MOSFETs

MOSFET basics are discussed here

1. MOSFETs
Mr. A. B. Shinde

2. Mr. A. B. Shinde
FET
2
FieldEffectTransistor(FET)
•Theconductivity(orresistivity)ofthepathbetweentwocontacts,the
sourceandthedrain,isalteredbythevoltageappliedtothegate.
–Deviceisalsoknownasavoltagecontrolledresistor.

3. Mr. A. B. Shinde
MOSFETs
3
Ametal–oxide–semiconductorfield-effecttransistor(MOSFET,MOS-
FET,orMOSFET)isafield-effecttransistorwherethevoltage
determinestheconductivityofthedevice.
Theabilitytochangeconductivitywiththeamountofappliedvoltagecan
beusedforamplifyingorswitchingelectronicsignals.
MOSFETsarenowevenmorecommonthanBJTs(bipolarjunction
transistors)indigitalandanalogcircuits.

4. Mr. A. B. Shinde
FET
4
n-channel
Enhancement Mode
(nMOSFET)
p-channel
Enhancement Mode
(pMOSFET)
n-channel
Depletion Mode
(nMOSFET)
p-channel
Depletion Mode
(pMOSFET)

5. Mr.A.B.Shinde
MOSFET:Structure
5
TypicallyL=0.03μmto1μm,W=0.05
μmto100μm,andthethicknessofthe
oxidelayer(tox)isintherangeof1to
10nm.
perspectiveview;
Crosssection.

6. Mr. A. B. Shinde
MOSFET : Operation
6
OperationwithZeroGateVoltage:
•Withzerovoltageappliedtothegate,twoback-to-backdiodesexistin
seriesbetweendrainandsource.
•n+drainregionandthep-typesubstrate,
•p-typesubstrateandthen+sourceregion.
•Theseback-to-backdiodespreventcurrentconductionfromdrainto
sourcewhenavoltageV
DSisapplied.
•Thepathbetweendrainandsourcehasaveryhighresistance(ofthe
orderof10
12
Ω).

7. Mr. A. B. Shinde
MOSFET : Operation
7
ChannelforCurrentFlow:
•Source&Drainaregroundedand
appliedapositivevoltagetothe
gate(V
GS).
•V
GSrepellsthefreeholesfromthe
regionofthesubstrateunderthe
gate(thechannelregion).
•Theseholesarepusheddownward
intothesubstrate,leavingbehinda
carrier-depletionregion.
•PositiveV
GSattractselectronsfrom
then+sourceanddrainregions
intothechannelregion
•Whenasufficientnumberofelectrons
accumulatenearthesurfaceofthe
substrateunderthegate,annregionis
ineffectcreated,connectingthesource
anddrainregions,

8. Mr. A. B. Shinde
MOSFET : Operation
8
ChannelforCurrentFlow:
•Ifavoltageisappliedbetween
drainandsource,currentflows
throughthisinducednregion.
•Theinducednregionthusformsa
channelforcurrentflowfrom
draintosource
•Thisiscalledann-channel
MOSFEToranNMOStransistor.
•Thechanneliscreatedbyinverting
thesubstratesurfacefromptype
tontype.Hencetheinduced
channelisalsocalledaninversion
layer.
Note:Ann-channelMOSFETisformedinap-type
substrate
TheexcessofV
GSoverV
tistermedthe
effectivevoltageortheoverdrive
voltageandisthequantitythat
determinesthechargeinthechannel.
Here,V
GS−V
t≡V
OV

9. Mr. A. B. Shinde
MOSFET
9
•GateandChannelregionoftheMOSFETformaparallel-plate
capacitor,withtheoxidelayeractingasthecapacitordielectric.
•Positivegatevoltagecausespositivechargetoaccumulateonthetop
plateofthecapacitor(thegateelectrode).
•Negativechargeonthebottomplateisformedbytheelectronsinthe
inducedchannel.
•Thisfieldcontrolstheamountofchargeinthechannelhenceit
determinesthechannelconductivitywhenavoltageV
DSisapplied.
•Thisistheoriginofthename“field-effecttransistor”(FET).

10. Mr. A. B. Shinde
MOSFET : Operation
10
ApplyingaSmallV
DS:
•ThevoltageV
DScausesa
currenti
Dtoflowthroughthe
inducednchannel.
•Currentiscarriedbyfree
electronstravelingfromsource
todrain
•AnNMOStransistorwithV
GS>
V
tandwithasmallV
DSapplied.
Thedeviceactsasaresistance
whosevalueisdeterminedby
V
GS.
•Specifically,the channel
conductanceisproportionalto
V
GS–V
t,andthusi
Dis
proportionalto(V
GS–V
t)V
DS. depletion region is not
shown for simplicity.

11. Mr. A. B. Shinde
MOSFET : Operation
11
•ApplyingaSmallV
DS:
•Thei
D–V
DScharacteristicsof
theMOSFETwhenthevoltage
appliedbetweendrainand
source,vDS,iskeptsmall.
•Thedeviceoperatesasa
linearresistancewhosevalue
iscontrolledbyV
GS.

12. Mr. A. B. Shinde
MOSFET : Operation
12
OperationoftheenhancementNMOS
transistorasV
DSisincreased.Theinduced
channelacquiresataperedshape,andits
resistanceincreasesasv
DSisincreased.
Here,v
GSiskeptconstantatavalue>V
t;
V
GS=V
t+V
OV.
Thedraincurrenti
DVsV
DSforan
enhancement-typeNMOStransistor
operatedwith
V
GS=V
t+V
OV.

13. Mr. A. B. Shinde
MOSFET : p-Channel
13
Physical structure of
the PMOS transistor

14. Mr. A. B. Shinde
FET
14
Circuitsymbol
forthen-channel
enhancement-
typeMOSFET
Modified symbol
n channel.
Simplified circuit
symbol

15. Mr. A. B. Shinde
MOSFET: I
D-V
DSCharacteristics
15

16. Mr. A. B. Shinde
MOSFET: I
D-V
DSCharacteristics
16
Thei
D−v
DScharacteristics
foranenhancement-type
NMOStransistor

17. Biasing in MOS Circuits

18. Mr. A. B. Shinde
Biasing in MOS Amplifier Circuits
18
•AnessentialstepinthedesignofaMOSFETamplifiercircuitisthe
establishmentofanappropriatedcoperatingpointforthetransistor.
•Thisstepisalsoknownasbiasingorbiasdesign.
•Anappropriatedcoperatingpointorbiaspointischaracterizedbya
stableandpredictabledcdraincurrentI
Dandbyadcdrain-to-source
voltageV
DSthatensuresoperationinthesaturationregionforall
expectedinput-signallevels.
•TypesofBiasing:
–BiasingbyFixingV
GS
–BiasingbyFixingVGandConnectingaResistanceintheSource
–BiasingUsingaDrain-to-GateFeedbackResistor
–BiasingUsingaConstant-CurrentSource

19. Mr. A. B. Shinde
Biasing in MOS Amplifier Circuits
19
BiasingbyFixingV
GS:
•ThemostcommonapproachtobiasingaMOSFETistofixitsgate-to-
sourcevoltageV
GStothevaluerequiredtoprovidethedesiredI
D.
•ThisvoltageisderivedfromthepowersupplyvoltageV
DDthroughthe
useofanappropriatevoltagedivider.
•IndependentofhowthevoltageV
GSmaybegenerated,thisisnota
goodapproachtobiasingaMOSFET.
Becauseweknowthat,
AndthethresholdvoltageV
Otheoxide-capacitanceC
OX,andtransistor
aspectratioW/Lvarywidelyamongdevicesofsamesizeandtype.

20. Mr. A. B. Shinde
Biasing in MOS Amplifier Circuits
20
•BiasingbyFixingV
GS:
•BiasingbyfixingV
GSisnota
goodtechnique.
•Figuretwoi
D-v
GScharacteristic
curvesrepresentingextreme
valuesinabatchofMOSFETs
ofthesametype.
•ForthefixedvalueofV
GS,the
resultantspreadinthevalues
ofthedraincurrentcanbe
substantial.

21. Mr. A. B. Shinde
Biasing in MOS Amplifier Circuits
21
•Anexcellentbiasingtechniquefordiscrete
MOSFETcircuitsconsistsoffixingthedcvoltage
atthegate,V
G,andconnectingaresistanceinthe
sourcelead,asshowninfigure.
We can write,
V
G= V
GS+ R
SI
D
BiasingbyFixingV
GandConnectinga
ResistanceintheSource:

22. Mr. A. B. Shinde
Biasing in MOS Amplifier Circuits
22
BiasingbyFixingV
GandConnectinga
ResistanceintheSource:
•IfV
G>>V
GS,I
Dwillbedeterminedbythevalues
ofV
GandR
S.
•IfV
G>V
GS,resistorR
sprovidesnegative
feedback,whichwillstabilizethevalueofthebias
currentI
D.
•Fromequation,whenI
Dincreases&V
Gis
constant,V
GSwilldecrease.Whichwillfurther
decreaseI
D.
•ThustheR
sworkstokeepI
Dasconstantas
possible.
•ThisnegativefeedbackactionofR
sgivesitthe
namedegenerationresistance.
V
G= V
GS+ R
SI
D

23. Mr. A. B. Shinde
Biasing in MOS Amplifier Circuits
23
•Figureshowsthei
D–v
GScharacteristics
fortwodevicesthatrepresentthe
extremesofabatchofMOSFETs.
•Astraightlinethatrepresentsthe
constraintimposedbythebiascircuit—
namely.
•Theintersectionofthisstraightlinewith
thei
D–v
GScharacteristiccurveprovides
thecoordinates(I
DandV
GS)ofthebias
point.
•Inthiscase,thevariabilityobtainedinI
D
ismuchsmaller.Also,notethatthe
variabilitydecreasesasV
GandR
sare
madelarger.
BiasingbyFixingV
GandConnectingaResistanceintheSource:

24. Mr. A. B. Shinde
Biasing in MOS Amplifier Circuits
24
BiasingbyFixingV
GandConnectinga
ResistanceintheSource:
Practicalimplementationusingasingle
supply:
•Thecircuitutilizesonepower-supplyV
DDand
derivesV
Gthroughavoltagedivider(R
G1,R
G2).
•SinceI
G=0,R
G1andR
G2canbeselectedtobe
verylarge(intheMΩrange),allowingthe
MOSFETtopresentalargeinputresistancetoa
signalsource

25. Mr. A. B. Shinde
Biasing in MOS Amplifier Circuits
25
BiasingUsingaDrain-to-GateFeedback
Resistor:
•Asimpleandeffectivebiasingarrangementutilizing
afeedbackresistorconnectedbetweenthedrain
andthegateisshowninfigure.
•HerethelargefeedbackresistanceR
G(usuallyin
theMΩrange)forcesthedcvoltageatthegateto
beequaltothatatthedrain(becauseI
G=0).
Thuswecanwrite
V
GS= V
DS= V
DD–R
DI
D
Which can be rewritten in the form
V
DD= V
GS+ R
DI
D

26. Mr. A. B. Shinde
Biasing in MOS Amplifier Circuits
26
•BiasingUsingaDrain-to-GateFeedback
Resistor:
•IfI
Dincreasesduetoanyreason,thenV
GSmust
decrease.
•ThedecreaseinV
GSinturncausesadecreasein
I
D.
•Thusthenegativefeedbackordegeneration
providedbyR
GworkstokeepthevalueofI
Das
constantaspossible.
V
DD= V
GS+ R
DI
D
V
GS= V
DS= V
DD–R
DI
D

27. Mr. A. B. Shinde
Biasing in MOS Amplifier Circuits
27
BiasingUsingaConstant-CurrentSource:
•ThemosteffectiveschemeforbiasingaMOSFET
amplifieristhatusingaconstant-currentsource,as
showninfigure.
•HereR
G(usuallyinMΩrange)establishesadc
groundatthegateandpresentsalargeresistance
toaninputsignalsourcethatcanbecapacitively
coupledtothegate.
•ResistorR
Destablishesanappropriatedcvoltage
atthedraintoallowfortherequiredoutputsignal
swingwhileensuringthatthetransistoralways
remainsinthesaturationregion.

28. Small Signal Operation Model

29. Mr. A. B. Shinde
Small-Signal Operation and Models
29
•Considertheconceptualamplifiercircuit
showninfigure.
•HeretheMOStransistorisbiasedby
applyingadcvoltageV
GS,andtheinput
signaltobeamplified,v
gs,issuperimposed
onthedcbiasvoltageV
GS.
•Theoutputvoltageistakenatthedrain.
Conceptualcircuitto
studytheoperation
oftheMOSFETasa
small-signalamplifier

30. Mr. A. B. Shinde
Small-Signal Operation and Models
30
•DCBiasPoint:
•ThedcbiascurrentI
Dcanbefoundbysettingthe
signalv
gstozero;
Thus,
Here,Itisassumedthatλ=0
Here,V
OV=V
GS−V
tistheoverdrivevoltageat
whichtheMOSFETisbiasedtooperate.
Thedcvoltageatthedrain,V
DS,willbe
V
DS= V
DD−R
DI
D

31. Mr. A. B. Shinde
Small-Signal Operation and Models
31
•DCBiasPoint:
•Toensuresaturation-regionoperation,we
musthave
V
DS> V
OV
•Furthermore,sincethetotalvoltageatthe
drainwillhaveasignalcomponent
superimposedonV
DS,V
DShastobe
sufficientlygreaterthanV
OVtoallowforthe
requirednegativesignalswing.

32. Mr. A. B. Shinde
Small-Signal Operation and Models
32
SignalCurrentintheDrainTerminal:
•Considerthesituationwiththeinputsignalv
gs
applied.
•Thetotalinstantaneousgate-to-sourcevoltage
willbe
v
GS= V
GS+ v
gs
resultinginatotalinstantaneousdraincurrent
i
D,
dc bias current I
D
current component that is directly proportional to the input signal v
gs
represents nonlinear distortion.

33. Mr.A.B.Shinde
Small-SignalOperationandModels
33
•SignalCurrentintheDrainTerminal:
•ToreducethenonlineardistortionintroducedbytheMOSFET,theinput
signalshouldbekeptsmallsothat
resultingin
or,equivalently,
Ifthissmall-signalconditionissatisfied,theni
Dcanbeexpressedas
i
D≈ I
D+i
d
where
i
d= k
n(V
GS−V
t)v
gs

34. Mr. A. B. Shinde
Small-Signal Operation and Models
34
Theparameterthatrelatesi
dandv
gsistheMOSFETtransconductance
g
m,
orintermsoftheoverdrivevoltageV
OV,
g
m= k
nV
OV

35. Mr. A. B. Shinde
Small-Signal Operation and Models
35
•Figure shows a
graphicalinterpretation
ofthesmall-signal
operationof the
MOSFETamplifier.
•Notethatg
misequalto
theslopeofthei
D–v
GS
characteristicatthebias
point,
Small-signal operation
of the MOSFET amplifier

36. Mr. A. B. Shinde
Small-Signal Operation and Models
36
VoltageGain:
•Totalinstantaneousdrainvoltagev
DSasfollows:
v
DS= V
DD−R
Di
D
•Underthesmall-signalcondition,wehave
v
DS= V
DD−R
D(I
D+i
d)
•whichcanberewrittenas
v
DS= V
DS−R
di
d
•Thusthesignalcomponentofthedrainvoltageis
V
ds=−i
dR
D=−g
mv
gsR
D
whichindicatesthatthevoltagegainisgivenby
Theminussignindicatesthattheoutputsignalv
dsis180°outofphasewithrespect
totheinputsignalv
gs.
Conceptual circuit to
study operation of the
MOSFET

37. Mr. A. B. Shinde
Small-Signal Operation and Models
37
VoltageGain:
•Theinputsignalisassumedtohaveatriangular
waveformwithanamplitudemuchsmallerthan
2(V
GS–V
t),thesmall-signalconditiontoensure
linearoperation.
•Foroperationinthesaturation(active)regionat
alltimes,theminimumvalueofv
DSshouldnot
fallbelowthecorrespondingvalueofv
GSby
morethanV
t.
•Themaximumvalueofv
DSshouldbesmaller
thanV
DD;otherwisetheFETwillenterthecutoff
regionandthepeaksoftheoutputsignal
waveformwillbeclippedoff.
Conceptual circuit to
study operation of
the MOSFET

38. Mr. A. B. Shinde
Small-Signal Operation and Models
38
VoltageGain:
Conceptual circuit to study
operation of the MOSFET
Total instantaneous voltages v
GSand v
DS

39. Mr. A. B. Shinde
Small-Signal Operation and Models
39
Small-SignalEquivalent-CircuitModels:
•TheFETbehavesasavoltage-controlledcurrentsource.
•Itacceptsasignalv
gsbetweengateandsourceandprovidesacurrent
g
m.v
gsatthedrainterminal.
•Theinputresistanceofthiscontrolledsourceisveryhigh—ideally,
infinite.
•Theoutputresistance—isalsohigh.

40. Mr. A. B. Shinde
Small-Signal Operation and Models
40
Small-SignalEquivalent-CircuitModels:
Small-signal models for the MOSFET:
Neglecting the dependence of i
D
on v
DSin the active region
includingtheeffectofchannel-length
modulation,modeled byoutput
resistance

41. Mr. A. B. Shinde
Small-Signal Operation and Models
41
Small-SignalEquivalent-CircuitModels:
•IntheanalysisofaMOSFETamplifiercircuit,
thetransistorcanbereplacedbythe
equivalent-circuitmodelshowninFigure.
•Therestofthecircuitremainsunchanged
exceptthatidealconstantdcvoltagesources
arereplacedbyshortcircuits.
•Mostseriousshortcomingofthismodelisthat
itassumesthedraincurrentinsaturationto
beindependentofthedrainvoltage,but
actuallydraincurrentdependsonv
DSina
linearmanner.
•Suchdependencewasmodeledbyafinite
resistancer
obetweendrainandsource,
Neglecting the
dependence of i
Don v
DSin
the active region
including the effect of channel-
length modulation,

42. Mr. A. B. Shinde
Small-Signal Operation and Models
42
Small-SignalEquivalent-CircuitModels:

43. MOSFET as Amplifier & Switch

44. Mr. A. B. Shinde
MOSFET as Amplifier
44
•Inthesaturationregion,theMOSFETactsasavoltage-controlled
currentsource:Changesinthegate-to-sourcevoltagev
GSgivesriseto
changesinthedraincurrenti
D.
•ThusthesaturatedMOSFET canbeusedtoimplementa
transconductanceamplifier

45. Mr. A. B. Shinde
MOSFET as Amplifier
45
Large-SignalOperation:
TheTransferCharacteristic:
•Groundedsourceterminaliscommonto
boththeinputandoutput.
•Here,changesinv
1(v
GS=v
1)giveriseto
changesini
D,weareusingaresistorR
Dto
obtainanoutputvoltagev
0
v
0= v
DS= V
DD–R
D.i
D
Inthiswaythetransconductanceamplifier
isconvertedintoavoltageamplifier.
•Todeterminethevoltagetransfer
characteristicoftheCSamplifier,wewill
assumev
jtobeintherangeof0toV
DD.
Basic structureof
common-sourceamplifier

46. Mr. A. B. Shinde
MOSFET as Amplifier
46
Large-SignalOperation-TheTransferCharacteristic:
Basic structureof
common-sourceamplifier
Transfer characteristic of the amplifier

47. Mr. A. B. Shinde
MOSFET as Amplifier
47
Large-SignalOperation:
TheTransferCharacteristic:
v
DS= V
DD–R
D.i
D
•Straightlineoni
D-v
DScharacteristics
curvesshowsthei
D-v
DSrelationship.
•Sincev
GS=v
1,forv
1<V
tthetransistor
willbecutoff,i
Dwillbezero,andv
0=
v
DS=V
DD(pointA).
•AsV
iexceedsV
tthetransistorturnson,
i
Dincreases,andv
0decreases.
•Thiscorrespondstopointsalongthe
segmentoftheloadlinefromAtoB.
•Wehaveidentifiedaparticularpointin
thisregionofoperationandlabeledit
Q.ItisobtainedforV
GS=V
IQandhas
thecoordinatesV
0Q=V
DSQandI
DQ.
Transfer characteristic
of the amplifier

48. Mr. A. B. Shinde
MOSFET as Amplifier
48
Large-SignalOperation:
TheTransferCharacteristic:
•Saturation-regionoperationcontinues
untilv
0decreasesbelowV
t.
•Atthispoint,v
DS=v
GS-V
Dandthe
MOSFETentersitstrioderegion.
•ThisisreferstopointBingraph.
PointBisdefinedbyv
0B=v
1B–V
t.
•ForV
i>V
IB,thetransistorisdriven
deeperintothetrioderegion.
•Thecharacteristiccurvesinthetriode
regionarebunchedtogether,theoutput
voltagedecreasesslowlytowardszero.
•Herewehaveidentifiedaparticular
operatingpointCobtainedforv
1=V
DD.
•ThecorrespondingoutputvoltageV
OCwill
usuallybeverysmall. Transfer characteristic
of the amplifier

49. Mr. A. B. Shinde
MOSFET as Amplifier
49
Large-SignalOperation-TheTransferCharacteristic:
Basic structureof
common-sourceamplifier
TransferCharacteristics

50. Mr. A. B. Shinde
MOSFET as Amplifier
50
Large-SignalOperation:
TheTransferCharacteristic:
PointCobtainedforv
i=V
DD.
Thecorrespondingoutputvoltage
V
OCwillusuallybeverysmall.
Thispoint-by-pointdeterminationof
thetransfercharacteristicresultsin
thetransfercurveshowninfigure.
Observethatwehavedelineated
itsthreedistinctsegments,each
correspondingtooneofthethree
regionsofoperationofMOSFET
Q
1.
TransferCharacteristics

51. Mr. A. B. Shinde
MOSFET as Amplifier
51
MOSFETasaSwitch:
•WhentheMOSFETisusedasaswitch,itisoperatedattheextreme
pointsofthetransfercurve.
•Thedeviceisturnedoffbykeeping,v<V
t.Here,v
0=V
DD.
•TheswitchisturnedonbyapplyingavoltageclosetoV
DD.Here,v
0is
verysmall.
•Thecommon-sourceMOScircuitcanbeusedasalogicinverterwith
the"low"voltagelevelcloseto0Vandthe"high"levelclosetoV
DD.

52. Mr. A. B. Shinde
MOSFET as Amplifier
52
MOSFETasaSwitch:
OperationasaLinearAmplifier
•TooperatetheMOSFETasanamplifier,saturation-modeismaintained.
•Thedeviceisbiasedatasomewhereneartothemiddleofthetransfer
curve.Thevoltagesignaltobeamplifiedv
tisthensuperimposedonthe
dcvoltageV
IQ.
•Bykeepingv
tsufficientlysmalltorestrictoperationtoanalmostlinear
segmentofthetransfercurve,theresultingoutputvoltagesignalv
0will
beproportionaltov
t.
•Thatis,theamplifierwillbeverynearlylinear,andv
Qwillhavethesame
waveformasv
texceptthatitwillbelargerbyafactorequaltothe
voltagegainoftheamplifieratQ.

53. Mr. A. B. Shinde
MOSFET as Amplifier
53
MOSFETasaSwitch:
OperationasaLinearAmplifier
Thusthevoltagegainisequaltotheslopeofthetransfercurveatthe
biaspointQ.
Theslopeisnegative,hencethebasicCSamplifierisinverting.
Iftheamplitudeoftheinputsignalv,theoutputsignalwillbecome
distortedsinceoperationwillnolongerberestrictedtoanalmostlinear
segmentofthetransferCurve.

54. Mr. A. B. Shinde
MOSFET as Amplifier
54
MOSFETasaSwitch:
AnalyticalExpressionsforthe
TransferCharacteristic:
•Fromthei-vrelationshipswecansee
that,theMOSFEToperatesinthree
regions—cutoff,saturation,and
triode.
•Cutoff–RegionSegment,XA:
Here,v
i<V
t,andv
0=V
DD.
•Saturation–RegionSegment,A
QB:
Here,v
i,≥V
tand
v
0≥v
i-V
t
•Triode-RegionSegment,BC:
Here,v
i≥V
tandv
0≤v
i–V
t
Transfer characteristic
of the amplifier

55. Single Stage MOS Amplifier

56. Mr. A. B. Shinde
Single Stage MOS Amplifier
56
•TheBasicStructure:
•Figureshowsthebasiccircuittoimplement
thevariousconfigurationsofdiscrete-circuit
MOSamplifiers.
•Duetoeffectivenessandsimplicityconstant-
currentbiasingtechniqueisusedforbiasing
theMOStransistor.
•Figureindicatesthedccurrentandthedc
voltagesresultingatvariousnodes.

57. Mr. A. B. Shinde
Single Stage MOS Amplifier
57
CharacterizingAmplifiers:
1.Theamplifieriswithasignalsourcehavinganopen-circuitvoltagev
sig
andaninternalresistanceR
sig.Thesearetheparametersofanactual
signalsource.Similarly,R
Lisanloadresistance.
2.ParametersR
i,R
0,A
vs,A
is,andG
mpertaintotheamplifierproper;thatis,
theydonotdependonthevaluesofR
sigandR
L.Bycontrast,R
in,R
out,
A
v,A
i,G
v0,andG
vmaydependononeorbothofR
sigandR
L.
3.Asmentionedabove,fornonunilateralamplifiers,R
inmaydependonR
L,
andR
outmaydependonR
sig.Nosuchdependenciesexistforunilateral
amplifiers,forwhichR
in=R
iandR
out=R
0.
4.Theloadingoftheamplifieronthesignalsourceisdeterminedbythe
inputresistanceR
in.ThevalueofR
indeterminesthecurrentthatthe
amplifierdrawsfromthesignalsource.Italsodeterminestheproportion
ofthesignalv
sigthatappearsattheinputoftheamplifier.

58. Mr. A. B. Shinde
Single Stage MOS Amplifier
58
•CharacterizingAmplifiers:
•Figureshowsanamplifierfedwithasignalsourcehavinganopen-
circuitvoltagev
sigandaninternalresistanceR
sig.
•Thesecanbetheparametersofanactualsignalsource,
•TheamplifierisshownwithaloadresistanceR
Lconnectedtotheoutput
terminal.
•Here,R
Lcanbeanactualloadresistanceortheinputresistanceofa
succeedingamplifierstageinacascadeamplifier.

59. Mr. A. B. Shinde
Single Stage MOS Amplifier
59
•CharacterizingAmplifiers:
•Figureshowstheamplifiercircuitwiththeamplifierblockreplacedbyits
equivalent-circuitmodel.
•TheinputresistanceR
inrepresentstheloadingeffectoftheamplifier
inputonthesignalsource.
R
inandR
sigformsavoltagedividerthatreducesv
sigtothevaluev
i

60. Mr. A. B. Shinde
Single Stage MOS Amplifier
60
•CharacterizingAmplifiers:
•Thesecondparameterincharacterizingamplifierperformanceisthe
open-circuitvoltagegainA
vo,definedas
ThelastparameteristheoutputresistanceR
o.Fromfigure,R
oisthe
resistanceseenlookingbackintotheamplifieroutputterminalwithv
iset
tozero.
AsR
oisdeterminedwithv
i=0,thevalueofR
odoesnotdependonR
sig.

61. Mr. A. B. Shinde
Single Stage MOS Amplifier
61
•CharacterizingAmplifiers:
Output voltage v
o
Voltage gain of the amplifier, A
v
Overall voltage gain, G
v

62. Mr. A. B. Shinde
Single Stage MOS Amplifier
62

63. Mr. A. B. Shinde
Single Stage MOS Amplifier
63
CSAmplifier:
•Figureshowsacommon-source(CS)amplifierfedwithasignalsource
v
sighavingasourceresistanceR
sig.
•AnalyzethiscircuittodetermineR
in,A
vo,andR
o.Here,assumeR
Dis
partoftheamplifier;thusifaloadresistanceR
Lisconnectedtothe
amplifieroutput,R
LappearsinparallelwithR
D.Insuchacase,wewish
todetermineA
vandG
vaswell.
•ReplacingtheMOSFETwithitshybrid-πmodel(withoutr
o),weobtain
theCSamplifierequivalentcircuitasshowninsecondfigure.

64. Mr. A. B. Shinde
Single Stage MOS Amplifier
64
CSAmplifier:

65. MOSFET Internal Capacitances

66. Mr. A. B. Shinde
MOSFET Internal Capacitances
66
•Various internal
capacitances,areshown
forn-channelMOSFET
operating in the
saturationregion.
•Therearefourinternal
capacitances:
•C
gsandC
gd,resultfrom
the gate-capacitance
effect;
•C
sbandC
db,arethe
depletioncapacitancesof
thepnjunctionsformed
bythesourceregionand
thesubstrate,andthe
drainregionandthe
substrate,respectively.

67. Mr. A. B. Shinde
MOSFET Internal Capacitances
67
•Thepolysilicongateformsaparallel-platecapacitorwiththechannel
region,whereoxidelayerworksasdielectric.
•Thegate(oroxide)capacitanceperunitgateareaisdenotedC
ox.When
thechannelistaperedandpinchedoff,thegatecapacitanceisgivenby
2/3WLC
ox.
•Therearetwoothersmallcapacitancesresultingfromtheoverlapofthe
gatewiththesourceregion(orsourcediffusion)andthedrainregion(or
draindiffusion).
•EachoftheseoverlapshasalengthL
ovandthustheresultingoverlap
capacitancesC
ovaregivenby
Typically,L
ov=0.05to0.1L.
Wecannowexpressthegate-to-sourcecapacitanceC
gsas

68. Mr. A. B. Shinde
MOSFET Internal Capacitances
68
•Forthegate-to-draincapacitance,wenotethatthechannelpinch-offat
thedrainendcausesC
gdtoconsistentirelyoftheoverlapcomponent
C
ov,
Thedepletion-layercapacitancesofthetworeverse-biasedpnjunctions
formedbetweeneachofthesourceandthedraindiffusionsandthep-
typesubstrate.
Thus,forthesourcediffusion,wehavethesource-bodycapacitance,
C
sb,
whereC
sb0isthevalueofC
sbatzerobody-sourcebias,V
SBisthe
magnitudeofthereverse-biasvoltage,andV
0isthejunctionbuilt-in
voltage(0.6Vto0.8V).

69. Mr. A. B. Shinde
MOSFET Internal Capacitances
69
•Similarly,forthedraindiffusion,wehavethedrain-bodycapacitance
C
db,
whereC
db0isthecapacitancevalueatzeroreverse-biasvoltageand
V
DBisthemagnitudeofthisreverse-biasvoltage.Notethatwehave
assumedthatforbothjunctions,thegradingcoefficientm=1/2.
Problem:Forann-channelMOSFETwitht
ox=10nm,L=1.0μm,
W=10μm,L
ov=0.05μm,C
sb0=C
db0=10fF,V
0=0.6V,V
SB=1Vand
V
DS=2V.Calculatethefollowingcapacitanceswhenthetransistoris
operatinginsaturation:C
ox,C
ov,C
gs,C
gd,C
sb,andC
db.
Ans: C
ox = 3.45 fF/μm
2
; C
ov= 1.72 fF; C
gs= 24.7 fF;
C
gd= 1.72 fF; C
sb= 6.1 fF; C
db= 4.1 fF

70. MOSFET High Frequency Model

71. Mr. A. B. Shinde
MOSFET High Frequency Model
71
•Figureshowsthesmall-signalmodeloftheMOSFET,includingthefour
capacitancesC
gs,C
gd,C
sb,andC
db.
•Thismodelisusedtopredictthehigh-frequencyresponseofMOSFET
amplifiers.
High-frequency, equivalent-circuit model for the MOSFET

72. Mr. A. B. Shinde
MOSFET High Frequency Model
72
•Whenthesourceisconnectedtothebody,themodelsimplifies
considerably,asshowninfigure.
•Inthismodel,C
gd,althoughsmall,playsasignificantroleindetermining
thehigh-frequencyresponseofamplifiersandthusmustbekeptinthe
model.
The equivalent circuit for source is connected to the substrate

73. Mr. A. B. Shinde
MOSFET High Frequency Model
73
•CapacitanceC
db,canusuallybeneglected,resultinginsignificant
simplificationofmanualanalysis.
•Theresultingcircuitisshowninfigure.
Theequivalent-circuitmodelwithC
dbneglected

74. Mr. A. B. Shinde
MOSFET High Frequency Model
74
•Figureshowsthehigh-frequencyTmodelinitssimplifiedform.
The simplified high-frequency T model

75. Mr. A. B. Shinde
MOSFET High Frequency Model
75
MOSFETHigh-FrequencyModel:Summary

76. Mr. A. B. Shinde
MOSFET High Frequency Model
76
•Problem:Calculatef
Tforthen-channelMOSFETwhosecapacitances
werefoundinExercise10.3.Assumeoperationat100μAandthat
k
n=160μA/V2.
•Ans.3.4GHz

77. Frequency Response of
CS Amplifier

78. Mr. A. B. Shinde
Frequency Response of CS Amplifier
78
Magnitude of the gain of a discrete-circuit MOS amplifier
versus frequency

79. Mr. A. B. Shinde
Frequency Response of CS Amplifier
79
•Figureshows,atlowerfrequencies,themagnitudeoftheamplifiergain
fallsoffduetocouplingandbypasscapacitors.Here,itisassumedthat
theirimpedancesweresmallenoughtoactasshortcircuits.
•Atmidbandfrequencies,asthefrequencyoftheinputsignalislowered,
thereactance1/jωCofeachofthesecapacitorsbecomessignificant,
thisresultsinadecreaseintheoverallvoltagegainoftheamplifier.
•Loweranduppercut-offfrequencyf
L&f
H,arethefrequenciesatwhich
thegaindropsby3dBbelowitsvalueinmidband.
•BW=f
H−f
L(discrete-circuitamplifiers)
•BW=f
H(integrated-circuitamplifiers)

80. CMOS Inverter

81. Mr. A. B. Shinde
CMOS Inverter
81
TheCMOSinverterisconstructedbyusing
nMOS&pMOStransistors.
AsthepMOStransistorpassesstrong1and
weak0,itisconnectedtothesupplyvoltage
V
DDand
nMOStransistorpassesstrong0andweak
1,itisconnectedtotheground.
CMOS inverter

82. Mr. A. B. Shinde
CMOS Inverter
82
CircuitOperation:
•Considerthetwoextremecases:
•Whenv
iisatlogic-0level,whichis0V,and
whenv
iisatlogic-1level,whichisV
DDvolts.
•Inbothcases,foreaseofexpositionwe
shallconsiderthen-channeldeviceQ
Ntobe
thedrivingtransistorandthep-channel
deviceQ
Ptobetheload.
•Ascircuitissymmetric,thisassumptionis
arbitrary,andthereversewouldleadto
identicalresults.

83. Mr. A. B. Shinde
CMOS Inverter
83
CircuitOperation:
Circuit with vi= V
DD
equivalent circuit

84. Mr. A. B. Shinde
CMOS Inverter
84
CircuitOperation:
Circuit with vi= 0 V
equivalent circuit

85. Mr. A. B. Shinde
CMOS Inverter
85
Voltage-
Transfer
Characteristic:

86. Mr. A. B. Shinde
Reference
86
•Microelectronic Circuits by Adel S. Sedra& Kenneth C. Smith, 7e

87. This presentation is published only for educational purpose
Frequency Response of CS Amplifier
87
ThankYou…!!!
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