BJT Biasing Part 2 electronics analog electronics

biasing bjt dc value analog electronics

1. DC Biasing of BJT

3. Fig. Common-emitter npntransistor configuration.

4. Relationship B/W α& β

5. •Inthisbiasingcircuit,anumberofnetworksareanalyzed,
thereisanunderlyingsimilarityintheanalysisofeach
configurationduetotherecurringuseofthefollowing
importantbasicrelationshipsforatransistor:

6. •Themaximumratingsareindicated
byahorizontallineforthemaximum
collectorcurrent.
•Averticallineatthemaximum
collector-to-emittervoltage.
•Themaximumpowerconstraintis
definedbythecurve
•Atthelowerendofthescalesare
thecutoffregion,definedby
I_B<=0mA,andthesaturation
region,definedbyVCE<=VCEsat.

7. •Linear-region operation or active region:
Base–emitter junction forward-biased
Base–collector junction reverse-biased
•Cutoff-region operation:
Base–emitter junction reverse-biased
Base–collector junction reverse-biased
•Saturation-region operation:
Base–emitter junction forward-biased
Base–collector junction forward-biased
•Reverse Active region:
Base–emitter junction reverse-biased
Base–collector junction forward-biased

8. •TheBJTdevicecouldbebiasedtooperateoutsidethese
maximumlimits,buttheresultofsuchoperationwouldbe
eitheraconsiderableshorteningofthelifetimeofthedevice
ordestructionofthedevice.
•Confiningourselvestotheactiveregion,wecanselectmany
differentoperatingareasorpoints.ThechosenQ-pointoften
dependsontheintendeduseofthecircuit.Still,wecan
considersomedifferencesamongthevariouspointsshownin
earlierslidetopresentsomebasicideasabouttheoperating
pointand,thereby,thebiascircuit.

9. •Ifnobiaswereused,thedevice
wouldinitiallybecompletely
off,resultinginaQ-pointatA—
namely,zerocurrentthroughthe
device(andzerovoltageacross
it).

10. •Sinceitisnecessarytobiasadevicesothatitcanrespondtotheentirerangeofan
inputsignal,pointAwouldnotbesuitable.
•ForpointB,ifasignalisappliedtothecircuit,thedevicewillvaryincurrentand
voltagefromoperatingpoint,allowingthedevicetoreactto(andpossibly
amplify)boththepositiveandnegativeexcursionsoftheinputsignal.

11. •PointCwouldallowsomepositiveandnegativevariationoftheoutputsignal,butthe
peak-to-peakvaluewouldbelimitedbytheproximityofVCE=0VorIC=0mA
•PointDsetsthedeviceoperatingpointnearthemaximumvoltageandpowerlevel.The
outputvoltageswinginthepositivedirectionisthuslimitedifthemaximumvoltageisnot
tobeexceeded.

12. •Temperaturecausesthedeviceparameterssuchasthe
transistorcurrentgain(βac)andthetransistorleakage
current(ICEO)tochange.
•Highertemperaturesresultinincreasedleakagecurrentsin
thedevice,therebychangingtheoperatingconditionsetby
thebiasingnetwork.
•Theresultisthatthenetworkdesignmustalsoprovidea
degreeoftemperaturestabilitysothattemperaturechanges
resultinminimumchangesintheoperatingpoint.
•Thismaintenanceoftheoperatingpointcanbespecifiedbya
stabilityfactor,S.

13. FIXED-BIAS CIRCUIT
Fixed-bias circuit. DC equivalent of Fixed-bias
circuit.

14. Base Emitter Loop

15. Collector Emitter Loop

17. Load-Line Analysis
•Itisreferredtoasload-lineanalysisbecausetheload(network
resistors)ofthenetworkdefinedtheslopeofthestraightline
connectingthepointsdefinedbythenetworkparameters.
•

18. DC load line Cont…

19. DC load line Cont..
Movement of the Q-point with increasing level of IB
IfthelevelofIBischangedby
varyingthevalueofRB,theQ-
pointmovesupordown
theloadlineasshowninFig.for
increasingvaluesofIB.

20. DC load line Cont..
Effect of an increasing level of RCon the load line
and the Q-point.
If VCCis held fixed and RCincreased,
the load line will shift as shown in side
Fig.

21. Effect of lower values of VCCon the load line and the Q-
point.
DC load line Cont..
If RCis fixed and VCC decreased,
the load line shifts as shown in
side Fig.

24. Fixed Bias with Emitter Register Configuration
BJT bias circuit with emitter
resistor
Themorestablea
configuration,thelessits
responsewillchangedueto
undesirablechangesin
temperatureandparameter
variations.
Emitter Register provide negative feedback
and try to maintain operating point .

25. Fixed Bias with Emitter Register

26. Fixed
Bias
Circuit

27. Output Loop

28. Example: For the emitter-bias network of as shown in figure, determine:

30. Example:Fixedbiascircuit(1)withoutemitterregisterand(2)withemitter
registerforthegivenvalueofβ50andforanewvalueofβ100.Comparethe
changesinICandVCEforthesameincreaseinβ.
Table:Effectofβvariationonthe
responseofthefixed-biasconfigurationof
showninsideFig.
The BJT collector current is seen to change
by 100% due to the 100% change in the
value of β. The value of IBis the same, and
VCEdecreased by 76% (1) Fixed Bias cktwithout emitter register

31. NowtheBJTcollectorcurrentincreasesby
about81%duetothe100%increaseinβ.
NoticethatIBdecreased,helpingmaintainthe
valueofIC—oratleastreducingtheoverall
changeinICduetothechangeinβ.The
changeinVCEhasdroppedtoabout35%.The
networkofsideFig.isthereforemorestable
thanthatofwithoutemitterregisterforthe
samechangeinβ.
Effect of βvariation on the response of the
emitter-bias configuration of side Fig. .
(2) Fixed Bias cktwith emitter register

32. Voltage-divider bias configuration

33. Voltage-divider bias configuration cont..
vInthepreviousbiasconfigurationsthebiascurrentICQandvoltageVCEQ
wereafunctionofthecurrentgainβofthetransistor.However,becauseβis
temperaturesensitive,especiallyforsilicontransistors.
vTheactualvalueofbetaisusuallynotwelldefined,itwouldbedesirableto
developabiascircuitthatislessdependenton,orinfactisindependentof,the
transistorbeta.
vThevoltage-dividerbiasconfigurationissuchanetwork.Ifanalyzedonan
exactbasis,thesensitivitytochangesinbetaisquitesmall.Ifthecircuit
parametersareproperlychosen,theresultinglevelsofICQandVCEQcanbe
almosttotallyindependentofbeta.
vThefirsttobedemonstratedistheexactmethod,whichcanbe
appliedtoanyvoltage-dividerconfiguration.Thesecondisreferredtoasthe
approximatemethodandcanbeappliedonlyifspecificconditionsare
satisfied.Theapproximateapproachpermitsamoredirectanalysiswitha
savingsintimeandenergy.

34. Exact Analysis:

35. Exact Analysis cont.:
vTheinputsideofthenetworkcanthenbe
redrawnasshowninsideFigureforthedc
analysis.
vTheThéveninequivalentnetworkforthe
networktotheleftofthebaseterminalcanthen
befound:
vRThThevoltagesourceisreplacedbyashort-
circuitequivalentasshowninsideFigure:

36. Exact Analysis cont.:
vThevoltagesourceVCCisreturnedtothe
networkandtheopen-circuitThévenin
voltageofFig.determinedbyApplyingthe
voltage-dividerrule:

37. Exact Analysis cont.:
vTheThéveninnetworkisthen
redrawnasshowninFig.,andIBQcan
bedeterminedbyfirstapplying
Kirchhoff’svoltagelawintheclockwise
directionfortheloopindicated:

38. vOnceIBisknown,theremaining
quantitiesofthenetworkcanbefound
inthesamemannerasdevelopedfor
theemitter-biasconfiguration.
EXAMPLE: Determine the DC bias
voltageVCEandthecurrentICfor
thevoltagedividerconfigurationof
asshowninsideFigure.

41. Approximate Analysis:

42. EX 4.9: Repeat the analysis of Fig. 4.35 using the approximate technique, and compare
solutions for ICQ and VCEQ.
EX4.11:DeterminethelevelsofICQandVCEQforthevoltage-dividerconfigurationof
Fig.4.37usingtheexactandapproximatetechniquesandcomparesolutions.
Ex: Repeat the exact analysis of Ex if βis reduced to 50, and compare solutions for ICQ
and VCEQ.
(Solved it form Boylestad11 edition.)

43. Table. Effect of βvariation on the response
of the voltage-divider configuration
Theresultsclearlyshowtherelativeinsensitivityofthecircuittothechangeinβ.Eventhough
βisdrasticallycutinhalf,from100to50,thelevelsofICQandVCEQareessentiallythesame.

44. Field-Effect Transistors

45. Field-Effect Transistors

46. Field-Effect Transistors
The BJT transistor is a current-controlled device as depicted in Fig. 6.1a , whereas the
JFET transistor is a voltage-controlled device as shown in Fig. 6.1b .

47. vBJT transistor is a bipolar device-the prefix bi indicates that the
conduction level is a function of two charge carriers, electrons and
holes.
vThe FETis a unipolardevice depending solely on either electron (n-channel) or hole (p-channel) conduction.
vFETsare more temperature stable than BJTs, and FETs are usually
smaller than BJTs, making them particularly useful in integrated-circuit
(IC) chips
vOne of the most important characteristics of the FETis its high
input impedance.

48. Types of FET

49. Junction Field Effect Transistor (JFET)

50. VGS=0V, VDSSome Positive Value:

52. •Pinch-off condition of JFET(VGS= 0 V, VDS= VP).
•IDSSisthemaximumdraincurrentfor
aJFETandisdefinedbytheconditions
VGS=0VandVDS>abs(VP).
•

53. qVGS< 0 V
Application of a negative voltage to
the gate of a JFET
qThe level of VGSthat results in ID=0mA
is defined by VGS=VP, with VPbeing a
negative voltage for n-channel devices and
a positive voltage for p-channel JFETs.

56. p-Channel JFET

57. p-Channel JFET

58. Symbols:
JFET symbols: (a) n-channel; (b) p-channel

59. Thank You