(DR)is90dB.TheOTAalsodemonstratesexcellentsettlingbehaviorwithasingle-poleroll-offfrequencyresponseandnoover-shootorringingisobservedattheoutputwhentherequiredmaximum4-Vtransientisappliedattheinput.Thesettlingtimeto0.05%accuracyfortheworstcaseis~19.5ns.1.INTRODUCTIONTOGAIN-BOOSTINGTECHNIQUEAsCMOSdesignscalesintolow-powerlow-voltageregime,designinganalogfunctionalblocksunderlimitedsupplyvoltagebecomesmoreandmoredifficult.Onetypicalexampleisthebasicgainstage.Cascodingisthemostlyusedtechniquetoachievehighgaincomparedto2-stagedesignsbecauseofitssuperiorfrequencyresponse.However,wequicklyrunintoheadroomproblemswhiletryingtocascodemoretransistorsinastackunderlimitedsupplyvoltage.Gain-boostingtechnique[1]wasintroducedtoremedythisproblem.ItallowsincreasingtheDCgainoftheoperationalamplifier(opamp)withoutsacrificingtheoutputswingofaregularcas-codestructure.Furthermore,ithasbeenpointedout[2]thatthegain-boostingtechniquedecouplestheDCgainandthefrequencyresponseoftheamplifier.Itisthereforepossibletoachievehighspeedandhighgainatthesametime.Thesefeaturesareespeciallydesirableinhigh-speed,high-dynamicrangeapplicationslikeswitched-capaci-torfiltersandA/Dconverters.Von,2In-VbpVbnFigure1CMOSgain-boostingtechniquesOut+VcnM2VcpIn+Out-VcnM1VcpVbpVbnOut+Out-Single-endedgainboostingFully-differentialgainboostingDiff.Pair2EE240,Spring2000FinalProjectReportTypicalgain-boostingstructuresareshowninFig.1.Thesingle-endedboostingarchitectureusesfourauxil-iaryopampstoregulatethetransconductance(gm)ofthecascodetransistorsandenhancesthegainrealizedbytheoriginalcascodingeffect.ItisfirstreportedbyK.BultandG.Geelen[1].However,thesingle-endedversionoftheboostersareassociatedwithsomeundesiredeffects,mainly,•Signalusuallytravelsalongerpathinsidetheboosterandseeanextrapolefromaninternalcurrentmirror.Therefore,frequencyresponseoftheboostersuffers,especiallywhenoptimizingthepole-zerodoubleteffect.•Noisesgeneratedbythebiasingcircuitryinsidetheboostersarenotcorrelated.Thismeansthenoiseover-headassociatedwithgainboostingishigher.Theproblemsareaddressedbyusingafullydifferentialgain-boostingscheme.Thisimprovesthesettlingbehaviorandreducesnoiseaswell.Butfullydifferentialamplifierrequirescommon-modefeedback.Inthisdesign,aninputcommon-modevoltagecontrolcircuitisadoptedandwillbeillustratedindetailslater.2.AMPLIFIERDESIGN2.1.OptimumArchitectureThreecandidatesareavailable,i.e.,telescopicdifferentialOTAwithgainboosting,folded-cascodeOTAwithgainboosting,and2-stageOTAwithclass-Aoutputstage.Thefolded-cascodearchitecturewithgainboostingischosenforthefollowingreasons,•Comparedtothefolded-cascodestructure,atelescopicamplifierhasaloweroutputswingrangebuttwicethepowerefficiency.Itsoutputhastoaccommodateaminimumof5Vdsat’sand1Vth(nottakingintoaccountthattailcurrentsourcemayneedtobecascodedaswell)whilethefolded-cascodestructurehasanoverheadofonly4Vdsat’s.GiventherequiredDRof90dB,let’sdoaquickcalculationtoseeifthetelescopicstructureoffersadvan-tage.AssumeVdsat=250mVandVth=650mV.Fora3-Vsupply,thisindicatesthatthetelescopicstructureyieldsamaximumoutputswingVo,max=3-5*0.25-0.65=1.1Vwhilethefolded-cascodearchitectureachievesVo,max=3-4*0.25=2V.Thismeansweneedtoknockdownthenoiseinthetelescopicamplifierbyabout4times!Takingintoaccountthatatelescopicamplifierhasmainly4transistorscontributingtotheoutputnoisewhileafolded-cas-codeonehas6,thelaterwillshowanoisefactor(N)ofabout5/3timeslarger.Combiningthetwoeffects,wecon-cludethat,althoughtelescopicstructurefeaturesahigherpowerefficiencyandalowernoisefactor,thedynamicrangeconstraintwelloffsetstheseadvantages.Butinasituationwherelessdynamicrangeisdemandedorenoughsupplyroomisavailable,thetelescopicarchitecturewillyieldabetterdesign.•2-stageOTA,ontheotherhand,offersevenlargeroutputswingwhereonly2Vdsat’shavetoberoomedfor.Ifatelescopicamplifierisusedforthe1ststage,thearchitecturealsooffersalowernoisefactor(ignoringnoisecon-tributionfromthe2ndstage).But,unlikecascoding,cascadinggainstageshasnegativeimpactonthefrequencyresponseoftheamplifier,andthusfrequencycompensation(Miller/cascodecompensation)techniqueshavetobeemployedtotradebandwidthforphasemargin/stability.T