Tutorial:3of8
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GORESET
ThePNPTransistor
ThePNPTransistoristheexactoppositetotheNPNTransistordevicewelookedatintheprevioustutorial.Basically,in
thistypeoftransistorconstructionthetwodiodesarereversedwithrespecttotheNPNtypegivingaPositive-Negative-
Positiveconfiguration,withthearrowwhichalsodefinestheEmitterterminalthistimepointinginwardsinthetransistor
symbol.
Also,allthepolaritiesforaPNPtransistorarereversedwhichmeansthatit"sinks"currentintoitsBaseasopposedto
theNPNtransistorwhich"sources"currentthroughitsBase.Themaindifferencebetweenthetwotypesoftransistorsis
thatholesarethemoreimportantcarriersforPNPtransistors,whereaselectronsaretheimportantcarriersforNPN
transistors.
Then,PNPtransistorsuseasmallbasecurrentandanegativebasevoltagetocontrolamuchlargeremitter-collector
current.InotherwordsforaPNPtransistor,theEmitterismorepositivewithrespecttotheBaseandalsowithrespectto
theCollector.
Theconstructionofa"PNPtransistor"consistsoftwoP-typesemiconductormaterialseithersideofanN-typematerial
asshownbelow.
APNPTransistorConfiguration
(Note:Arrowdefinestheemitterandconventionalcurrentflow,"in"foraPNPtransistor.)
TheconstructionandterminalvoltagesforanNPNtransistorareshownabove.ThePNPTransistorhasverysimilar
characteristicstotheirNPNbipolarcousins,exceptthatthepolarities(orbiasing)ofthecurrentandvoltagedirections
arereversedforanyoneofthepossiblethreeconfigurationslookedatinthefirsttutorial,CommonBase,Common
EmitterandCommonCollector.
ThevoltagebetweentheBaseandEmitter(V),isnownegative
attheBaseandpositiveattheEmitterbecauseforaPNPtransistor,
theBaseterminalisalwaysbiasednegativewithrespecttothe
Emitter.AlsotheEmittersupplyvoltageispositivewithrespectto
theCollector(V).SoforaPNPtransistortoconducttheEmitter
isalwaysmorepositivewithrespecttoboththeBaseandthe
Collector.
ThevoltagesourcesareconnectedtoaPNPtransistorareas
shown.ThistimetheEmitterisconnectedtothesupply
voltageVwiththeloadresistor,RLwhichlimitsthemaximum
BE
CE
CC
PNPTransistorConnectioncurrentflowingthroughthedeviceconnectedtotheCollectorterminal.TheBasevoltageVwhichisbiasednegativewith
respecttotheEmitterandisconnectedtotheBaseresistorR,whichagainisusedtolimitthemaximumBasecurrent.
TocausetheBasecurrenttoflowinaPNPtransistortheBaseneedstobemorenegativethantheEmitter(currentmust
leavethebase)byapprox0.7voltsforasilicondeviceor0.3voltsforagermaniumdevicewiththeformulasusedto
calculatetheBaseresistor,BasecurrentorCollectorcurrentarethesameasthoseusedforanequivalentNPN
transistorandisgivenas.
Generally,thePNPtransistorcanreplaceNPNtransistorsinmostelectroniccircuits,theonlydifferenceisthepolarities
ofthevoltages,andthedirectionsofthecurrentflow.PNPtransistorscanalsobeusedasswitchingdevicesandan
exampleofaPNPtransistorswitchisshownbelow.
APNPTransistorCircuit
TheOutputCharacteristicsCurvesforaPNPtransistorlookverysimilartothoseforanequivalentNPNtransistor
exceptthattheyarerotatedby180totakeaccountofthereversepolarityvoltagesandcurrents,(thecurrentsflowingout
oftheBaseandCollectorinaPNPtransistorarenegative).ThesamedynamicloadlinecanbedrawnontotheI-V
curvestofindthePNPtransistorsoperatingpoints.
TransistorMatching
YoumaythinkwhatisthepointofhavingaPNPTransistor,whenthereareplentyofNPNTransistorsavailablethatcan
beusedasanamplifierorsolid-stateswitch?.Well,havingtwodifferenttypesoftransistors"PNP"and"NPN",canbea
greatadvantagewhendesigningamplifiercircuitssuchastheClassBAmplifierwhichuses"Complementary"or
"MatchedPair"transistorsinitsoutputstageorinreversibleH-Bridgemotorcontrolcircuitswerewewanttocontrolthe
flowofcurrentevenlythroughthemotorinbothdirections.
ApairofcorrespondingNPNandPNPtransistorswithnearidenticalcharacteristicstoeachotherare
calledComplementaryTransistorsforexample,aTIP3055(NPNtransistor)andtheTIP2955(PNPtransistor)aregood
examplesofcomplementaryormatchedpairsiliconpowertransistors.TheybothhaveaDCcurrentgain,Beta,(Ic/Ib)
matchedtowithin10%andhighCollectorcurrentofabout15Amakingthemidealforgeneralmotorcontrolorrobotic
applications.
Also,classBamplifiersusecomplementaryNPNandPNPintheirpoweroutputstagedesign.TheNPNtransistor
conductsforonlythepositivehalfofthesignalwhilethePNPtransistorconductsfornegativehalfofthesignal.This
B
B
o
ComplementaryTransistors
allowstheamplifiertodrivetherequiredpowerthroughtheload
loudspeakerinbothdirectionsatthestatednominalimpedanceandpower
resultinginanoutputcurrentwhichislikelytobeintheorderofseveral
ampssharedevenlybetweenthetwocomplementarytransistors.
IdentifyingthePNPTransistor
Wesawinthefirsttutorialofthistransistorssection,thattransistorsare
basicallymadeupoftwoDiodesconnectedtogetherback-to-back.Wecan
usethisanalogytodeterminewhetheratransistorisofthePNPtypeor
NPNtypebytestingitsResistancebetweenthethreedifferent
leads,Emitter,BaseandCollector.Bytestingeachpairoftransistorleads
inbothdirectionswithamultimeterwillresultinsixtestsintotalwiththe
expectedresistancevaluesinOhm''sgivenbelow.
1.Emitter-BaseTerminals-TheEmittertoBaseshould
actlikeanormaldiodeandconductonewayonly.
2.Collector-BaseTerminals-TheCollector-Basejunctionshouldactlikeanormaldiode
andconductonewayonly.
3.Emitter-CollectorTerminals-TheEmitter-Collectorshouldnotconductineither
direction.
TransistorResistanceValuesforaPNPTransistorandaNPNTransistor
BetweenTransistorTerminalsPNPNPN
CollectorEmitterRR
CollectorBaseRR
EmitterCollectorRR
EmitterBaseRR
BaseCollectorRR
BaseEmitterRR
ThenwecandefineaPNPTransistorasbeingnormally"OFF"butasmalloutputcurrentandnegativevoltageatits
Base(B)relativetoitsEmitter(E)willturnit"ON"allowingamuchlargeEmitter-Collectorcurrenttoflow.PNP
transistorsconductwhenVeismuchgreaterthanVc.
InthenexttutorialaboutBipolarTransistorsinsteadofusingthetransistorasanamplifyingdevice,wewilllookatthe
operationofthetransistorinitssaturationandcut-offregionswhenusedasasolid-stateswitch.Bipolartransistor
switchesareusedinmanyapplicationstoswitchaDCcurrent"ON"or"OFF"suchasLED’swhichrequireonlyafew
milliampsatlowDCvoltages,orrelayswhichrequirehighercurrentsathighervoltages.
HIGHHIGH
LOWHIGH
HIGHHIGH
LOWHIGH
HIGHLOW
HIGHLOW
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