Transistors

Transistorscangenerate,amplify,modulate,andmixAFandRFsignals.Sometransistorscanalsoactashigh-speedswitchesforDCanddigitalapplications.

BipolarTransistorsBipolartransistorscontainthreesectionsofsemiconductormaterialwithtwoP-Njunctions.Twomaintypesprevail:aPtypelayerbetweentwoNtypelayers(calledanNPNtransistor),andanNtypelayerbetweentwoPtypelayers(calledaPNPtransistor).

NPNversusPNPFigure7-1AisafunctionaldrawingofanNPNtransistor,andFig.7-1Bshowstheschematicsymbol.ThePtype,orcenter,layerformsthebase.OneoftheNlayersformstheemitter,andtheotherNlayerformsthecollector.Thebase,theemitter,andthecollectorareabbreviatedasB,E,andC.

FIGURE7-1FunctionaldiagramofanNPNtransistor(A),theschematicsymbolforanNPNtransistor(B),functionaldiagramofaPNPtransistor(C),andtheschematicsymbolforaPNPtransistor(D).

APNPtransistorhastwoPtypelayers,oneoneithersideofathinNtypelayer,asshowninFig.7-1C.TheschematicsymbolforthisdeviceappearsinFig.7-1D.TheNlayerformsthebase.OneofthePlayersformstheemitter,andtheotherPlayerformsthe

collector.AschematicwilltellyouwhetherthecircuitdesignerintendsatransistortobeNPNor

PNP.Thearrowalwaysgoeswiththeemitter,soyoucanidentifythethreeelectrodeswithouthavingtolabelthem.InanNPNtransistor,thearrowattheemitterpointsoutward.InaPNPtransistor,thearrowpointsinward.

Inpracticalcircuits,PNPandNPNtransistorscanperformthesamefunctions.However,theyrequireoppositevoltagepolarities,andthecurrentsflowintheoppositedirections.OftenyoucanreplaceanNPNdevicewithaPNPdeviceorviceversa,reversethepower-supplypolarity,andthenewcircuitwillbehaveastheoldonedid.

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SomebipolartransistorsworkbestinRFamplifiersandoscillators;othersworkbetterforAFapplications.Somecanhandlehighpower,andothersaremeanttodealwithweaksignals.Somearemanufacturedfordigitaloperations,suchason/offswitchingandlogicfunctions,insteadofanalogoperations,suchasoscillationandamplification.

PowerSupplyforNPNInordertomakeatransistorwork,youmustapplycertainvoltagescalledbiasvoltages(orsimplybias).InanNPNtransistor,youllusuallyconnecttheemittertothenegativebatteryorpower-supplyterminalandthecollectortothepositiveterminal,oftenthroughcoilsand/orresistors.Figure7-2showsthebasicemitter-collectorbiasingschemeforanNPNtransistor.Typicalbatteryorpower-supplyvoltagesrangefrom3Vto50V.

FIGURE7-2PowersupplyconnectiontotheemitterandcollectorofanNPNtransistor.

InFig.7-2,youcanlabelthebaseControlbecause,withconstantcollector-to-emitter(C-E)voltage(symbolizedECorVC),themagnitudeofthecurrentthroughthetransistordependsonthecurrentthatflowsacrosstheemitter-base(E-B)junction,calledthebasecurrentandlabeledIB.Whathappensatthebase,frominstanttoinstantintime,dictateshowthedevicebehavesasawhole.

ZeroBiasWhenyoudontconnectabipolartransistorsbasetoanything,orwhenyoushort-circuitittotheemitterforDC,thetransistoroperatesatzerobasebias(orzerobias).Noappreciablecurrentcanflowbetweentheemitterandthecollectorwithzerobiasunderno-signalconditions.Inordertocausecurrenttoflowbetweentheemitterandcollector,

youmustapplyaDCvoltageequaltoorgreaterthantheforwardbreakovervoltageattheE-Bjunction,orelseintroduceanACsignalatthebaseoremitterthatcausesthevoltageattheE-Bjunctiontoreachorexceedtheforward-breakovervoltageforatleastasmallpartofeachsignalcycle.

ReverseBiasInthesituationofFig.7-2,imaginethatyouconnectasecondbatterybetweenthebaseandtheemitteroftheNPNtransistor,forcingthebasetoacquireanegativevoltagewithrespecttotheemitter.TheadditionofthisnewbatterywillcausetheE-Bjunctiontooperateinaconditionofreversebias.(AssumethatthisnewbatterydoesnthaveahighenoughvoltagetocauseavalanchebreakdownattheE-Bjunction.)

Whenyoureverse-biastheE-Bjunctionofatransistor,nocurrentflowsbetweentheemitterandthecollectorunderno-signalconditions.Youmightinjectasignalatthebaseoremittertoovercomethecombinedreverse-biasbatteryvoltageandforwardbreakovervoltageoftheE-Bjunction,butsuchasignalmusthavepositivevoltagepeakshighenoughtocauseconductionattheE-Bjunctionforpartoftheinputsignalcycle.Otherwise,thetransistorwillremaininastateofcutofffortheentirecycle.

ForwardBiasNowsupposethat,inthesituationofFig.7-2,youmakethebiasvoltageatthebaseoftheNPNtransistorpositiverelativetotheemitter,startingatsmalllevelsandgraduallyincreasing.Youmightdothisbyconnectingaresistorbetweenthebaseandtheexistingpositivebatteryterminal,andanotherresistorbetweenthebaseandthenegativebatteryterminal.Youvarythebasebiasbytweakingthevaluesofthetworesistors,whichformacircuitcalledavoltagedivider.

Alternatively,youcangetbasebiaswithasecondbattery.ThisactioncausesforwardbiasattheE-Bjunction.Ifthisbiasremainssmallerthantheforwardbreakovervoltage,nocurrentflows.ButoncetheDCvoltagereachesorexceedstheforwardbreakoverthreshold,theE-Bjunctionconductsunderno-signalconditions.

Despiteanormalconditionofreversebiasatthebase-collector(B-C)junction,someemitter-collectorcurrent,moreoftencalledcollectorcurrentanddenotedIC,flowswhentheE-Bjunctionconducts.AsmallinstantaneousriseinthepositivepolarityofanACsignalatthebase,attendedbyasmallinstantaneousriseinthebasecurrentIB,willcausealargeinstantaneousincreaseinthecollectorcurrentIC.Conversely,asmallinstantaneousriseinthenegativepolarityofanACsignalatthebase,attendedbyasmallinstantaneousdropinIB,willcausealargeinstantaneousdecreaseinIC.Thecollectorcurrentvariesmorethanthebasecurrentdoes,sothetransistoractsasanalternating-current(AC)amplifier.

SaturationIfyouadjusttheconditionsattheE-BjunctionsothatIBcontinuestorise,youlleventuallyreachapointwhereICnolongerincreasesrapidly.Ultimately,theICversusIBfunction,orcharacteristiccurve,levelsoff.Figure7-3showsafamilyofcharacteristic

curvesforatypicalbipolartransistor(eitherNPNorPNP;thegraphisnotpolarity-specific).Eachcurveshowsthesituationforacertainfixedcollector-to-emittervoltage,usuallycalledthecollectorvoltageandsymbolizedasEC.Theactualcurrentlevelsdependontheinternalstructureofthetransistor.Wherethecurvesleveloff,thetransistoroperatesinastateofsaturation.Undertheseconditions,itcannotamplify,butitcanoperateasaswitchifyouchangebackandforthbetweenthesaturatedstate(switchon)andthecutoffstate(switchoff).

FIGURE7-3Afamilyofcharacteristiccurvesforabipolartransistor.

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Whenworkingwithanalogcircuitssuchasamplifiers,engineersrarelyoperatebipolartransistorsinthesaturatedstate.However,theyoftenbiasbipolartransistorsatsaturationindigitalcircuitsinwhichthesignalisalwayseitherfull-on(high,orlogic1) orfull-off(low,orlogic0).

PowerSupplyforPNPForaPNPtransistor,theDCpowersupplyconnectiontotheemitterandcollectorisamirrorimageofthecaseforanNPNdevice,asshowninFig.7-4.YoureversethebatterypolaritycomparedwiththeNPNcircuit.ToovercomeforwardbreakoverattheE-Bjunction,anappliedvoltageorsignalatthebase(labeledControl)mustattainsufficientnegativepeakpolarity.IfyouapplyapositiveDCvoltageatthebase,thedevicewilloperateinastateofcutoff.

FIGURE7-4PowersupplyconnectiontotheemitterandcollectorofaPNPtransistor.

Eithertypeoftransistor,PNPorNPN,worksasacurrentvalve.SmallchangesinthebasecurrentIBinducelargefluctuationsinthecollectorcurrentICwhenyouoperatethedeviceintheregionofthecharacteristiccurveinwhichthegraphhasasteepriseoverrun(technicallycalledslope).ThedetailsoftheinternalatomicactivitydifferinthePNPdeviceascomparedwiththeNPNdevice,butinmostpracticalcases,theexternalcircuitrycanttellthedifference.

BasicBipolar-TransistorCircuitsYoucanconnectabipolartransistortoexternalcomponentstobuildcircuitsforspecializedtasks.Threegeneralarrangementsprevail:

1.Thecommon-emittercircuit,inwhichyougroundtheemitterforsignal

2.Thecommon-basecircuit,inwhichyougroundthebaseforsignal

3.Thecommon-collectorcircuit,inwhichyougroundthecollectorforsignal

CommonEmitterFigure7-5isaschematicdiagramofagenericNPNcommon-emittercircuit.CapacitorC1presentsashortcircuittotheACsignal,placingtheemitteratsignalground(butnotnecessarilyDCground).ResistorR1givestheemitterasmallpositiveDCvoltagewithrespecttoground.TheexactDCvoltageattheemitterunderno-signalconditionsdependsonthevalueofR1andalsoonthebasebias,determinedbytheratioofthevaluesofresistorsR2andR3.Thebasebiascanrangefrom0V(groundpotential)allthewayuptothesupplyvoltage,inthiscase+12VDC.Normally,youllwanttosettheno-signalbasebiasacoupleofvoltspositivewithrespecttoDCground.

FIGURE7-5Common-emitterbipolar-transistorconfiguration.

CapacitorC2isolates,orblocks,DCfromtheinput,whileallowingtheACsignaltogetthrough.CapacitorC3blocksDCattheoutput,whilelettingtheACsignalpass.Engineerscallcapacitors,suchasC2andC3,blockingcapacitors.ResistorR4keepstheoutputsignalfromshortingthroughthepowersupply,while,nevertheless,allowingapositiveDCvoltagetoexistatthecollector.

AsignalentersthecircuitthroughC2,whereitcausesthebasecurrentIBtovary.SmallfluctuationsinIBcauselargevariationsinIC.ThiscurrentpassesthroughR4,causingafluctuatingDCvoltagetoappearacrossthatresistor.TheACsignal,superimposedontheDCflowingthroughR4,passesthroughC3totheoutputterminal.

Thecommon-emitterconfigurationoffersexcellentgainwhenproperlydesignedtooperateasanamplifier.TheACoutputwaveappearsinphaseoppositiontotheinputwaveifyouinputasinewavetothecircuit.Ifcircuitryoutsidethetransistorinvertsthesignalagain,ahigh-gaincommon-emittercircuitwillsometimesoscillateratherthanamplify.

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Youcanminimizetheriskofoscillationinacommon-emitteramplifierbymakingsurethatthecircuitdoesnthavetoomuchgain.(Anamplifiercansometimesworktoowellforitsowngood!)Alternatively,youcanuseadifferentcircuitconfiguration,suchasthecommon-basedesigndescribednext.

CommonBaseInthecommon-basecircuit(Fig.7-6),youplacethebaseatsignalground.TheDCbias

onthetransistoristhesameforthiscircuitasforthecommon-emittercircuit.YouapplytheACinputsignaltotheemitter,producingfluctuationsinthevoltageacrossR1,inturncausingsmallvariationsinIB.Asaresult,yougetalargechangeinIC,thecollectorcurrentthatflowsthroughR4,soamplificationoccurs.Theoutputwaveappearsinphasecoincidence(lockstep)withtheinputwave.

FIGURE7-6Common-basebipolar-transistorconfiguration.

ThesignalentersthroughC1.ResistorR1keepstheinputsignalfromshortingtoground.BasebiasisprovidedbyR2andR3.CapacitorC2keepsthebaseatsignalground.ResistorR4keepsthesignalfromshortingthroughthepowersupply.TheACpartoftheoutputsignalgoesthroughC3.

Acommon-basecircuitcantproduceasmuchgainasacommon-emittercircuitdoes,butthecommon-baseamplifierislesspronetooscillatebecauseitslesssusceptibletotheundesirableeffectsofpositivefeedback,whichcancauseanamplifiertogetoutofcontrolinmuchthesamewayasapublic-addresssystemhowlsorrumbleswhenthemicrophonepicksuptoomuchsoundfromthespeakers.

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Common-basecircuitsworkwellashigh-poweramplifiersinradiotransmitters,wheretoomuchpositivefeedbackcancauseparasiticoscillationsocalledbecauseitrobsthetransmitterofusefulsignaloutputpoweronitsdesignfrequency.Parasiticscanwreakhavocontheairwavesbyinterferingwithotherwirelesscommunications.

CommonCollectorAcommon-collectorcircuit(Fig.7-7)operateswiththecollectoratsignalground.TheinputsignalpassesthroughC2ontothebaseofthetransistor.ResistorsR2andR3provide

thebasebias.ResistorR4limitsthecurrentthroughthetransistor.CapacitorC3keepsthecollectoratsignalground.AfluctuatingcurrentflowsthroughR1,andafluctuatingvoltage,therefore,appearsacrossit.TheACcomponentpassesthroughC1totheoutput.Becausetheinstantaneousoutputsignallevelfollowsalongwiththeinstantaneousemittercurrent,thiscircuitissometimescalledanemitterfollower.Theoutputwaveappearsinphasewiththeinputwave.Whenwelldesigned,anemitterfollowerworksoverawiderangeoffrequencies,andoffersalow-costalternativetoanRFtransformer.

FIGURE7-7Common-collectorbipolar-transistorconfiguration.

DidYouKnow?

Anemitter-followercircuitwontamplifysignals,butitcanhelptoprovideisolationbetweentwodifferentpartsofanelectronicsystem.Engineerscallthissortofisolationcircuitabuffer.

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Figures7-5,7-6,and7-7showNPNtransistorcircuits.YoucanobtaintheequivalentdiagramsforPNPcircuitsbyreplacingtheNPNtransistorswithPNPdevices,andbyreversingthepower-supplypolarityineachcase(providingabout12VDC,ratherthan+12VDC,atthenon-collectorendofresistorR4).

Field-EffectTransistorsTheothermajorformofsemiconductortransistor(besidesthebipolartype)isthefield-effecttransistor(FET).Twomainversionsexist:thejunctionFET(JFET)andthemetal-oxide-semiconductorFET(MOSFET).

PrincipleoftheJFETInaJFET,afluctuatingelectricfieldcausesthecurrenttovarywithinthesemiconductormedium.Chargecarriers(electronsorholes)movealongapathcalledthechannelfromthesource(S)electrodetothedrain(D)electrode.Asaresult,youobserveadraincurrentIDequaltothesourcecurrentIS,andalsoequaltothecurrentatanypointalongthechannel.Thecurrentthroughthechanneldependsontheinstantaneousvoltageatthegate(G)electrode.

IfyoudesignaJFETcircuitproperly,thensmallchangesinthegatevoltage,EG,causelargechangesinthecurrentthroughthechannel,andtherefore,inID.Whenthefluctuatingdraincurrentpassesthroughanexternalresistance,yougetlargevariationsintheinstantaneousDCvoltageacrossthatresistance.YoucandrawofftheACpartofthefluctuatingDC,therebyobtaininganoutputsignalmuchstrongerthantheinputsignal.ThatshowanFETproducesvoltageamplification.

N-ChannelversusP-ChannelFigure7-8AisasimplifiedfunctionaldrawingofanN-channelJFET.Figure7-8Bshowsitsschematicsymbol.TheNtypematerialformsthepathforthecurrent.Electronsconstitutemostofthechargecarriers,sobydefinition,electronsarethemajoritycarriers.Thedrainisconnectedtothepositivepower-supplyterminal,usuallythrougharesistor,acoil,orsomeothercombinationofcomponents.ThegatecomprisesPtypematerial.Another,largersectionofPtypematerial,calledthesubstrate,formsaboundaryonthesideofthechanneloppositethegateandservesasafoundationfortheJFETphysicalstructure.Thevoltageonthegateproducesanelectricfieldthatinterfereswiththeflowofchargecarriersthroughthechannel.AsEGbecomesmorenegative,theelectricfieldchokesoffthecurrentthoughthechannelmoreandmore,sothedraincurrentIDdecreases.

FIGURE7-8FunctionaldiagramofanN-channelJFET(A),theschematicsymbolforanN-channelJFET(B),functionaldiagramofaP-channelJFET(C),andtheschematicsymbolforaP-channelJFET(D).

AP-channelJFET(Figs.7-8CandD)hasacurrentpathwayofPtypesemiconductormaterial.Themajoritycarriersareholes.Thedrainisconnectedtothenegativepower-supplyterminal.ThegateandsubstrateconsistofNtypematerial.ThemorepositiveEGgets,themoretheelectricfieldchokesoffthecurrentthroughthechannel,andthesmallerIDbecomes.

YoucanusuallyrecognizeanN-channelJFETinschematicdiagramsbyanarrowpointinginwardatthegate,andaP-channelJFETbyanarrowpointingoutwardatthegate.SomediagramslackarrowsinJFETsymbols,butthepower-supplypolaritygivesawaythedevicetype.Whenthedraingoestothepositivepower-supplyvoltage(withthenegativepower-supplyterminalconnectedtoground),itindicatesanN-channelJFET.Whenthedraingoestothenegativepower-supplyvoltage(withthepositivepower-supplyterminalconnectedtoground),itindicatesaP-channelJFET.

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IfyoureplaceanN-channelJFETwithaP-channelJFETandreversethepower-supplypolarity,inmostcasesthenewcircuitwilloperateprettymuchthesamewayastheoldonedid,aslongasthenewJFEThastheproperspecifications.

DepletionandPinchoffAJFETworksbecausethevoltageatthegateproducesanelectricfieldthatinterferes,moreorless,withtheflowofchargecarriersalongthechannel.

AsthedrainvoltageEDincreases,sodoesthedraincurrentID(uptoacertainmaximumleveling-offvalue)aslongasthegatevoltageEGremainsconstant,andaslongasEGdoesntgettoohigh.AsEGincreases(negativelyinanNchannelorpositivelyinaPchannel),adepletionregionformsinthechannel.Chargecarrierscantflowinthedepletionregion,sotheymustpassthroughanarrowedchannel.Becauseoftherestrictedpathway,thecurrentgoesdown.

AsthegatevoltageEGincreases(negativelyforanN-channeldeviceorpositivelyforaP-channeldevice),thedepletionregionwidensandthechannelnarrows.IfEGgetshighenough,thedepletionregionclosesthechannel,preventinganyflowofchargecarriersfromthesourcetothedrain.Engineerscallthisconditionpinchoff.

JFETBiasingFigure7-9showstwobiasingarrangementsforanN-channelJFET.AtA,thegateisgroundedthroughresistorR2.ThesourceresistorR1limitsthecurrentthroughthedevice.ResistorsR1andR2determinethegatebias.ThedraincurrentIDflowsthroughR3,producingavoltageacrossit.TheACoutputsignalpassesthroughC2.

FIGURE7-9TwomethodsofbiasinganN-channelJFET.AtA,fixedgatebias;atB,variablegatebias.

AtB,thegateisconnectedthroughpotentiometerR2toanegativeDCvoltagesource.Whenyouadjustthispotentiometer,youvarythenegativegatevoltageEGatthepointbetweenresistorsR2andR3.ResistorR1limitsthecurrentthroughtheJFET.ThedraincurrentIDflowsthroughR4,producingavoltageacrossit.TheACoutputsignalpassesthroughC2.

Inbothofthesecircuits,youwouldconnectthedraintoapositiveDCvoltagesourcerelativetoground.InthecaseofaP-channelJFETcircuit,thepolaritiesmustbereversed,soyouwouldconnectthedraintoanegativeDCvoltagesourcerelativetoground.

ThebiasingarrangementinFig.7-9Aiscommonlyusedforweak-signalamplifiers,

low-levelamplifiers,andoscillators.YouwouldmorelikelyusetheschemeshowninFig.7-9Binpoweramplifiersrequiringasubstantialinputsignal.

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TypicalJFETpower-supplyvoltagesarecomparabletothosewithbipolartransistors.ThevoltageEDbetweenthedrainandgroundcanrangefromapproximately3Vto50V;mostoftenits6Vto12V.

HowtheJFETAmplifiesFigure7-10showstherelativedraincurrentIDasafunctionoftheno-signalgatebiasvoltageEGforahypotheticalN-channelJFET,assumingthatthedrainvoltageEDremainsconstant.WhenaJFEToperatesproperly,thedraincurrentisthesameasthecurrentthroughthechannel,soyoucanthinkofthedraincurrentasthechannelcurrenttoo.

FIGURE7-10RelativedraincurrentasafunctionofgatevoltageforahypotheticalN-channelJFET.

WhenEGisfairlylargeandnegative,theJFEToperatesinastateofpinchoff,sonocurrentflowsthroughthechannel.AsEGgetslessnegative,thechannelopensupandcurrentbeginsflowing.AsEGgetsstilllessnegative,thechannelgrowswiderandthedraincurrentIDincreases.AsEGapproachesthepointwherethesource-gate(S-G)junctionreachestheforwardbreakovervoltage,thechannelconductsaswellasitcan;itswideopen.IfEGgetsstillmorepositive,exceedingtheforwardbreakovervoltageandcausingtheS-Gjunctiontoconduct,someofthecurrentinthechannelleaksoutthroughthegate.InatypicalJFETcircuit,S-Gjunctioncurrentshouldneverflow.

ThegreatestamplificationforweaksignalsoccurswhenyousetEGsothatthecurveinFig.7-10hasitssteepestslope,asshownbytherangemarkedX.Inahigh-powerRFtransmittingamplifierinwhichtheinputsignalisrelativelypowerfultobeginwith,youlloftengetthebestresultswhenyoubiasaJFETatorbeyondpinchoff,intherangemarkedY.

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Fromprecalculus,youmightrecallthatinarectangularcoordinategraph,suchasFig.7-10,thepartofacurvethatrampsupasyoumovetowardtherighthasapositiveslopebydefinition;thepartofacurvethatrampsdownasyoumovetowardtherighthasanegativeslopebydefinition.

InapracticalJFETamplifiercircuit,thedraincurrentpassesthroughthedrainresistor,asshowninFig.7-9AorFig.7-9B.SmallfluctuationsinEGcauselargechangesinID,andthesevariations,inturn,producewideswingsintheDCvoltageacrossR3(atA)orR4(atB).TheAC(signal)componentofthisvoltagegoesthroughcapacitorC2,andappearsattheoutputwithgreaterpeak-to-peakvoltagethanthatoftheinputsignalatthegate.Therefore,theJFEToperatesasavoltageamplifier.

DrainCurrentversusDrainVoltageWithanyJFET,youcantestandgraphthedraincurrentIDasafunctionofthedrainvoltageEDforvariousvaluesofgatevoltageEG.Theresultinggraphiscalledafamilyofcharacteristiccurvesforthedevice.Fig.7-11showsafamilyofcharacteristiccurvesforahypotheticalN-channelJFET.

FIGURE7-11AfamilyofcharacteristiccurvesforanN-channelJFET.

Metal-OxideFETsTheacronymMOSFET(pronouncedMOSS-fet)standsformetal-oxide-semiconductor

field-effecttransistor.ThistypeofcomponentcanbeconstructedwithachannelofNtypematerial,orwithachannelofPtypematerial.EngineerscalltheformertypeanN-channelMOSFETandthelattertypeaP-channelMOSFET.Figure7-12Aisafunctionalcross-sectiondrawingofanN-channelMOSFET.Figure7-12Bshowstheschematicsymbol.TheP-channelcross-sectiondrawingandsymbolappearatCandD.

FIGURE7-12PictorialdiagramofanN-channelMOSFET(A),theschematicsymbolforanN-channelMOSFET(B),pictorialdiagramofaP-channelMOSFET(C),andtheschematicsymbolforaP-channelMOSFET(D).

TheInsulatedGateWhensemiconductorengineersconceivedanddevelopedtheMOSFET,theycalleditaninsulated-gateFETorIGFET.Somepeoplethinkthatthisexpressiondescribesthedevicebetterthanthecurrentlyacceptedtermdoes.Thegateelectrodeisactuallyinsulated,byathinlayerofdielectricmaterial,fromthechannel.

TheinputimpedanceforaMOSFETexceedsthatofaJFETwhenyouapplyaninputsignalatthegateelectrode.Infact,thegate-to-source(G-S)resistanceofatypicalMOSFETcomparesfavorablytotheleakageresistanceofacapacitor.Itssolargethatyoucanconsideritinfiniteinmostapplications.

Figure7-13showsafamilyofcharacteristiccurvesforahypotheticalN-channelMOSFET.NotethatthecurvesrisesteeplyatfirstforrelativelysmallvaluesofdrainvoltageED,butasEDincreasesbeyondacertainthreshold,thecurvesleveloffmorequicklythantheydoforaJFET.

FIGURE7-13AfamilyofcharacteristiccurvesforatypicalN-channelMOSFET.

BewareandTakeCare!

Metal-oxidedevicesareeasilydestroyedbyelectrostaticdischarges,evensmallones.Whenyoubuild,test,orserviceacircuitcontainingMOStransistorsorintegratedcircuits,youmustusespecialequipmenttoensurethatyourhandsdontacquireanyelectrostaticcharge.Ifanystraydischargeoccursthroughthethin,fragiledielectriclayerinaMOSFET,theresultingcurrentcandestroythedevice.IveseenitoccureveninthehumidsummerclimateofSouthFlorida.Sobewareandtakecare!

DepletionversusEnhancementModesInaJFET,thechannelconductswithzerogatebias,whenthegatehasthesamevoltageasthesource(EG=0).AsEGincreases(negativelyforanN-channeldeviceandpositivelyforaP-channeldevice),thedepletionregiongrowswiderandwider,sothechargecarriersmustpassthroughanarrowerandnarrowerchannel.Thisconditionisknownasthedepletionmode.SomeMOSFETscanfunctioninthedepletionmode.ThedrawingsandschematicsymbolsofFig.7-12depicttheinternalconstructionandschematicsymbolsfordepletion-modeMOSFETs.

Metal-oxide-semiconductortechnologyallowsforanalternativeelectricalenvironmentthatradicallydiffersfromthedepletionmode.Anenhancement-modeMOSFEThasapinched-offchannelatzerobias.Youmustapplyagatebiasvoltage,EG,tocreateachannelinthistypeofdevice.IfEG=0,thenID=0intheabsenceofsignalinput.Youapplygatebiasandsignalstowiden,ratherthanconstrict,thechannel.

Figure7-14showstheschematicsymbolsforN-channelandP-channelenhancement-

modeMOSFETs.Notethatinthesesymbols,theright-handverticallinesarebroken,ratherthansolidasinthesymbolsfordepletion-modeMOSFETs.Thatdifferenceallowsyoutodistinguishbetweenthetwotypesofdevicewhenyouseethemincircuitdiagrams.

FIGURE7-14AtA,theschematicsymbolforanN-channelenhancement-modeMOSFET.AtB,theschematicsymbolforaP-channelenhancement-modeMOSFET.

BasicFETCircuitsThreegeneralcircuitconfigurationsexistforFETs.Theyretheequivalentsofthecommon-emitter,common-base,andcommon-collectorbipolar-transistorcircuits,andtheybreakdownasfollows:

1.Thecommon-sourcecircuit,inwhichyougroundthesourceforACsignals.

2.Thecommon-gatecircuit,inwhichyougroundthegateforACsignals.

3.Thecommon-draincircuit,inwhichyougroundthedrainforACsignals.

CommonSourceInacommon-sourcecircuit,youapplytheinputsignaltothegate,asshowninFig.7-15.ThisdiagramshowsanN-channelJFET,butyoucouldsubstituteanN-channel,depletion-modeMOSFETandgetthesameresultsinapracticalcircuit.YoucouldalsouseanN-channel,enhancement-modeMOSFETandaddanextraresistorbetweenthegateandthepositivepower-supplyterminal.

FIGURE7-15Common-sourceFETconfiguration.

ForP-channeldevices,thepowersupplywouldprovideanegativevoltageratherthanapositivevoltage.Otherwise,thecircuitdetailswouldcorrespondtothoseshowninFig.7-15.

CapacitorC1andresistorR1placethesourceatsignalground,whileelevatingthesourceabovegroundforDC.TheACsignalentersthroughcapacitorC2.ResistorsR1andR2providebiasforthegate.TheACsignalpassesoutofthecircuitthroughcapacitorC3.ResistorR3keepstheoutputsignalfromshortingthroughthepowersupply.

ThecircuitofFig.7-15canofferastartingpointforthedesignofweak-signalamplifiersandlow-poweroscillators,especiallyinRFsystems.Thecommon-sourcearrangementprovidesthegreatestgainofthethreeFETcircuitconfigurations.Theoutputwaveisinverted(appearsinphaseopposition)withrespecttotheinputwave.

CommonGateThecommon-gatecircuit(Fig.7-16)operateswiththegateatsignalground.Youapplytheinputsignaltothesource.ThisillustrationshowsanN-channelJFET.ForothertypesofFETs,thesameconsiderationsapplyasinthecaseofthecommon-sourcecircuit.AnN-channelenhancement-modedevicerequiresaresistorbetweenthegateandthepositivepower-supplyterminal.ForP-channeldevices,youreversethepolarityofthepowersupply.

FIGURE7-16Common-gateFETconfiguration.

TheDCbiasforthecommon-gatecircuitresemblesthatforthecommon-sourcearrangement,butthesignalfollowsadifferentpath.TheACinputsignalentersthroughcapacitorC1.ResistorR1keepstheinputfromshortingtoground.ResistorsR1andR2providethegatebias.CapacitorC2placesthegateatsignalground,whileallowingDCbiasvoltagetoexistonthatelectrode.TheoutputsignalexitsthecircuitthroughC3.ResistorR3keepstheoutputsignalfromshortingthroughthepowersupply.

Thecommon-gatearrangementproduceslessgainthanitscommon-sourcecounterpart.However,acommon-gateamplifierislesslikelythanacommon-sourceamplifiertobreakintounwantedoscillation.Theoutputwaveoccursinphasecoincidencewiththeinputwave.

CommonDrainFigure7-17showsacommon-draincircuit,inwhichyouplacethedrainatsignalground.Engineerssometimescallthiscircuitasourcefollowerbecausetheoutputsignalwaveformfollowstheinstantaneousvoltageatthesourceelectrode.

FIGURE7-17Common-drainFETconfiguration.

TouseanFETinthecommon-drainconfiguration,youbiasitforDCinthesamewayasyoudointhecommon-sourceandcommon-gatecircuits.ThecircuitshowninFig.7-17employsanN-channelJFET,butyoucansubstituteanyotherkindofFET,reversingthepolarityforP-channeldevices.Enhancement-modeMOSFETsrequirearesistorbetweenthegateandthepositivepower-supplyterminal(orthenegativeterminalforaP-channeldevice).

TheinputsignalpassesthroughcapacitorC2tothegate.ResistorsR1andR2providegatebias.ResistorR3limitsthemaximumcurrentthatcanflowthroughthechannel.CapacitorC3keepsthedrainatsignalground.FluctuatingDC(thechannelcurrent)flowsthroughR1asaresultoftheinputsignal;thiscurrentcausesafluctuatingDCvoltagetoappearacrossR1.YoutaketheACoutputfromthesourcethroughcapacitorC1.

Theoutputwaveofthecommon-draincircuitexistsinphasecoincidencewiththeinputwave.Thiscircuit,likethecommon-collectorarrangement,canserveasalow-costalternativetoawidebandtransformer,especiallyinRFapplications.

SomeTechTalk

Intheprecedingdiscussions(accompaniedbyFigs.7-15,7-16,and7-17),youmightgettheideathatyoucandirectlyinterchangeJFETsandMOSFETsinpracticalcircuits.Sometimesyoucandothat,butnotalways.AlthoughthecircuitdiagramsforJFETanddepletion-modeMOSFETdeviceslookidenticalwhenyoudontspecifyactualcomponentvalues,theresusuallyadifferenceintheoptimumresistancesandcapacitances.TheseoptimumvaluescanvarynotonlybetweenJFETsandMOSFETs,butalsobetweendepletion-modeMOSFETsandenhancement-modeMOSFETs.Specificdesignexampleswouldsurpassthescopeofthisbook,butyoushouldrememberthatJFETsandMOSFETsaresometimes,butnotalways,directlyinterchangeable.

Experiment1:CheckaBipolarTransistorYoucanusesome(butnotall)ohmmeterstotestdiodes,bipolartransistors,andJFETsbyusingthemeterasapolarity-sensitivecontinuitytester.Butyoumustmakesurethatyourmeterisuptothetask.

TesttheProbePolarityFirst,youmustdeterminewhichohmmeterprobeproduceswhatpolarity.Youmightimaginethatelectronswouldemergefromtheprobewiththeblackwire(thenegativemeterleadforcurrentandvoltage),andentertheprobewiththeredwire(thepositivemeterleadforcurrentandvoltage),sotheelectroncurrentwouldflowfromblacktored.Maybetheydothatinyourmeter.ButwhenItestedmyanalogohmmeterusingaseparatemicrovoltmeter,andalsousingacoupleofrectifierdiodesknowntobegood,theoppositesituationprevailed.Becauseofthewaythatparticularohmmetersinternalbatteryisconnected,theredmeterleadproducesnegativeDCandtheblackoneproducespositiveDC,sotheelectronsactuallymovefromredtoblack!

HeadsUp!

Notallmultimetersarepolarity-reversedforcurrentproductionasopposedtomeasurement,butmyanalogmeteris.ImgladthatIfoundoutaboutitbeforestartingthecomponenttests.Youshoulddothesamething.

ChecktheProbeVoltageTheotherlurkingbugaboothatmightmessupthisexperimentinvolvestheactualvoltageproducedatthetestleadsofyourohmmeter.IfitslessthantheforwardbreakovervoltageofatypicalP-Njunction(about0.6V),yourmeterwontindicateconductionineitherdirectionthroughthejunction,andyoumightmistakenlyconcludethatthecomponentisbad.Mydigitalmeterfailedtoproduceenoughvoltage,andshowedextremelyhighresistanceineitherdirectionthroughtheP-Njunctionsinseveraldifferentcomponents.Myanalogmeterdidproducethenecessaryvoltage,andIfoundthatoutbytestingarectifierdiodeknowntobegood.Withthediodesanodepositiveandthecathodenegative,mymetershowedabout40ohms.Withtheanodenegativeandthecathodepositive,mymetershowedinfinityohms.

Tip

Mostdiodeshavealineonthecasetoindicatethecathodeside.Myrectifierdiodes,havingblackcases,havewhitelinesonthecathodesides.

GetaTransistorYoullfindNPNbipolartransistorsatmostRadioShackretailoutlets,andalsoontheirwebsite(www.radioshack.com).Asmalldevicewillsuffice;youdontneedabigpowertransistor.Iuseda2N222transistor,RadioShackcatalognumber276-1617,butany

similarcomponentwillworkaswell.Onceyougetthetransistor,youllprobablydecidetogetapairofstrongreadingglassestoo.Thattransistorissmall,andunlessyouhavekeeneyesightandasteadyhand,youllhavetroubleholdingthemeterprobetipsupagainstthetransistorleads.Worseyet,youmightfinditdifficulttoseetheleadsthemselves.

PrepareforTestingBeforeyoutestthetransistor,makecertainthatyouknowwhichleadgoestotheemitter,whichonegoestothebase,andwhichonegoestothecollector.Thewrappingpackageforthecomponentshouldhaveaprintedguideontheback,tellingyouhowtolocatethem.Onceyouknowthatinformation,dontconnectthetransistorintoacircuit.Testitinisolationfirst.

GoforIt!Thetestsinvolveholdingthemeterprobetipsupagainstthetransistorleadsinallsixwaysthatitcanbedone.Yourbodyresistanceishighenoughsothatyoushouldnothavetoweargloves.Figures7-18through7-23showtheconnectionsandindicatetheresultsyoushouldseewithagoodNPNtransistor.Moderateresistanceontheorderofafewohmsortensofohms(theexactvaluedoesntmatter)indicatesconduction.Infiniteresistanceindicatesnon-conduction.

FIGURE7-18Electronsshouldflowfromtheemittertothebase.

FIGURE7-19Electronsshouldnotflowfromthebasetotheemitter.

FIGURE7-20Electronsshouldflowfromthecollectortothebase.

FIGURE7-21Electronsshouldnotflowfromthebasetothecollector.

FIGURE7-22Electronsshouldnotflowfromtheemittertothecollector.

FIGURE7-23Electronsshouldnotflowfromthecollectortotheemitter.

1.Electronsshouldflowfromtheemittertothebase(Fig.7-18).

2.Electronsshouldnotflowfromthebasetotheemitter(Fig.7-19).

3.Electronsshouldflowfromthecollectortothebase(Fig.7-20).

4.Electronsshouldnotflowfromthebasetothecollector(Fig.7-21).

5.Electronsshouldnotflowfromtheemittertothecollector(Fig.7-22).

6.Electronsshouldnotflowfromthecollectortotheemitter(Fig.7-23).

Experiment2:CheckaJFETYoullfindJFETsatmostRadioShackretailoutlets,andalsoontheirwebsite(www.radioshack.com),justasyoucandowithbipolartransistors.Atthetimeofthiswriting,mostJFETdeviceswereavailableonlythroughthewebsite.

GettheComponentIusedaP-channelJFETcalledNTE489,withRadioShackcatalognumber55052761.Itsatinydevice,likethe2N222,soyoumightneedstrongglassesand,forsure,asteadyhand.Ifyoucantfindtheexactcomponent,anythingsimilarwillworkfine.MakesureitsaP-channeldevice,notanN-channeldevice.

HeadsUp!

ThistestwillworkwithJFETs,butnotwithMOSFETs.AllJFETshaveP-Njunctionsbetweenthegateandthechannel.However,MOSFETsdonothaveP-Njunctionsatall.Youllseeinfinityohmsinbothdirectionsbetweenthegateandthechannelina

MOSFETbecausethegateisinsulatedfromthechannel.

PrepareforTestingBeforeyougiveyourohmmeteranotherworkout,makesurethatyouknowthelocationsofthesource,gate,anddrainontheJFETcase.RadioShacksJFETscomeinpackageswiththeleadplacementguideprintedontheback.Onceyouknowwheretheleadsare,keepthecomponentinisolationsothatyoullgettrueresultswhenyoucheckforconduction.

DotheDeed!Asyoudidwiththebipolartransistor,holdthemeterprobetipsupagainsttheJFETleadsinallsixcombinations.Figures7-24through7-29showtheconnectionsandindicatetheresultsyoushouldseewithagoodP-channelJFET.Moderateresistanceindicatesconduction,justaswithanyotherP-Njunction.Dontworryabouttheexactohmicvalue;yourmetergeneratescurrentthatcreatesanapparentresistanceinthejunction.Infiniteresistance,inwhichthemeterneedledoesnotbudge(ifyouuseananalogmeter,asIdid),however,definitelyindicatesnon-conduction.

FIGURE7-24Electronsshouldflowfromthegatetothesource.

FIGURE7-25Electronsshouldnotflowfromthesourcetothegate.

FIGURE7-26Electronsshouldnotflowfromthedraintothegate.

FIGURE7-27Electronsshouldflowfromthegatetothedrain.

FIGURE7-28Electronsshouldflowfromthesourcetothedrain.

FIGURE7-29Electronsshouldflowfromthedraintothesource.

1. Electronsshouldflowfromthegatetothesource(Fig.7-24).

2. Electronsshouldnotflowfromthesourcetothegate(Fig.7-25).

3. Electronsshouldnotflowfromthedraintothegate(Fig.7-26).

4. Electronsshouldflowfromthegatetothedrain(Fig.7-27).

5. Electronsshouldflowfromthesourcetothedrain(Fig.7-28).

6. Electronsshouldflowfromthedraintothesource(Fig.7-29).