MOS
Operation with Zero GateVoltage
two back to back diodes in series
pn junctions atand drain
between
no current willflow when Vds is applied R 10112
Creating a Channel Current Flow
Source and Drain gnu
Positive voltage Ugs applied
freipositive holes or pushed downward
carrier Depletion regionwithacceptor atoms n region
elections an attracted
Indeed n region forms a channel for current flow between
drain und Sour
NMOS
Position threshold voltage
for a sufficientamountofmobileekibous
Ut 0,3 1W
Vos Vz Von IQ Cox WLvor
Thedrain isalwayspositiverelativtothe
Con E EMI C KWL FI L.EE
Applying a smallVos
Vos is small so Ugs is still constant alongthechannel
with effective voltage Vor
unit Lmleugin CoxWvor IE
Electron
driftvelocity Mu EI Mn
in MrCox E WasV1 Uns
For smalluns thechannel behaves as a linear resistance
controlled
by Von and therefore by Vos
Gas penCox E vor ros
hansconcherkner
process paranuw
Ein penCox AN
Was Ux ros 8 is is
Eu tunCent WK AN
Operation as Uns is increased
at source Vos VetVon
at drain Von Vos Vos UttVor Uns
AsVos is increased thechannel becomes moretop
resistance increases
n E Vorvos vs
Ein Y Vor Eun uns
in Ein E IvesVt uns Vos
Operation
forUss Vor
channelpinch
off when VosreachesVor
channel depth decreases to 0
drain current saturates as highervoltagewon't have
an Kit
i Edi E Voi uns Vol
Vossat Vor Vos Vt
caratstill flows through depletion region
CMOS and PMOS
subthnshold operation
for vors Ut with small draincurrent
Current Voltage Characteristics
in Vos MOSFET as switch
ros.uae
in Vos MOSFET as am
Operated in saturation region
MOSFET thenoperates as a voltagecontrolledconstant currentsource
determined
byVos orVor ofUn
independent
is Eu E Was Val
in 1k E vor
FinikOutput Resistance
increasingVos does effect the channel further
pickoff point ismovingauch fromdein
Chanel lengthmodulation
1 Avos
is En E was Ver 1 100
VA mit Us VAL
rot 1 ElvasVal
ro D
7 wouldntNo
PMOS
Applying the MOSFET in Amp Design
Obtaining a Voltage Amp
Insaturation voltage controlled currentsource
transcondultane
amp
pass current through resistor toget a voltage
Vas input voltage Ro loadresistance Uno supply voltage
Vos Von isRo
VoltageTransfer Characteristic UTC
Vos Vt transistor
is outoff in 0 Vos V00
Saturation Vt CVas 2 Vos Vos Vt Vos decreases
Triode Vos decreases moreslowly
ABregionwithgreatest slope in Eu Ucs Vel
Vos Vor duRoVos Vt
Vosis Vt 124EUR57 VG Vosis uns Vos VaslisVt
Biasing for Liner Amp
DCvoltage Vos der point Q
Vos V00 InRoVcs Vt quiescentpoint
4 A signal is superimposed on Vos
Vos t Vos Vost
Vos t be obtained graphically
can
theshark thesegment of VonH
thegreat the linearity
The Small Signal Voltage Gain
4 for a small Vos Vos α Uns
Voltage
gain is theslopeofthetangent
Au Fülasvos
Au En Vers Vt Ro EnVorRo
1 The gain is negative 180 phaseshift
2 The gain is proportional to RoEnandVon
Decurrent 10 1 InVö Au
IAumen
IT as bla Von
UTCby Graphical Analysis
in KF uns Load hin
intersects
x axis at Uns V00
slope NRO
A Vos Vt outoff so
operation P
Q Vas Vas Vas Vos lamp
B SaturationtoTriode
C Vas Von inTrioderegion switch closed
withlowros
Small Signal Operation und Models
The DCBiasPaint
DCbias
45 Vos A duVoi vg 0
Vos VoD RoIo
Ensure saturation Vos Vor
TheSignal Carrot in theDrainTerminal
Vos Vos Vgs in Eu Vostrgs Vt
T.EE tEEEEtEEE
Lastcomponent represents nonlinear distortion
Redend
by keeping V9 small
Eng kn Vos Vt rgs
rgs k 2 Us Vt 2Vor
Then smallsignalcondition is satisfied in Ist id
with id kn Vcs Vt vgs
Transcondentance
gm Äs En Vos Vt EuVor
9m vorvos
TheVoltageCain
Vos VoD Roin
smallsignalcondition
Vos V00 RD Ist id
Vos Raid
drainvoltage
PL gmvg.ro vgs 2 Vos Vt
Vgs Vt VD VDD
Ar F gmRo Triode tutoff
Small Signal Equivalent Circuit Models
FET behaves as voltage controlled currentsource
it applied idealconstantdevoltage sources an short
circuted
assumes drincurrent in saturation is independent of drainvoltage
modelled with to
go II VA Fo knVö
Av Ei gm Rollo
Transconductance
Im
9m Ein
WL Vos Vt hi WhVor
large transconduckner device must be short and mich
9m 12TVWIVET
S fünotosdebia wat
9m v
threedesign parameters Wh Vor Io two canbechosen independently
The T Equivalent Circuit Model
knVor
Basic MOSFET Amp Configs
TheThreeBasicConfigs
Connecteach
ofthe terminals to thegud
CommonSource
S to go
between Gand
Vi gut
wo across Ro between Dundgud
Common Gak
G togo
Vi between sourceandgnu
Vo across Ro between Dundgud
Common Drain SaraFollower
to gru
between Gand
Vi gut
wo across Ra between S undgud
Charatenizing Amps
Signal Using
Rigofsignal sara GILG
internalresistente
loadresistanceRa
input resistance
Rin Vi RinfksigUsig
Opencircuitvoltagegain Avo Rao
outputresistance Ro Er forVi 0
lookingintoampoutput
output voltage Vo MIR Avovi
voltage gain Av AvoREM
overallvoltagegain Gu Eig Ri RigAvoRIEN
The CommonScarceAmp CS
Rin D Vi Usig
Vo 9mVgs rollRo just for disenh circuits
Avo gm rollRo 5 Avo gmRD
Ro rollRD 5 Ro RD
1 Theinput resistance isideallyinfinite
2 Theoutput resistance is moderatetohigh reducingRo would decreasegain
lowoutput resistance requires
a sourcefollowstage
3 Theopencircuitvoltagegain Avo is high bandwidth is limited though
OverallVoltage Cain Gu Av gm Rollkur
With Rin s D Vi Usig K 2Vor
Rawillbeparallel toDoand Ro
TheCommon SourceAmp with SaurerResistance
T Model preferred with source resistance inseries
with Ngm
Rin D Vi Vsig
Vgs Fis Vi 1 YmRs
Rs can controlthemagnitudeofUss no distortion
Vo i Ro mit i Ins Vi 19Ham
Avo GER 9779m
Voltagegain
fromsaletodrein
Ro Ro mit Ra Av ILE Gu
The CommonGakAmp CG
Rin fm lowinput res
Vo IRD mit i Ägm
Avo gmRo same as CSbut Vi is attenuated
Ro RD
Eig REIN Isis
v.inin EI Gv 254am
EHER
R s
The Common DrainAmp Follow
Source
seven attenuation of signal b
amp withunity gain c
largeinput resistance Ui 0,9Usig
Raincludes actualloadandotherresistances
rossRa canbe omitted
Rin α
Ar pffh Av 1
Ro Igm Vi 0
Rafugm
G Au
highinput resistance
Summary
1 The CSanfig is thebestsuited for realizingtheballofthegainrequired in an amp
Dependingon the magnitude ofthegainrequired either a singlestage or a cascade
of
two or threestages canbe used
2 Including a resistor Rs in the source lead of theCSstage provides a numberof
performance improvements at theexpanseof gain reduction
3 The lowinput resistance the CGamp makes it useful in highfree scenarios
of
especially combined with a CS circuit
4 The source follower finds application as a voltagebuffer for connecting a high resistant
source to a lowresistance load and astheoutputstage in a multistageamp whereits
purpose is toequiptheampwith a lowoutput resistance
Biasingin MOS Amp Circuits
byFixing Voand Connecting a Resistance
intheSource
Vo Vast Rss
It Vo Vos Io ismostly by RsandVG
determined
16 Vg Vcs Rs provides negative feedback tostabiliaEs
As VG is constant Vos needsto decrease as In increases
butthatmakes Fo decreaseagain Rt degenerationresistan
Power supply VoD
C
Vo through voltagedivider RonRoz
IG 0 Ron are large forlang Rin
Ro as largeas possible forhighgain butsmall
enoughfor signalswingandsaturation
d Capocitator blocks du and couples using
must behighenoughfor desired frequencies
e Twopoorer supplies Uss VG
Reforhigh input resistance
Using a Brain to GateFeedback
Resistor
Vas Vos VoD ROID VDD Was RDID
RG MR forces devoltageatgahtobeequaltodrain Fo 0
When Fo increases Vormust decreasewhich causes In to decrease
Constant Current Source
using a
RG MR forlogsRin
Ro forappropriatedevoltageallowingtheoutput signal
tosavingund keepingtransistor saturated
b Q1 drainshortedtogateoperatinginsaturation
Ion Edi Elvas Val mit 1 0
VoD V
Ines
Q2 Voss Voss
7
I Ins lwal.tw aIo 1kilEh vas K Ä
Discrete Circuit MOSAmps
TheBasicStructure
s MF for ac ground forfrequencies ofinterest to bypass outputresistanceof arrestsource
bypasscapacitor
Usingwith Rsig is decoupledfrom circuit through Can blocking de and AC isshortcircuited
couplingcapacitor
ifSignalsource isprovidingdepathtogud gahand source can be connected directly
s Can coupling capacitor for Ra Vo Vd
Rin RG MRI Gv RoFRsig 9m rollRDIIRz
TheCommon SourceAmp withSour Resistance
Gv Rofksig
11
Biasresistance Ro makes RinJinik
The CommonGakAmp The SourceFollower
DC AC are zero at gab so Gu Rotksig RIEHL
its commented directlyatgut
eliminating RG Mo RD as drain is signalgro
Coupling capacitators working similar Coupling capacitors
to CS circuit
essentially theshipped downversion
TheAmpFreyResponse
BW fix Ja BW da
war
GB AnBW
