Unit3TheBipolarJunc3onTransistor

Bipolarjunc-ontransistors(BJTs)

Contents•  BasicBipolarJunc3onTransistor,•  TransistorStructures(NPNandPNP)•  ModesofOpera3on•  SymbolandConven3ons•  CurrentVoltageCharacteris3cs•  NonidealTransistor•  Leakagecurrent•  Breakdown•  DCanalysisoftransistorcircuit•  CE,CB,CCConfigura3ons•  BasicTransistorac3onasaswitch,asanamplifier•  TransistorBiasing•  BiasStability•  DifferentBiasingCircuits•  UnderstandingManufacturersSpecifica3on

BasicBipolarJunc3onTransistorTransistorStructures

Construc-onofBipolarjunc-ontransistors

Baseregion(verynarrow)

Emi=erregion

Collectorregion

Collector

Base

Emi=er

EmiMer-basejunc3on

Collector-basejunc3on

NPNBJTshown•3terminals:emiMer,base,andcollector•2junc3ons:emiMer-basejunc3on(EBJ)andcollector-basejunc3on(CBJ)

Construc-onofBipolarjunc-ontransistors

UnbiasednpnTransistor

BJT

StandardTransistorSymbols

Q:Explainthesatura1on,cutoffandac1vemodesoftransistor

Biasedtransistor•  Inordertooperatethetransistorasanamplifierorswitch,

thetwopnjunc3onshastobebiasedproperlywithexternalvoltages.

•  Dependingontheexternalvoltagepolarityused,thetransistorworksinoneofthethreeregions.

• TransistorasAmplifier–Ac3veRegion

• TransistorasONswitch–Satura3onRegion

• TransistorasOFFswitch–CutoffRegion

TransistorBiasing

WorkingofanpntransistorinAc3vemode

Circuitconnec3onsforac-veregionfornpnandpnptransistor

BiasedTransistor

Workingofanpntransistor

Workingofapnptransistor

TransistorCurrentRela3on

TransistorCurrentRela3on

IC=αdcIE

αdciscalledemiMertocollectorcurrentgainInmanycircuits,Ic≈IE,αdc=1

Typicalvalueofαdc=0.96to0.995

TransistorCurrentRela3on(Cont.)IC=αdcIE+ICBO

ICBOiscalledthereverseleakagecurrentwhichflowsastheCBjunc3onisreversebiased.

TransistorCurrentDirec3ons

TransistorVoltageSourceConnec3onsandTerminalVoltages

Ac3veCBjunc3on:ForwardBiasedEBjunc3on:Reversebiased

Vcc>Vbb:Ac3veModeVcc<Vbb:Satura3onmode

TransistorVoltageSourceConnec3onsandTerminalVoltages

TransistorVoltageSourceConnec3onsandTerminalVoltages

VCC>VBB(ACTIVE)

-VCB-VBE+VCE=0

VCB=VCE-VBE

VBE>0.7V

VCE>0.7V,CBjunc3onisReversebiased,ACTIVEMODE

VCE<0.7V,CBjunc3onisForwardbiased,SATURATION

TransistorConfigura3ons

TransistorConfigura3ons

TransistorCharacteris3c

Characteris3cofCB

InputCharacteris3c

OutputCharacteris3c

Opera3ngRegions

•  Ac3veRegion•  CutoffRegion•  BreakdownRegion•  Satura3onRegion

Parameters

•  DynamicInputResistant(ri)•  DynamicOutputResistance(ro)•  Dccurrentgain(αdc)•  ShortCircuitCurrentGain(αorhi)

COMMONBASECONFIGURATION

CBcurrentgainorcurrenttransferra-o:

For example, α = 0.99, indicates that 99%of the electrons from the emitter arrive at the collector to form Ic while only 1% combined with the hole in the base to form IB

The relationship between the output and the input can be found as:

The base current is:

Summary :

CommonBaseConfigura3on

Characteris3cofCE

37

Common-EmiMerNPNTransistor

ForwardbiastheBEJ

ReversebiastheCBJ

InputCharacteris3cofCE•  PlotIBasf(VBE,VCE)•  AsVCEincreases,moreVBErequiredtoturntheBEonsothatIB>0.

•  Lookslikeapnjunc3onvolt-amperecharacteris3c.

OutputCharacteris3cofCE

OutputCharacteris3cs

•  PlotICasf(VCE,IB)•  Cutoffregion(off)

–  bothBEandBCreversebiased

•  Ac3veregion–  BEForwardbiased–  BCReversebiased

•  Satura3onregion(on)–  bothBEandBCforwardbiased

Q:Explainthethreeregionsofopera1onofCEBJT

CEConfigura3on

Q:Derivetherela1onbetweenαandβhencefindthevalueofβifα=0.95

Q:Findthevalueofαifβofthetransistoris49.FindthevalueofIcofthetransistorusingbothαandβra1ngofthetransistor

Q:Acertaintransistorhasα=.98,Ico=5μA,IB=100μA.FindthevaluesofcollectorcurrentandemiQercurrent.

CommonCollector(CC)Configura3on

Q:Derivetherela1onbetweenαandβhencefindthevalueofβifα=0.95

Comparisonoftheconfigura3ons

CEConfigura-onwidelyusedinamplifiercircuit?

CEConfigura3onismostlyuseddueto:• HighVoltageGain• HighCurrentGain• HighPowerGain

Q:Withthehelpofneatdiagram,explaintheconstruc1on,characteris1csandworkingoftypicalBJTusedforvoltageamplifica1on

Ans:(points)Construc1onofTransistor.Transistorasanamplifierusesac1vemodeofbiasing.Ac1vemodeofbiasing–EmiQerBasejunc1onisForwardBiasedandCollectorBasejunc1onisReverseBiased.Transistorconfigura1onusedforamplifierisCommonEmiQerconfigura1on.Drawthecircuitdiagram.Drawtheoutputcharacteris1cofTransistorinCEmodeandexplaintheworking

Transistorasanamplifier

AsignalsourceVsisconnectedininputcircuit.

Inputimpedanceofthetransistoris1KΩ,outputimpedanceis10KΩ.

Q:DrawandExplaintheTransistorasanamplifier

VBEvarieswith3me.IBvarieswith3measaresultcollectorcurrentIcandemiMercurrentIEvarieswith3me.

Thisvarying,collectorcurrentIcpassesthroughRLandavaryingoutputvoltageisdevelopedacrossit.

SignalVsissuperimposedondcvoltageVBB

Q:DrawandExplaintheTransistorasanamplifier

Example:Tounderstandhowasignalvoltageismagnified

InputsignalvoltageVs=20mV(rms)

Inputimpedanceofthetransistoris1KΩ,outputimpedanceis10KΩ.

Dccurrentgainoftransistor(β)ofCEtransistoris100

Outputresistanceoftransistorisveryhigh,thereforeoutputvoltageVoisapproximately:

Q:DrawandExplaintheTransistorasanamplifier

VoltageAmplifica3on:

Thus,TransistorAmplifyingac-onisbasicallyduetoitscapabilityoftransferringitssignalcurrentfromlowresistance

circuittoahighresistancecircuit

Transfer+Resistor=TRANSISTOR

Q:DrawandExplaintheTransistorasanamplifier

Graphicalmethod

TransistorasaSwitch

Opera3ngPointValuesofVCEandIcintheabsenceofacinputsignal(Vs)iscalledOPERATINGPOINT

OPERATINGPOINT=(Ic,VCE)valuesofthegivencircuitwiththeappliedDCbiasfornoinputsignal

Example:Tofindopera3ngpointforthegivencircuit.

ApplyingKVLtotheinputsec3onandneglec3ngVBE

Contd:

ApplyingKVLtotheoutputsec3on:

Vcc–IcRc–VcE=0

Opera3ngpointdefinedby:Ic=20mAandVcE=6V

Q:ExplaintheDCloadlineanalysisofatransistor,DependencyofQpoint,temperature,β

DCLoadLine•  Itisastraightlinedrawnontheoutputcharacteris3cofthetransistor.

•  DCLoadLineisusedforanalyzingtheperformanceofanAMPLIFIER,

•  ItisagraphofCollectorcurrent(Ic)versusCollectorEmiMervoltage(Vce)foragivenvalueofRcandVcc.

•  Itisagraphicalrepresenta3onofallthepossibleopera3ngpointsforthegivencircuitwiththegivenDCbiasing.

HowtoplotaDCloadline?

HowtoplotaDCloadline?

TofindtheQpointforpreviousproblem?

ApplyingKVLtotheinputsec3onandneglec3ngVBE

ApplyingKVLtotheoutputsec3on:

STABILIZATION

OPERATINGPOINTisthezerosignalvaluesof(Ic,VCE).

Ifeitherofthetwochanges,theopera3ngpointisshised.

Q:DependencyofQpointontemperatureandβ

Causesofunstabiliza3on?

•  Atransistorissaidtobeunstabilizedwhenitsopera3ngpointisshised,duetochangesincollectorcurrent.

•  TheCollectorcurrentinatransistorchangesrapidlyduetofollowingreasons:

1.   Inherentvaria1onsoftransistorparameters:Ifthetransistorisreplacedbyanotheroneofthesametype.(Becausenotwotransistorcanhavesametransistor

parameters(i.e.β)

2.ThetemperaturechangesaffectsIc.

HowdoesTemperaturechangeaffectIc.

•  LeakagecurrentIcoincreasesduetoincreaseintemperature.

•  IncreaseinIcoincreasesIc.

•  RiseinIcincreasestemperature.•  RiseinTemp.furtherincreasesIc.

•  RiseinIcfurtherincreasesTemp.,Temp.risefurtherincreasesIco,increaseinIcofutherincreasesIcandhencetempandsoon.

Cumula-veEffect:THERMALRUNAWAY

Selfdestruc1onoftransistor

Ic=βIB+ICEO

Opera3onpointstabiliza3on

Theprocessofmakingopera3ngpointindependentoftemperaturechangesorinherentvaria3onintransistorparametersiscalledSTABILIZATION

TRANSISTORBIASING

WhatistransistorDCbiasing?Biasingisselec3onofopera3ngpoint(Quiescentpoint)tooperateatransistorinadesiredregion(Ac3ve,cutoff,satura3on)

BiasingCircuit:Thecircuitrywhichprovidesthenecessarycondi3onsoftransistorbiasingisknownasbiasingcircuit.

NeedofTransistorBiasingForTransistorasanamplifier:

AserapplyinginputACsignalOpera3ngpointshouldalwaysremaininACTIVEREGION

Opera3ngPointinACTIVEREGION?

Opera3ngPointinACTIVEREGION?

Opera3ngPointinACTIVEREGION?

RequirementsofTransistorBiasingCircuit?

1.ItshouldensureproperzerosignalIcandVcE

2.Itshouldmaketheopera3ngpointindependentoftransistorparameters.

Sothatitdoesnotshi[whenthetransistorisreplacedwithanotherofthesametypeinthecircuit.

i.e.opera-ngpointshouldbeestablishedinthecenteroftheac1veregionofthecharacteris-cs,sothatsignalmaynotcutoffatanypart.

3.Stabilizethecollectorcurrentagainsttemperaturevaria3on.

DifferentBiasingCircuits

1.  FixedBiasingorBaseResistorBias.2.  CollectortobaseBiasing.3.  EmiMerresistorbiasing4.  Poten3aldividerbiasing

Analysisoffixedbiascircuit

Opera3ngPoint:

TofindIcandVCE

TofindIc?

ApplyKVLtotheinputsideandfindIB

FindICbyequa3on:Ic=βIB

FindVCEbyapplyingKVLtotheouterloop

FixedBias(BaseResistorBiasing)1.  Inthisbiasingahighvaluedresistorisconnectedbetween

thesupplyandbaseofthetransistor.

2.TheBaseemiMerjunc3onisforwardbiasedandthebasecollectorjunc3onisreversebiasedbythesupplyvoltageVcc.

3.Forapnptransistorthenega3veendofthesupplyistobeconnectedtotheRcandRb.

Q:DrawandExplainBaseresistormethodofbiasing

Ic=βIB+ICEO

Neglec3ng,ICEO

Ic=βIB

TocalculateIc:

TocalculateVcE:ApplyKVLtoouterloop:

Vcc=IcRc+VcEVcE=VcE-IcRc

Opera1ngpoint:(Ic,VCE)

AdvantagesofFixedBias:

•  Calcula3onsaresimple•  Simpleinconstruc3onOnlytworesistorsandabaKeryisneeded•  Easytofixtheopera3onpointanywhereintheac3veregionofthecharacteris3c.

BysimplychangingthevalueofRb.

Q:DrawandExplainBaseresistormethodofbiasing

DisadvantagesofFixedBias:

•  Opera3ngpointcanbefixedinthecenteroftheac3veregion.

Butitmiserablyfailsintheothertworequirementsofthebiasingcircuit.

•  Opera3ngpointisnotstablew.r.ttemperatureriseandtransistorparameterβ

Q:DrawandExplainBaseresistormethodofbiasing

CollectortoBaseBiasing1.  InthisbiasingaveryhighvalueresistanceRbisconnected

betweenthecollectorandbase,andRcisconnectedbetweenSupplyvoltageandcollectoroftransistor.

2.ThebasecurrentIbfeedbacksfromtheoutput.

ThebasecurrentIbdependsuponthecollectorvoltage.

ThiscircuitisalsocalledasVOLTAGEFEEDBACKBIASINGCIRCUIT.

AnalysisofCollectortobasebias

ApplyKVLtoinnerloop:

TocalculateIc:

TocalculateVcE:

ApplyKVLtoouterloop:

Vcc=(Ic+IB)Rc+VcE

CollectortoBaseBiasing

Advantagesofcollectortobasebias

Tendencyofthecircuittostabilizetheopera1ngpointagainsttemperaturevaria1onsandtransistorparametervaria1ons

•  Ifthereisanincreaseinβduetopiecevaria3on.OR

•  IfthereisaincreaseinβandIcoduetothechangeintemperature.

Ic=βIB+ICEO Icincreases

VcE=Vcc-IcRc VcEdecreasesIBdecreases

Icdecreases

Advantagesofcollectortobasebias

RisingtendencyinIcischecked

Advantagesofcollectortobasebias

•  IcdependsonIb.•  DecreaseinIbreducestheoriginalincreaseinIc(Nega3vefeedback).

•  Thereforethecircuitmaintainsastablevalueofcollectorcurrent,keepingQpointfixed.

Advantagesofcollectortobasebias

CollectortoBasebiasisseldomused?

•  TheresistorRBalsoprovidesacfeedback.

Thisreducesthevoltagegainoftheamplifier.

EmiMerResistorBiasing• Itisjustamodifica3ontotheBaseResistorBiasingcircuit.• Inthisbiasinganaddi3onalresistorREisconnectedintheemiMer.• Hence,thiscircuitcontainsthreeresistorsRB,RCandRE.

Analysisofopera3ngpoint

Analysisofopera3ngpoint

Stabiliza3on

• Hence,thestablilityoftheopera1ngpointisimprovedtolargeextentbyinser1ngaresistorinemiQercircuit.

• Thus,anincreaseincollectorcurrentisbroughtbacktoitspreviousvaluebythesequenceofevents.

Advantages• Thestabilityfactorofthisbiasingisquitehigh.• Hence,theopera3ngpointdoeschange,althoughtolowerextent,duetotemperatureriseorinherentvaria3onsinparameters.• TheDCfeedbackhelpsinthestabiliza3onofopera3ngpoint.• Butatthesame3meACfeedbackreducesthevoltagegainoftheamplifier.• Thisisanundesirablefeature.• However,thisdrawbackcanberemediedbypuWngacapacitorCEacrosstheresistorRE.ThecapacitorCEoffersverylowimpedancetotheACcurrentanddoesnotallowittopassthroughREandhenceACfeedbackisrestricted.Thus,theprocessofamplifica3onremainsunaffected

DrawbackofEmiMerbias

Thisbiasingcircuitisalsonotprac-callyusedbecauseofthefollowingreason:

Forstability,Icshouldbeindependentofβi.edenominatorshouldbeindependentofβ

• Thiscondi3oncanonlybeobtainedeitherusingREofverylargevalueorbyusingRBofverysmallvalue.

• Now,alargevalueofREwillcausealargevoltagedropacrossitandtoobtainarequiredopera3ngpoint,wehavetoapplyavoltagesourceVCCofhighvalue.

• Ontheotherhand,ifRBisofverylowvalue,aseparatelowvoltagesupplyhastobeusedforbasecircuit.

• Boththesealterna1vesarequiteimprac1cal.Hence,thisbiasingcircuitisnotusedprac1cally.

VoltageDividerBias

• Mostwidelyusedbiasingcircuit.

Q:DrawandExplainVoltageDividermethodofbiasing

AnalysisofOpera3ngPoint:(IcandVCE)InputCircuit

OutputCircuit

VoltageDividerBias

Opera1ngpoint:(Ic,VCE)

Problems

1.ForthecircuitshownplottheDCloadline

2.

3.

4.

DCLoadLine

4.

Opera-ngPoint

5.

5.

6.

6.

6.

QuiscentPoint=(1.228mA,2.326V)

7.

7.