新型CCII电流传输器

NewCCIICurrentConveyorBy:JohnRobinsonMar27,2008Abstract:Offeringhigherbandwidththanitsvoltage-feedbackcounterpart,thesecond-generationcurrentconveyoroperationalamplifier(CCII)canbeusedinRFmixers,high-frequencyprecisionrectifiers,andmedicalapplicationssuchaselectricalimpedancetomography.Theconventionaloperationalamplifierscannotbeusedinthehigh-frequencyapplicationsduetotheirlimitedgain-bandwidthproduct.IntroductionThecurrentconveyorhasbeenaroundsincetheoriginaldesign,orCCI(whichcanberegardedasanidealtransistor),wasinitiallyproposedbySmithandSedrain1968¹,².CCIwasthenreplacedbyamoreversatilesecond-generationdevicein1970,theCCII³.CurrentconveyordesignshavemainlybeenBJTduetotheirhightransconductancevaluescomparedtotheirCMOScounterparts.Theyareusedascurrent-feedbackoperationalamplifierssuchastheMAX4223low-poweramplifier,whichfeaturescurrentfeedbackratherthantheconventionalvoltagefeedbackusedbystandardoperationalamplifiers.Thismeansthatthecurrentfeedbackoperationalamplifierisnotrestrictedbytheconventionalgainbandwidthsofastandardoperationalamplifier,andcanofferamuchhigherbandwidthsolutionthanitsvoltage-feedbackcounterpart.Currentconveyorsareusedinhigh-frequencyapplicationswheretheconventionaloperationalamplifierscannotbeused,becausetheconventionaldesignsarelimitedbytheirgain-bandwidthproduct.Intheory,thecurrentconveyorisonlylimitedbytheftofthetransistorsusedinitsdesign.SomeapplicationswherecurrentconveyorsareusedtodayincludeRFmixers,high-frequencyprecisionrectifiers,andmedicalapplicationssuchasElectricalImpedanceTomography(EIT).ABipolarConveyorThediagraminFigure1belowshowsacurrentconveyorimplementedusingbipolardevices.Figure1.AbipolarCCII.FromFigure1itcanbeseenthattheCCIIconveyorcanbemodelledasanidealtransistor:Ybeingthebase/gateXbeingtheemitter/sourceZbeingthecollector/drainThistypeofcircuitworkswellasacircuitwithBJTs,asthetransconductanceandEarlyvoltagesofBJTsaremuchgreaterthanthatofCMOSdevices.Therefore,currentconveyorsworkwellassourcefollowers.GainX/Yiscloseto1;Zhasanaturalhigh-outputimpedancewhichcannotbemimickedbytheirCMOScounterparts.TheCMOSSourceFollowerAspreviouslyexplained,themajorproblemwithaCMOSfolloweristhelowgmandpoorEarlyvoltage(1/lambda).Thisequatestopoorgain,becausethegainforavoltagefollowerheavilyreliesonthesetwoparameterstobelarge.Thiscanbeobservedintheequationbelow:WheregListheloadconductance,gdsisthedrainsourceconductanceandgmisthetransconductanceoftheCMOSdevice.AtypicalsimulatedgainwithaTSMC0.18µmwithaloadof1kΩgaveagainof0.7.Comparedtotheidealgainof1,thisrepresentsa30%lossinoutputgain.CurrentConveyorSourceFollowerOnecanuseanunbufferedamplifier(Figure2a)tomimicasourcefollowerwithagainofone.ThenthismodificationcanbeaddedtothebasicdesigninFigure1tomakeaCCIIcurrentconveyor.Figure2a.Asimplesourcefollower.Figure2acanbeimplementedasshownbelowinFigure2b.Figure2b.TheCCIIunbufferedsourcefollowerandimplementation.FromFigure2bitcanbeseenthatoutputXisfedbacktooneofthelongtailpairsofinputs(X').TheotherinputtothelongtailpairisY,asinputYchangesthecurrentthroughM1.M2differsasM3,andM4isacurrentmirror.ThereisacurrentdifferencebetweenM2andM4.Thisimbalanceisaddressedbypullingcurrentfrom,orto,thegate/sourcecapacitanceCgsofdeviceM5.UntiltheoutputX'matchesthatofY,thebandwidthlimitisdefinedastherateatwhichthistransistorcanbedischargedandcharged.Thus,thebandwidthlimitcanbedefinedas:CurrentConveyor(CCII+)UsinganUnbufferedAmplifierFromFigure2,thefirstpartofthecurrentconveyor(CCII+)canberealized.Tobuildtherestofthecurrentconveyor(CCII+),thecurrentfromoutputX'simplyhastobemirrored.TogiveZ'soutput,seetheexampleinFigure3.Figure3.Thecurrentconveyor(CCII+)usinganunbufferedamplifier.ThecurrentfromM5/M6issimplymirroredbyM7/M8,givingtheoutputZ(-)oftheCCII+.TheoutputimpedanceofZcanbeimprovedbyaddinginacascodetoM7/M8ifnecessary.Onemustbeawarethattomimicthecurrentsuccessfully,theoutputimpedanceofXmustmatchthatofZ,i.e.,thesametransistortypesandconfirmationmustbeusedonM5/M6asonM7/M8.ThegainoftheCCIIissimply:ConvertingfromaCCII+toaCCII-TakingthebiaspointYb'(Figure3),simplyaddtheextraconnectionsasshowninFigure4.Figure4.ThecurrentconveyorconfiguredasaCCII-.FromFigure4,ifalltransistordimensionsarethesameandifYb'(thebiaspointfromFigure3)istaken,thenacurrent2iisgeneratedfromM10andM11.ThisismirroredbyM9togiveacurrentof2ithroughM13.M12providesacurrentofiandgivesacurrentof-ithroughZ(+),thusgivingatrueCCII-output.Thereisaproblemwiththisapproach:theZ(+)nowhasan-iDCterminsteadofa+iterm.Consequently,a2iDCtermneedstobeaddedtotheoutputofZ(+)tocompensatefor-i.Figure5belowshowstheaddition.Figure5.TheCCII-outputwithaddedDCbias.FromFigure5,transistorsM14andM15providetheappropriatecurrenttocompensatefortheDCcurrenttakenbyM13.(NotethatM14andM15mustmatchM12).MakethecurrentthroughR3equaltoi(DC)-i'.RememberthatR3andR2mustmatch;anymismatchintheirvalueswillmeanthattheiroutputDCvalueswilldiffer.TheVBIASCircuitTocreatethenecessaryandappropriatevoltage,VDCBIAS,thatwillmaintaintheDCcurrentsinM14andM15,thenVDCBIAS(Figure5)musthavethesameDCvalueasNodeYb'(Figures4an