NPNTransistorConnection
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GORESET
TheNPNTransistor
IntheprevioustutorialwesawthatthestandardBipolarTransistororBJT,comesintwobasicforms.AnNPN(Negative-
Positive-Negative)typeandaPNP(Positive-Negative-Positive)type,withthemostcommonlyusedtransistortypebeing
theNPNTransistor.Wealsolearntthatthejunctionsofthebipolartransistorcanbebiasedinoneofthreedifferentways
-CommonBase,CommonEmitterandCommonCollector.
Inthistutorialaboutbipolartransistorswewilllookmorecloselyatthe"CommonEmitter"configurationusingNPN
TransistorswithanexampleoftheconstructionofaNPNtransistoralongwiththetransistorscurrentflow
characteristicsisgivenbelow.
AnNPNTransistorConfiguration
(Note:Arrowdefinestheemitterandconventionalcurrentflow,"out"foranNPNtransistor.)
TheconstructionandterminalvoltagesforanNPNtransistorareshownabove.ThevoltagebetweentheBaseand
Emitter(V),ispositiveattheBaseandnegativeattheEmitterbecauseforanNPNtransistor,theBaseterminalis
alwayspositivewithrespecttotheEmitter.AlsotheCollectorsupplyvoltageispositivewithrespecttotheEmitter
(V).SoforanNPNtransistortoconducttheCollectorisalwaysmorepositivewithrespecttoboththeBaseandthe
Emitter.
ThenthevoltagesourcesareconnectedtoanNPNtransistoras
shown.TheCollectorisconnectedtothesupplyvoltageVvia
theloadresistor,RLwhichalsoactstolimitthemaximumcurrent
flowingthroughthedevice.TheBasesupplyvoltageVis
connectedtotheBaseresistorR,whichagainisusedtolimit
themaximumBasecurrent.
Weknowthatthetransistorisa"current"operateddevice(Beta
model)andthatalargecurrent(Ic)flowsfreelythroughthe
devicebetweenthecollectorandtheemitterterminalswhenthe
transistorisswitched"fully-ON".However,thisonlyhappenswhen
asmallbiasingcurrent(Ib)isflowingintothebaseterminalof
thetransistoratthesametimethusallowingtheBasetoactasasortofcurrentcontrolinput.
ThetransistorcurrentinanNPNtransistoristheratioofthesetwocurrents(Ic/Ib),calledtheDCCurrentGainofthe
deviceandisgiventhesymbolofhfeornowadaysBeta,(β).Thevalueofβcanbelargeupto200forstandard
transistors,anditisthislargeratiobetweenIcandIbthatmakestheNPNtransistorausefulamplifyingdevicewhen
BE
CE
CC
B
B
usedinitsactiveregionasIbprovidestheinputandIcprovidestheoutput.NotethatBetahasnounitsasitisaratio.
Also,thecurrentgainofthetransistorfromtheCollectorterminaltotheEmitterterminal,Ic/Ie,iscalledAlpha,(α),and
isafunctionofthetransistoritself(electronsdiffusingacrossthejunction).AstheemittercurrentIeisthesumofavery
smallbasecurrentplusaverylargecollectorcurrent,thevalueofalphaα,isveryclosetounity,andforatypicallow-
powersignaltransistorthisvaluerangesfromabout0.950to0.999
αandβRelationshipinaNPNTransistor
Bycombiningthetwoparametersαandβwecanproducetwomathematicalexpressionsthatgivestherelationship
betweenthedifferentcurrentsflowinginthetransistor.
ThevaluesofBetavaryfromabout20forhighcurrentpowertransistorstowellover1000forhighfrequencylowpower
typebipolartransistors.ThevalueofBetaformoststandardNPNtransistorscanbefoundinthemanufactures
datasheetsbutgenerallyrangebetween50-200.
TheequationaboveforBetacanalsobere-arrangedtomakeIcasthesubject,andwithazerobasecurrent(Ib=0)
theresultantcollectorcurrentIcwillalsobezero,(βx0).Alsowhenthebasecurrentishighthecorresponding
collectorcurrentwillalsobehighresultinginthebasecurrentcontrollingthecollectorcurrent.Oneofthemostimportant
propertiesoftheBipolarJunctionTransistoristhatasmallbasecurrentcancontrolamuchlargercollectorcurrent.
Considerthefollowingexample.
ExampleNo1
AnNPNTransistorhasaDCcurrentgain,(Beta)valueof200.CalculatethebasecurrentIbrequiredtoswitcharesistive
loadof4mA.
Therefore,β=200,Ic=4mAandIb=20μA.
OneotherpointtorememberaboutNPNTransistors.Thecollectorvoltage,(Vc)mustbegreaterandpositivewith
respecttotheemittervoltage,(Ve)toallowcurrenttoflowthroughthetransistorbetweenthecollector-emitterjunctions.
Also,thereisavoltagedropbetweentheBaseandtheEmitterterminalofabout0.7v(onediodevoltdrop)forsilicon
devicesastheinputcharacteristicsofanNPNTransistorareofaforwardbiaseddiode.Thenthebasevoltage,(Vbe)of
aNPNtransistormustbegreaterthanthis0.7Votherwisethetransistorwillnotconductwiththebasecurrentgivenas.
Where:Ibisthebasecurrent,Vbisthebasebiasvoltage,Vbeisthebase-emittervoltdrop(0.7v)andRbisthebase
inputresistor.IncreasingIb,Vbeslowlyincreasesto0.7VbutIcrisesexponentially.
ExampleNo2
AnNPNTransistorhasaDCbasebiasvoltage,Vbof10vandaninputbaseresistor,Rbof100kΩ.Whatwillbethe
valueofthebasecurrentintothetransistor.
Therefore,Ib=93μA.
TheCommonEmitterConfiguration.
Aswellasbeingusedasasemiconductorswitchtoturnloadcurrents"ON"or"OFF"bycontrollingtheBasesignaltothe
transistorinetheritssaturationorcut-offregions,NPNTransistorscanalsobeusedinitsactiveregiontoproducea
circuitwhichwillamplifyanysmallACsignalappliedtoitsBaseterminalwiththeEmittergrounded.IfasuitableDC
"biasing"voltageisfirstlyappliedtothetransistorsBaseterminalthusallowingittoalwaysoperatewithinitslinearactive
region,aninvertingamplifiercircuitcalledasinglestagecommonemitteramplifierisproduced.
OnesuchCommonEmitterAmplifierconfigurationofanNPNtransistoriscalledaClassAAmplifier.A"ClassA
Amplifier"operationisonewherethetransistorsBaseterminalisbiasedinsuchawayastoforwardbiastheBase-
emitterjunction.Theresultisthatthetransistorisalwaysoperatinghalfwaybetweenitscut-offandsaturationregions,
therebyallowingthetransistoramplifiertoaccuratelyreproducethepositiveandnegativehalvesofanyACinputsignal
superimposeduponthisDCbiasingvoltage.Withoutthis"BiasVoltage"onlyonehalfoftheinputwaveformwouldbe
amplified.ThiscommonemitteramplifierconfigurationusinganNPNtransistorhasmanyapplicationsbutiscommonly
usedinaudiocircuitssuchaspre-amplifierandpoweramplifierstages.
Withreferencetothecommonemitterconfigurationshownbelow,afamilyofcurvesknownastheOutput
CharacteristicsCurves,relatestheoutputcollectorcurrent,(Ic)tothecollectorvoltage,(Vce)whendifferentvaluesof
Basecurrent,(Ib).Outputcharacteristicscurvesareappliedtothetransistorfortransistorswiththesameβvalue.
ADC"LoadLine"canalsobedrawnontotheoutputcharacteristicscurvestoshowallthepossibleoperatingpoints
whendifferentvaluesofbasecurrentareapplied.ItisnecessarytosettheinitialvalueofVcecorrectlytoallowtheoutput
voltagetovarybothupanddownwhenamplifyingACinputsignalsandthisiscalledsettingtheoperatingpoint
orQuiescentPoint,Q-pointforshortandthisisshownbelow.
SingleStageCommonEmitterAmplifierCircuit
OutputCharacteristicsCurvesofaTypicalBipolarTransistor
ThemostimportantfactortonoticeistheeffectofVceuponthecollectorcurrentIcwhenVceisgreaterthanabout1.0
volts.WecanseethatIcislargelyunaffectedbychangesinVceabovethisvalueandinsteaditisalmostentirely
controlledbythebasecurrent,Ib.Whenthishappenswecansaythenthattheoutputcircuitrepresentsthatofa
"ConstantCurrentSource".
ItcanalsobeseenfromthecommonemittercircuitabovethattheemittercurrentIeisthesumofthecollector
current,Icandthebasecurrent,Ib,addedtogethersowecanalsosaythatIe=Ic+Ibforthecommonemitter(CE)
configuration.
ByusingtheoutputcharacteristicscurvesinourexampleaboveandalsoOhm′sLaw,thecurrentflowingthroughthe
loadresistor,(RL),isequaltothecollectorcurrent,Icenteringthetransistorwhichinturncorrespondstothesupply
voltage,(Vcc)minusthevoltagedropbetweenthecollectorandtheemitterterminals,(Vce)andisgivenas:
Also,astraightlinerepresentingtheDynamicLoadLineofthetransistorcanbedrawndirectlyontothegraphofcurves
abovefromthepointof"Saturation"(A)whenVce=0tothepointof"Cut-off"(B)whenIc=0thusgivingusthe
"Operating"orQ-pointofthetransistor.Thesetwopointsarejoinedtogetherbyastraightlineandanypositionalongthis
straightlinerepresentsthe"ActiveRegion"ofthetransistor.Theactualpositionoftheloadlineonthecharacteristics
curvescanbecalculatedasfollows:
Then,thecollectororoutputcharacteristicscurvesforCommonEmitterNPNTransistorscanbeusedtopredictthe
Collectorcurrent,Ic,whengivenVceandtheBasecurrent,Ib.ALoadLinecanalsobeconstructedontothecurvesto
determineasuitableOperatingorQ-pointwhichcanbesetbyadjustmentofthebasecurrent.Theslopeofthisloadline
isequaltothereciprocaloftheloadresistancewhichisgivenas:-1/R
ThenwecandefineaNPNTransistorasbeingnormally"OFF"butasmallinputcurrentandasmallpositivevoltageat
itsBase(B)relativetoitsEmitter(E)willturnit"ON"allowingamuchlargeCollector-Emittercurrenttoflow.NPN
transistorsconductwhenVcismuchgreaterthanVe.
InthenexttutorialaboutBipolarTransistors,wewilllookattheoppositeorcomplementaryformoftheNPN
TransistorcalledthePNPTransistorandshowthatthePNPTransistorhasverysimilarcharacteristicstotheirNPN
transistorexceptthatthepolarities(orbiasing)ofthecurrentandvoltagedirectionsarereversed.
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