Transistor_Biasing.pdf

Transistor Biasing

1. 1
TRANSISTOR BIASING
&
STABILIZATION

2. 2
Transistor Biasing
Thebasicfunctionoftransistorisamplification.Theprocessofraisingthe
strengthofweaksignalwithoutanychangeinitsgeneralshapeis
referredasfaithfulamplification.Forfaithfulamplificationitisessential
that:-
1.Emitter-Base junction is forward biased
2.Collector-Base junction is reversed biased
3.Proper zero signal collector current
Theproperflowofzerosignalcollectorcurrentandthe
maintenanceofpropercollectoremittervoltageduringthe
passageofsignaliscalledtransistorbiasing.

3. 3
WHY BIASING?
Ifthetransistorisnotbiasedproperly,itwouldworkinefficientlyand
producedistortioninoutputsignal.
HOW A TRANSISTOR CAN BE BIASED?
Atransistorisbiasedeitherwiththehelpofbatteryorassociatinga
circuitwiththetransistor.Thelatermethodismoreefficientandis
frequentlyused.Thecircuitusedfortransistorbiasingiscalledthe
biasingcircuit.

4. 4
BIASSTABILITY
Throughproperbiasing,adesiredquiescentoperatingpointofthetransistor
amplifierintheactiveregion(linearregion)ofthecharacteristicsisobtained.Itis
desiredthatonceselectedtheoperatingpointshouldremainstable.The
maintenanceofoperatingpointstableiscalledStabilisation.
Theselectionofaproperquiescentpointgenerallydependsonthefollowing
factors:
(a)Theamplitudeofthesignaltobehandledbytheamplifieranddistortion
levelinsignal
(b)Theloadtowhichtheamplifieristoworkforacorrespondingsupply
voltage
Theoperatingpointofatransistoramplifiershiftsmainlywithchangesin
temperature,sincethetransistorparameters—β,I
COandV
BE(wherethe
symbolscarrytheirusualmeaning)—arefunctionsoftemperature.

5. 5
The DC Operating Point
Foratransistorcircuittoamplifyitmustbeproperlybiasedwithdc
voltages.Thedcoperatingpointbetweensaturationandcutoffis
calledtheQ-point.ThegoalistosettheQ-pointsuchthatthatit
doesnotgointosaturationorcutoffwhenanaacsignalisapplied.

6. 6
Requirements of biasing network
•Ensuring proper zero signal collector current.
•Ensuring VcE not falling below 0.5V for Ge transistor and 1V for Silicon
transistor at any instant.
•Ensuring Stabilization of operating point. (zero signal ICand VcE)

7. 7
The Thermal Stability of Operating Point (S
Ico)
StabilityFactorS:-ThestabilityfactorS,asthechangeofcollector
currentwithrespecttothereversesaturationcurrent,keepingβand
VBEconstant.Thiscanbewrittenas:
The Thermal Stability Factor : S
Ico
S
Ico= ∂I
c
∂I
co
This equation signifies that I
cChanges S
Icotimes as fast as I
co
DifferentiatingtheequationofCollectorCurrentI
C=(1+β)Ico+βIb&
rearrangingthetermswecanwrite
S
Ico ═ 1+β
1-β(∂I
b/∂I
C)
It may be noted that Lower is the value of S
Icobetter is the stability
V
be,β

8. 8
Various Biasing Circuits
•Fixed Bias Circuit
•Fixed Bias with Emitter Resistor
•Collector to Base Bias Circuit
•Potential Divider Bias Circuit

9. 9
The Fixed Bias Circuit15 V
C

E

B

15 V
200 k 1 k
The Thermal Stability Factor : S
Ico
S
Ico= ∂I
c
∂I
co
General Equation of S
Ico
Comes out to be
S
Ico═ 1 + β
1-β(∂I
b/∂I
C)
V
be, β
Applying KVL through Base Circuit we can
write, I
b R
b+ V
be= V
cc
Diff w. r. t. I
C, we get (∂I
b/ ∂I
c) = 0
S
Ico= (1+β) is very large
Indicating high un-stability
I
b
R
b
R
C
R
C

10. 10
Merits:
•Itissimpletoshifttheoperatingpointanywhereintheactiveregionby
merelychangingthebaseresistor(RB).
•Averysmallnumberofcomponentsarerequired.
Demerits:
•Thecollectorcurrentdoesnotremainconstantwithvariationin
temperatureorpowersupplyvoltage.Thereforetheoperatingpointis
unstable.
•Whenthetransistorisreplacedwithanotherone,considerablechangein
thevalueofβcanbeexpected.Duetothischangetheoperatingpointwill
shift.
•Forsmall-signaltransistors(e.g.,notpowertransistors)withrelativelyhigh
valuesofβ(i.e.,between100and200),thisconfigurationwillbeproneto
thermalrunaway.Inparticular,thestabilityfactor,whichisameasureof
thechangeincollectorcurrentwithchangesinreversesaturationcurrent,
isapproximatelyβ+1.Toensureabsolutestabilityoftheamplifier,a
stabilityfactoroflessthan25ispreferred,andsosmall-signaltransistors
havelargestabilityfactors.

11. 11
Usage:
•Duetotheaboveinherentdrawbacks,fixedbiasisrarelyusedinlinear
circuits(i.e.,thosecircuitswhichusethetransistorasacurrentsource).
Instead,itisoftenusedincircuitswheretransistorisusedasaswitch.
However,oneapplicationoffixedbiasistoachievecrudeautomaticgain
controlinthetransistorbyfeedingthebaseresistorfromaDCsignal
derivedfromtheACoutputofalaterstage.

12. 12
Thefixedbiascircuitismodified
byattachinganexternalresistor
totheemitter.Thisresistor
introducesnegativefeedback
thatstabilizestheQ-point.
Fixed bias with emitter resistor

13. 13
Merits:
•Thecircuithasthetendencytostabilizeoperatingpointagainst
changesintemperatureandβ-value.
Demerits:
•Asβ-valueisfixedforagiventransistor,thisrelationcanbesatisfied
eitherbykeepingREverylarge,ormakingRBverylow.
IfREisoflargevalue,highVCCisnecessary.Thisincreasescost
aswellasprecautionsnecessarywhilehandling.
IfRBislow,aseparatelowvoltagesupplyshouldbe
usedinthebasecircuit.Usingtwosuppliesofdifferent
voltagesisimpractical.
•Inadditiontotheabove,REcausesacfeedbackwhichreducesthe
voltagegainoftheamplifier.
Usage:
Thefeedbackalsoincreasestheinputimpedanceoftheamplifierwhen
seenfromthebase,whichcanbeadvantageous.Duetotheabove
disadvantages,thistypeofbiasingcircuitisusedonlywithcareful
considerationofthetrade-offsinvolved.

14. 14
The Collector to Base Bias CircuitVCC
RC
C

E

B

RF
I
c
I
b
V
BE
+
-
I
E
Thisconfigurationemploysnegative
feedbacktopreventthermalrunawayand
stabilizetheoperatingpoint.Inthisformof
biasing,thebaseresistorRFisconnectedto
thecollectorinsteadofconnectingittothe
DCsourceVcc.Soanythermalrunawaywill
induceavoltagedropacrosstheRcresistor
thatwillthrottlethetransistor'sbasecurrent.

15. 15
Applying KVL through base circuit
we can write (I
b+ I
C) R
C+ I
b R
f+ V
be= V
cc
Diff. w. r. t. I
Cwe get
(∂I
b/ ∂I
c) = -R
C /(R
f+ R
C)
Therefore, S
Ico═ (1+ β)
1+ [βR
C/(R
C+ R
f)]
Which is less than (1+β), signifying better thermal stability

16. 16
Merits:
•Circuitstabilizestheoperatingpointagainstvariationsintemperature
andβ(i.e.replacementoftransistor)
Demerits:
•As β -value is fixed (and generally unknown) for a given transistor, this
relation can be satisfied either by keeping Rcfairly large or making Rfvery
low.
IfRcislarge,ahighVccisnecessary,whichincreases
costaswellas
precautionsnecessarywhilehandling.
IfRfislow,thereversebiasofthecollector–baseregionis
small,whichlimitstherangeofcollectorvoltageswingthat
leavesthetransistorinactivemode.
•TheresistorRfcausesanACfeedback,reducingthevoltage
gainoftheamplifier.Thisundesirableeffectisatrade-offfor
greaterQ-pointstability.
Usage:Thefeedbackalsodecreasestheinputimpedanceoftheamplifier
asseenfromthebase,whichcanbeadvantageous.Duetothegain
reductionfromfeedback,thisbiasingformisusedonlywhenthetrade-off
forstabilityiswarranted.

17. 17
Thisisthemostcommonlyusedarrangementforbiasingasitprovidegood
biasstability.Inthisarrangementtheemitterresistance‘RE’provides
stabilization.Theresistance‘RE’causeavoltagedropinadirectionsoasto
reversebiastheemitterjunction.Sincetheemitter-basejunctionistobe
forwardbiased,thebasevoltageisobtainedfromR1-R2network.Thenet
forwardbiasacrosstheemitterbasejunctionisequaltoVB-dcvoltagedrop
across‘RE’.ThebasevoltageissetbyVccandR1andR2.Thedcbias
circuitisindependentoftransistorcurrentgain.Incaseofamplifier,toavoid
thelossofacsignal,acapacitoroflargecapacitanceisconnectedacross
RE.Thecapacitoroffersaverysmallreactancetoacsignalandsoitpasses
throughthecondensor.
The Potential Divider Bias Circuit

18. 18VCC
RC
C

E

B

VCC
R1
RE R2
I
E
I
C
I
b
The Potential Divider Bias Circuit
Tofindthestabilityofthiscircuitwehave
toconvertthiscircuitintoitsThevenin’s
Equivalentcircuit
R
th=
R
1*R
2 &V
th=
Vcc R
2
R
1+R
2 R
1+R
2VCC
RC
C

E

B

VCC
R1
RE R2
R
th=
R
1*R
2 &V
th=
Vcc R
2
R
1+R
2 R
1+R
2
R
th=
R
1*R
2 &V
th=
Vcc R
2
R
1+R
2 R
1+R
2
R
th=
R
1*R
2 &V
th=
Vcc R
2
R
1+R
2 R
1+R
2

19. 19
Applying KVL through input base circuit
we can write I
bR
Th+ I
E R
E+ V
be= V
Th
Therefore, I
bR
Th+ (I
C+ I
b) R
E+ V
BE= V
Th
Diff. w. r. t. I
C& rearranging we get
(∂I
b/ ∂I
c) = -R
E /(R
Th+ R
E)
Therefore,
This shows that S
Ico
is inversely proportional to R
E
and It is less than (1+β), signifying better thermal
stability
Thevenin
Equivalent Ckt









ThRR
R
E
E
IcoS


1
1
The Potential Divider Bias CircuitVCC
RC
C

E

B

RE
RTh
VTh _
+
Thevenin
Equivalent Voltage
Self-bias Resistor
IE
I
b
I
C

20. 20
Merits:
•Operating point is almost independent of β variation.
•Operating point stabilized against shift in temperature.
Demerits:
•Asβ-valueisfixedforagiventransistor,thisrelationcanbesatisfiedeither
bykeepingREfairlylarge,ormakingR1||R2verylow.
IfREisoflargevalue,highVCCisnecessary.Thisincreases
costaswell
asprecautionsnecessarywhilehandling.
IfR1||R2islow,eitherR1islow,orR2islow,orbothare
low.AlowR1raisesVBclosertoVC,reducingtheavailable
swingincollectorvoltage,andlimitinghowlargeRCcanbe
madewithoutdrivingthetransistoroutofactivemode.Alow
R2lowersVbe,reducingtheallowedcollectorcurrent.
Loweringbothresistorvaluesdrawsmorecurrentfromthe
powersupplyandlowerstheinputresistanceoftheamplifier
asseenfromthebase.
ACaswellasDCfeedbackiscausedbyRE,whichreduces
theACvoltagegainoftheamplifier.AmethodtoavoidAC
feedbackwhileretainingDCfeedbackisdiscussedbelow.
Usage:
Thecircuit'sstabilityandmeritsasabovemakeitwidelyusedforlinear
circuits.

21. 21
Summary
•TheQ-pointisthebestpointforoperationofa
transistorforagivencollectorcurrent.
•Thepurposeofbiasingistoestablishastable
operatingpoint(Q-point).
•Thelinearregionofatransistoristheregionof
operationwithinsaturationandcutoff.
•Outofallthebiasingcircuits,potentialdivider
biascircuitprovideshigheststabilitytooperating
point.

22. 22
THANK YOU