ATypical
BipolarTransistor
Tutorial:1of8
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GORESET
BipolarTransistorBasics
IntheDiodetutorialswesawthatsimplediodesaremadeupfromtwopiecesofsemiconductormaterial,eithersilicon
orgermaniumtoformasimplePN-junctionandwealsolearntabouttheirpropertiesandcharacteristics.Ifwenowjoin
togethertwoindividualsignaldiodesback-to-back,thiswillgiveustwoPN-junctionsconnectedtogetherinseriesthat
shareacommonPorNterminal.Thefusionofthesetwodiodesproducesathreelayer,twojunction,threeterminal
deviceformingthebasisofaBipolarJunctionTransistor,orBJTforshort.
Transistorsarethreeterminalactivedevicesmadefromdifferentsemiconductormaterialsthatcanactaseitheran
insulatororaconductorbytheapplicationofasmallsignalvoltage.Thetransistor''sabilitytochangebetweenthesetwo
statesenablesittohavetwobasicfunctions:"switching"(digitalelectronics)or"amplification"(analogueelectronics).
Thenbipolartransistorshavetheabilitytooperatewithinthreedifferentregions:
?ActiveRegion-thetransistoroperatesasanamplifierandIc=β.Ib
?Saturation-thetransistoris"Fully-ON"operatingasaswitchandIc=I(saturation)
?Cut-off-thetransistoris"Fully-OFF"operatingasaswitchandIc=0
ThewordTransistorisanacronym,andisacombinationofthewordsTransferVaristorused
todescribetheirmodeofoperationwaybackintheirearlydaysofdevelopment.Therearetwo
basictypesofbipolartransistorconstruction,PNPandNPN,whichbasicallydescribesthe
physicalarrangementoftheP-typeandN-typesemiconductormaterialsfromwhichtheyare
made.
TheBipolarTransistorbasicconstructionconsistsoftwoPN-junctionsproducingthree
connectingterminalswitheachterminalbeinggivenanametoidentifyitfromtheothertwo.
ThesethreeterminalsareknownandlabelledastheEmitter(E),theBase(B)and
theCollector(C)respectively.
BipolarTransistorsarecurrentregulatingdevicesthatcontroltheamountofcurrentflowing
throughtheminproportiontotheamountofbiasingvoltageappliedtotheirbaseterminal
actinglikeacurrent-controlledswitch.Theprincipleofoperationofthetwotransistor
typesPNPandNPN,isexactlythesametheonlydifferencebeingintheirbiasingandthe
polarityofthepowersupplyforeachtype.
BipolarTransistorConstruction
TheconstructionandcircuitsymbolsforboththePNPandNPNbipolartransistoraregivenabovewiththearrowinthe
circuitsymbolalwaysshowingthedirectionof"conventionalcurrentflow"betweenthebaseterminalanditsemitter
terminal.ThedirectionofthearrowalwayspointsfromthepositiveP-typeregiontothenegativeN-typeregionforboth
transistortypes,exactlythesameasforthestandarddiodesymbol.
BipolarTransistorConfigurations
AstheBipolarTransistorisathreeterminaldevice,therearebasicallythreepossiblewaystoconnectitwithinan
electroniccircuitwithoneterminalbeingcommontoboththeinputandoutput.Eachmethodofconnectionresponding
differentlytoitsinputsignalwithinacircuitasthestaticcharacteristicsofthetransistorvarywitheachcircuit
arrangement.
?CommonBaseConfiguration-hasVoltageGainbutnoCurrentGain.
?CommonEmitterConfiguration-hasbothCurrentandVoltageGain.
?CommonCollectorConfiguration-hasCurrentGainbutnoVoltageGain.
TheCommonBase(CB)Configuration
Asitsnamesuggests,intheCommonBaseorgroundedbaseconfiguration,theBASEconnectioniscommontoboth
theinputsignalANDtheoutputsignalwiththeinputsignalbeingappliedbetweenthebaseandtheemitterterminals.
Thecorrespondingoutputsignalistakenfrombetweenthebaseandthecollectorterminalsasshownwiththebase
terminalgroundedorconnectedtoafixedreferencevoltagepoint.
Theinputcurrentflowingintotheemitterisquitelargeasitsthesumofboththebasecurrentandcollectorcurrent
respectivelytherefore,thecollectorcurrentoutputislessthantheemittercurrentinputresultinginacurrentgainforthis
typeofcircuitof"1"(unity)orless,inotherwordsthecommonbaseconfiguration"attenuates"theinputsignal.
TheCommonBaseTransistorCircuit
Thistypeofamplifierconfigurationisanon-invertingvoltageamplifiercircuit,inthatthesignal
voltagesVinandVoutare"in-phase".Thistypeoftransistorarrangementisnotverycommonduetoitsunusuallyhigh
voltagegaincharacteristics.Itsoutputcharacteristicsrepresentthatofaforwardbiaseddiodewhiletheinput
characteristicsrepresentthatofanilluminatedphoto-diode.
Alsothistypeofbipolartransistorconfigurationhasahighratioofoutputtoinputresistanceormoreimportantly"load"
resistance(RL)to"input"resistance(Rin)givingitavalueof"ResistanceGain".Thenthevoltagegain(Av)fora
commonbaseconfigurationisthereforegivenas:
CommonBaseVoltageGain
Where:Ic/Ieisthecurrentgain,alpha(α)andRL/Rinistheresistancegain.
Thecommonbasecircuitisgenerallyonlyusedinsinglestageamplifiercircuitssuchasmicrophonepre-amplifieror
radiofrequency(Rf)amplifiersduetoitsverygoodhighfrequencyresponse.
TheCommonEmitter(CE)Configuration
IntheCommonEmitterorgroundedemitterconfiguration,theinputsignalisappliedbetweenthebase,whiletheoutput
istakenfrombetweenthecollectorandtheemitterasshown.Thistypeofconfigurationisthemostcommonlyused
circuitfortransistorbasedamplifiersandwhichrepresentsthe"normal"methodofbipolartransistorconnection.
Thecommonemitteramplifierconfigurationproducesthehighestcurrentandpowergainofallthethreebipolar
transistorconfigurations.ThisismainlybecausetheinputimpedanceisLOWasitisconnectedtoaforward-biasedPN-
junction,whiletheoutputimpedanceisHIGHasitistakenfromareverse-biasedPN-junction.
TheCommonEmitterAmplifierCircuit
Inthistypeofconfiguration,thecurrentflowingoutofthetransistormustbeequaltothecurrentsflowingintothe
transistorastheemittercurrentisgivenasIe=Ic+Ib.Also,astheloadresistance(RL)isconnectedinserieswith
thecollector,thecurrentgainofthecommonemittertransistorconfigurationisquitelargeasitistheratioofIc/Ibandis
giventheGreeksymbolofBeta,(β).Astheemittercurrentforacommonemitterconfigurationisdefined
asIe=Ic+Ib,theratioofIc/IeiscalledAlpha,giventheGreeksymbolofα.Note:thatthevalueofAlphawillalwaysbe
lessthanunity.
Sincetheelectricalrelationshipbetweenthesethreecurrents,Ib,IcandIeisdeterminedbythephysicalconstructionof
thetransistoritself,anysmallchangeinthebasecurrent(Ib),willresultinamuchlargerchangeinthecollectorcurrent
(Ic).Then,smallchangesincurrentflowinginthebasewillthuscontrolthecurrentintheemitter-collectorcircuit.
Typically,Betahasavaluebetween20and200formostgeneralpurposetransistors.
BycombiningtheexpressionsforbothAlpha,αandBeta,βthemathematicalrelationshipbetweentheseparameters
andthereforethecurrentgainofthetransistorcanbegivenas:
Where:"Ic"isthecurrentflowingintothecollectorterminal,"Ib"isthecurrentflowingintothebaseterminaland"Ie"isthe
currentflowingoutoftheemitterterminal.
Thentosummarise,thistypeofbipolartransistorconfigurationhasagreaterinputimpedance,currentandpowergain
thanthatofthecommonbaseconfigurationbutitsvoltagegainismuchlower.Thecommonemitterconfigurationisan
invertingamplifiercircuit.Thismeansthattheresultingoutputsignalis180"out-of-phase"withtheinputvoltagesignal.
TheCommonCollector(CC)Configuration
IntheCommonCollectororgroundedcollectorconfiguration,thecollectorisnowcommonthroughthesupply.Theinput
signalisconnecteddirectlytothebase,whiletheoutputistakenfromtheemitterloadasshown.Thistypeof
configurationiscommonlyknownasaVoltageFollowerorEmitterFollowercircuit.Thecommoncollector,oremitter
followerconfigurationisveryusefulforimpedancematchingapplicationsbecauseoftheveryhighinputimpedance,in
theregionofhundredsofthousandsofOhmswhilehavingarelativelylowoutputimpedance.
TheCommonCollectorTransistorCircuit
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Thecommonemitterconfigurationhasacurrentgainapproximatelyequaltotheβvalueofthetransistoritself.Inthe
commoncollectorconfigurationtheloadresistanceissituatedinserieswiththeemittersoitscurrentisequaltothatof
theemittercurrent.AstheemittercurrentisthecombinationofthecollectorANDthebasecurrentcombined,theload
resistanceinthistypeoftransistorconfigurationalsohasboththecollectorcurrentandtheinputcurrentofthebase
flowingthroughit.Thenthecurrentgainofthecircuitisgivenas:
TheCommonCollectorCurrentGain
Thistypeofbipolartransistorconfigurationisanon-invertingcircuitinthatthesignalvoltagesofVinandVoutare"in-
phase".Ithasavoltagegainthatisalwayslessthan"1"(unity).Theloadresistanceofthecommoncollectortransistor
receivesboththebaseandcollectorcurrentsgivingalargecurrentgain(aswiththecommonemitterconfiguration)
therefore,providinggoodcurrentamplificationwithverylittlevoltagegain.
BipolarTransistorSummary
Thentosummarise,thebehaviourofthebipolartransistorineachoneoftheabovecircuitconfigurationsisverydifferent
andproducesdifferentcircuitcharacteristicswithregardstoinputimpedance,outputimpedanceandgainwhetherthis
isvoltagegain,currentgainorpowergainandthisissummarisedinthetablebelow.
BipolarTransistorConfigurations
withthecharacteristicsofthedifferenttransistorconfigurationsgiveninthefollowingtable:
CharacteristicCommonBaseCommonEmitterCommonCollector
InputImpedanceLowMediumHigh
OutputImpedanceVeryHighHighLow
PhaseAngle01800
VoltageGainHighMediumLow
CurrentGainLowMediumHigh
PowerGainLowVeryHighMedium
InthenexttutorialaboutBipolarTransistors,wewilllookattheNPNTransistorinmoredetailwhenusedinthe
commonemitterconfigurationasanamplifierasthisisthemostwidelyusedconfigurationduetoitsflexibilityandhigh
gain.Wewillalsoplottheoutputcharacteristicscurvescommonlyassociatedwithamplifiercircuitsasafunctionofthe
collectorcurrenttothebasecurrent.
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