RF Circuit Design

210 Pages·2008·8.911 MB·English

by  Bowick, Christopher, Blyler, John, Ajluni, Cheryl

RF CIRCUIT DESIGN
CHRISTOPHER BOWICK
WITH
JOHN BLYLER AND CHERYL AJLUNI
AMSTERDAM • BOSTON • HEIDELBERG • LONDON • NEW YORK • OXFORD
PARIS • SAN DIEGO • SAN FRANCISCO • SINGAPORE • SYDNEY • TOKYO
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PREFACE
A great deal has changed since Chris Bowick’s RF Circuit Design was first published, some 25 years ago. In fact, we could just
say that the RF industry has changed quite a bit since the days of Marconi andTesla—both technological visionaries woven into
thefabricofhistoryasthemenwhoenabledradiocommunications.Whocouldhaveenvisionedthattheirinnovationsinthelate
1800’swouldlaythegroundworkfortheeventualcreationoftheradio—akeycomponentinallmobileandportablecommunications
systems that exist today? Or, that their contributions would one day lead to such a compelling array of RF applications, ranging
fromradartothecordlesstelephoneandeverythinginbetween. Today, theradiostandsasthebackboneofthewirelessindustry.
Itisinvirtuallyeverywirelessdevice,whetheracellularphone,measurement/instrumentationsystemusedinmanufacturing,satellite
communicationssystem,televisionortheWLAN.
Of course, back in the early 1980s when this book was first written, RF was generally seen as a defense/military technology. It
was utilized in the United States weapons arsenal as well as for things like radar and anti-jamming devices. In 1985, that image
of RF changed when the FCC essentially made several bands of wireless spectrum, the Industrial, Scientific, and Medical (ISM)
bands,availabletothepubliconalicense-freebasis.Bydoingso—andperhapswithoutevenfullycomprehendingthemomentum
itsactionswouldeventuallycreate—theFCCplantedtheseedsofwhatwouldonedaybeamultibillion-dollarindustry.
Todaythatindustryisbeingdrivennotbyaerospaceanddefense,butratherbytheconsumerdemandforwirelessapplicationsthat
allow“anytime,anywhere”connectivity.And,itisbeingenabledbyarangeofnewandemergingradioprotocolssuchasBluetooth®,
Wi-Fi(802.11WLAN),WiMAX,andZigBee®,inadditionto3Gand4GcellulartechnologieslikeCDMA,EGPRS,GSM,andLong
TermEvolution(LTE).Forevidenceofthisfact, oneneedslooknofurtherthanthecellularhandset.Withinonedecade, between
roughly the years 1990 and 2000, this application emerged from a very small scale semiprofessional niche, to become an almost
omnipresentdevice,withthenumberofusersequalto18%oftheworldpopulation.Today,nearly2billionpeopleusemobilephones
onadailybasis—notjustfortheirvoiceservices,butforagrowingnumberofsocialandmobile,data-centricInternetapplications.
Thankstothemobilephoneandservicetelecommunicationsindustryrevolution,averageconsumerstodaynotonlyexpectpervasive,
ubiquitousmobility,theyaredemandingit.
ButwhatwillthefutureholdfortheconsumerRFapplicationspace?Theanswertothatquestionseemsfairlywell-definedasthe
RFindustrynowfindsitselfrallyingbehindasinglegoal:torealizetrueconvergence.Inotherwords,thefutureoftheRFindustry
liesinitsabilitytoenablenext-generationmobiledevicestocrossalloftheboundariesoftheRFspectrum. Essentiallythen, this
convergedmobiledevicewouldbringtogethertraditionallydisparatefunctionality(e.g.,mobilephone,television,PCandPDA)on
themobileplatform.
Again, nowhere is the progress of the converged mobile device more apparent than with the cellular handset. It offers the ideal
platformonwhichRFstandardsandtechnologiescanconvergetodeliverawholehostofnewfunctionalityandcapabilitiesthat,as
asociety, wemaynotevenyetbeabletoimagine. Movementinthatdirectionhasalreadybegun.Accordingtoanalystswiththe
IDCWorldwideMobilePhoneTrackerservice,theconvergedmobiledevicemarketgrewanestimated42percentin2006foratotal
ofover80millionunits.Inthefourthquarteralone,vendorsshippedatotalof23.5milliondevices,33percentmorethanthesame
quarterayearago.That’safairlyremarkableaccomplishmentconsideringthat,priortothemid-nineties,thepossibilityoftrueRF
convergencewasthoughtunreachable. Themixing, samplinganddirect-conversiontechnologiesweresimplydeemedtooclunky
andlimitedtoprovidethefoundationnecessaryforimplementationofsuchavision.
x Preface
Regardlessofhowandwhenthegoaloftrueconvergenceisfinallyrealized,onethinghasbecomeimminentlyclearinthemidstof
allthegrowthandinnovationofthepasttwentyfiveyears—theRFindustryisaliveandwell.Moreimportantly,itiswellprimed
forafuturefullofcontinuinginnovationandmarketgrowth.
Ofcourse,whileallofthesechangescreatedawealthofbusinessopportunitiesintheRFindustry,theyalsocreatednewchallenges
for RF engineers pushing the limits of design further and further. Today, new opportunities signal new design challenges which
engineers—whetherexpertsinRFtechnologyornot—willlikelyhavetoface.
Onekeychallengeishowtoaccommodatetheneedformulti-bandreceptionincellularhandsets.Anotherstemsfromtheneedfor
higherbandwidthathigherfrequencieswhich,inturn,meansthatthecriticaldimensionsofrelevantparasiticelementsshrink.Asa
result,layoutelementsthatoncecouldbeignored(e.g.,interconnect,contactareasandholes,andbondpads)becomenon-negligible
andinfluencecircuitperformance.
Inresponsetotheseandotherchallenges,theelectronicsindustryhasinnovated,andcontinuestoinnovate.Consider,forexample,
that roughly 25 years ago or so, electronic design automation (EDA) was just an infant industry, particularly for high-frequency
RF and microwave engineering. While a few tools were commercially available, rather than use these solutions, most companies
opted to develop their own high-frequency design tools.As the design process became more complex and the in-house tools too
costlytodevelopandmaintain,engineersturnedtodesignautomationtoaddresstheirneeds.Thankstoinnovationfromavariety
ofEDAcompanies,engineersnowhaveaccesstoafullgamutofRF/microwaveEDAproductsandmethodologiestoaidthemwith
everythingfromdesignandanalysistoverification.
But the innovation doesn’t stop there. RF front-end architectures have and will continue to evolve in step with cellular handsets
sportingmulti-bandreception.Multi-bandsubsystemsandshrinkingelementsizeshavecoupledwithongoingtrendstowardlower
costanddecreasingtime-to-markettocreatetheneedfortightlyintegratedRFfront-endsandtransceivercircuits.Thesehighlevels
of system integration have in turn given rise to single-chip modules that incorporate front-end filters, amplifiers and mixes. But
implementing single-chip RF front-end designs requires a balance of performance trade-offs between the interfacing subsystems,
namely, theantennaanddigitalbasebandsystems.AchievingtherequiredsystemperformancewhenimplementingintegratedRF
front-endsmeansthatanalogdesignersmustnowworkmorecloselywiththeirdigitalbasebandcounterpart,thusleadingtogreater
integrationofthetraditionalanalog–digitaldesignteams.
OtherareasofinnovationintheRFindustrywillcomefromimprovedRFpowertransistorsthatpromisetogivewirelessinfrastructure
poweramplifiersnewlevelsofperformancewithbetterreliabilityandruggedness.RFICshopetoextendtheroleofCMOStoenable
emergingmobilehandsetstodelivermultimediafunctionsfromacompactpackageatlowercost.Incumbentslikegalliumarsenide
(GaAs) have moved to higher voltages to keep the pace going.Additionally, power amplifier-duplexer-filter modules will rapidly
displaceseparatecomponentsinmulti-bandW-CDMAradios.Single-chipmultimodetransceiverswilldisplaceseparateEDGEand
W-CDMA/HSDPA transceivers inW-EDGE handsets.And, to better handle parasitic and high-speed effects on circuits, accurate
modelingandback-annotationofever-smallerlayoutelementswillbecomecritical,aswillaccurateelectromagnetic(EM)modeling
ofRFon-chipstructureslikecoilsandinterconnect.
StillfurtherinnovationwillcomefromemergingtechnologiesinRFsuchasgalliumnitrideandmicro-electro-mechanicalsystems
(MEMS).Inthelattercase,theseadvancedmicromachineddevicesarebeingintegratedwithCMOSsignalprocessingandcondi-
tioning circuits for high-volume markets such as mobile phones and portable electronics.According to market research firmABI
Research,by2008useofMEMsinmobilephoneswilltakeoff.Thisisduetothetechnology’ssmallsize,flexibilityandperformance
advantages,allofwhicharecriticaltoenablingtheadaptive,multifunctionhandsetsofthefuture.
It is this type of innovation, coupled with the continuously changing landscape of existing application and market opportunities,
which has prompted a renewed look at the content in RF Circuit Design. It quickly became clear that, in order for this book
to continue to serve its purpose as your hands-on guide to RF circuit design, changes were required. As a result, this new 25th
anniversaryeditioncomestoyouwithupdatedinformationonexistingtopicslikeresonantcircuits, impedancematchingandRF
amplifierdesign, aswellasnewcontentpertainingtoRFfront-enddesignandRFdesigntools. Thisinformationisapplicableto
anyengineerworkingintoday’sdynamicallychangingRFindustry,aswellasforthosetruevisionariesworkingonthecuspofthe
information/communication/entertainmentmarketconvergencewhichtheRFindustrynowinspires.
CherylAjluniandJohnBlyler
CONTENTS
Preface ix
Acknowledgments xi
CHAPTER1 1
Components and Systems
Wire–Resistors–Capacitors–Inductors–Toroids–ToroidalInductorDesign–PracticalWindingHints
CHAPTER2 23
Resonant Circuits
SomeDefinitions–Resonance(LosslessComponents)–LoadedQ–InsertionLoss–ImpedanceTransformation–
CouplingofResonantCircuits–Summary
CHAPTER3 37
Filter Design
Background–ModernFilterDesign–NormalizationandtheLow-PassPrototype–FilterTypes–Frequencyand
ImpedanceScaling–High-PassFilterDesign–TheDualNetwork–BandpassFilterDesign–Summaryofthe
BandpassFilterDesignProcedure–Band-RejectionFilterDesign–TheEffectsofFiniteQ
CHAPTER4 63
Impedance Matching
Background–TheLNetwork–DealingWithComplexLoads–Three-ElementMatching–Low-QorWideband
MatchingNetworks–TheSmithChart–ImpedanceMatchingontheSmithChart–SoftwareDesignTools–Summary
CHAPTER5 103
The Transistor at Radio Frequencies
RFTransistorMaterials–TheTransistorEquivalentCircuit–YParameters–SParameters–UnderstandingRF
TransistorDataSheets–Summary
CHAPTER6 125
Small-Signal RF Amplifier Design
SomeDefinitions–TransistorBiasing–DesignUsingYParameters–DesignUsingSParameters
viii Contents
CHAPTER7 169
RF (Large Signal) Power Amplifiers
RFPowerTransistorCharacteristics–TransistorBiasing–RFSemiconductorDevices–PowerAmplifierDesign–
MatchingtoCoaxialFeedlines–AutomaticShutdownCircuitry–BroadbandTransformers–PracticalWindingHints–
Summary
CHAPTER8 185
RF Front-End Design
HigherLevelsofIntegration–BasicReceiverArchitectures–ADC’SEffectonFront-EndDesign–
SoftwareDefinedRadios–CaseStudy—ModernCommunicationReceiver
CHAPTER9 203
RF Design Tools
DesignToolBasics–DesignLanguages–RFICDesignFlow–RFICDesignFlowExample–SimulationExample1–
SimulationExample2–Modeling–PCBDesign–Packaging–CaseStudy–Summary
APPENDIXA 227
APPENDIXB 229
BIBLIOGRAPHY 233
INDEX 237
C
H
A
COMPONENTS
P
T
E
and Systems R
1
Components, those bits and pieces which make up
a radio frequency (RF) circuit, seem at times to EXAMPLE1-1
be taken for granted. A capacitor is, after all, a
GiventhatthediameterofAWG50wireis1.0mil(0.001
capacitor—isn’tit?A1-megohmresistorpresents
inch),whatisthediameterofAWG14wire?
an impedance of at least 1 megohm—doesn’t it?
The reactance of an inductor always increases with frequency, Solution
right?Well,asweshallseelaterinthisdiscussion,thingsaren’t AWG50=1mil
alwaysastheyseem.Capacitorsatcertainfrequenciesmaynot
AWG44=2×1mil=2mils
becapacitorsatall,butmaylookinductive,whileinductorsmay
looklikecapacitors,andresistorsmaytendtobealittleofboth. AWG38=2×2mils=4mils
AWG32=2×4mils=8mils
Inthischapter,wewilldiscussthepropertiesofresistors,capac-
itors,andinductorsatradiofrequenciesastheyrelatetocircuit AWG26=2×8mils=16mils
design.But,first,let’stakealookatthemostsimplecomponent AWG20=2×16mils=32mils
ofanysystemandexamineitsproblemsatradiofrequencies.
AWG14=2×32mils=64mils(0.064inch)
WIRE
WireinanRFcircuitcantakemanyforms.Wirewoundresistors,
inductors,andaxial-andradial-leadedcapacitorsalluseawire Thedepthintotheconductoratwhichthecharge-carriercurrent
ofsomesizeandlengtheitherintheirleads,orintheactualbody density falls to 1/e, or 37% of its value along the surface, is
ofthecomponent,orboth.Wireisalsousedinmanyinterconnect knownastheskindepthandisafunctionofthefrequencyand
applicationsinthelowerRFspectrum.Thebehaviorofawirein thepermeabilityandconductivityofthemedium.Thus,differ-
theRFspectrumdependstoalargeextentonthewire’sdiameter ent conductors, such as silver, aluminum, and copper, all have
andlength.Table1-1lists,intheAmericanWireGauge(AWG) differentskindepths.
system, each gauge of wire, its corresponding diameter, and
Thenetresultofskineffectisaneffectivedecreaseinthecross-
other characteristics of interest to the RF circuit designer. In
sectionalareaoftheconductorand,therefore,anetincreasein
the AWG system, the diameter of a wire will roughly double
the ac resistance of the wire as shown in Fig. 1-1. For copper,
every six wire gauges. Thus, if the last six gauges and their
theskindepthisapproximately0.85cmat60Hzand0.007cm
correspondingdiametersarememorizedfromthechart,allother
at 1MHz. Or, to state it another way: 63% of the RF current
wire diameters can be determined without the aid of a chart
flowinginacopperwirewillflowwithinadistanceof0.007cm
(Example1-1).
oftheouteredgeofthewire.
SkinEffect Straight-WireInductors
Aconductor,atlowfrequencies,utilizesitsentirecross-sectional Inthemediumsurroundinganycurrent-carryingconductor,there
areaasatransportmediumforchargecarriers.Asthefrequency exists a magnetic field. If the current in the conductor is an
is increased, an increased magnetic field at the center of the alternatingcurrent, thismagneticfieldisalternatelyexpanding
conductor presents an impedance to the charge carriers, thus andcontractingand,thus,producingavoltageonthewirewhich
decreasing the current density at the center of the conductor opposesanychangeincurrentflow. Thisoppositiontochange
and increasing the current density around its perimeter. This iscalledself-inductanceandwecallanythingthatpossessesthis
increasedcurrentdensityneartheedgeoftheconductorisknown qualityaninductor.Straight-wireinductancemightseemtrivial,
asskineffect.Itoccursinallconductorsincludingresistorleads, but as will be seen later in the chapter, the higher we go in
capacitorleads,andinductorleads. frequency,themoreimportantitbecomes.
2 RF CIRCUIT DESIGN
A (cid:1)pr2 electriccurrent.Bydefinition:
1 1
A (cid:1)pr2 1voltacross1ohm=1coulombpersecond
2 2
Skin Depth Area (cid:1)A2(cid:2)A1 =1ampere
(cid:1)p(r2(cid:2)r2)
2 1 Thethermaldissipationinthiscircumstanceis1watt.
P=EI
r
2 =1volt×1ampere
r1 =1watt
Resistorsareusedeverywhereincircuits,astransistorbiasnet-
RF current flow works,pads,andsignalcombiners.However,veryrarelyisthere
in shaded region any thought given to how a resistor actually behaves once we
departfromtheworldofdirectcurrent(DC).Insomeinstances,
suchasintransistorbiasingnetworks,theresistorwillstillper-
formitsDCcircuitfunction,butitmayalsodisruptthecircuit’s
FIG.1-1. Skindepthareaofaconductor. RFoperatingpoint.
Theinductanceofastraightwiredependsonbothitslengthand
ResistorEquivalentCircuit
itsdiameter,andisfoundby:
The equivalent circuit of a resistor at radio frequencies is
(cid:1) (cid:2) (cid:3) (cid:4)
4l shown in Fig. 1-2. R is the resistor value itself, L is the lead
L=0.002l 2.3log −0.75 µH (Eq.1-1)
d inductance, and C is a combination of parasitic capacitances
whichvariesfromresistortoresistordependingontheresistor’s
where, structure. Carbon-composition resistors are notoriously poor
L=theinductanceinµH, high-frequencyperformers.Acarbon-compositionresistorcon-
l=thelengthofthewireincm, sistsofdenselypackeddielectricparticulatesorcarbongranules.
Between each pair of carbon granules is a very small parasitic
d=thediameterofthewireincm.
capacitor. These parasitics, in aggregate, are not insignificant,
however,andarethemajorcomponentofthedevice’sequivalent
ThisisshownincalculationsofExample1-2.
circuit.
L L
R
EXAMPLE1-2
Findtheinductanceof5centimetersofNo.22copper C
wire.
Solution
FromTable1-1,thediameterofNo.22copperwireis FIG.1-2. Resistorequivalentcircuit.
25.3mils.Since1milequals2.54×10−3cm,thisequals
Wirewoundresistorshaveproblemsatradiofrequenciestoo.As
0.0643cm.SubstitutingintoEquation1-1gives
maybeexpected,theseresistorstendtoexhibitwidelyvarying
(cid:1) (cid:2) (cid:3) (cid:4)
4(5) impedances over various frequencies. This is particularly true
L=(0.002)(5) 2.3log 0.0643 −0.75 of the low resistance values in the frequency range of 10MHz
to 200MHz. The inductor L, shown in the equivalent circuit
=50nanohenries
of Fig. 1-2, is much larger for a wirewound resistor than for
acarbon-compositionresistor.Itsvaluecanbecalculatedusing
thesingle-layerair-coreinductanceapproximationformula.This
Theconceptofinductanceisimportantbecauseanyandallcon- formula is discussed later in this chapter. Because wirewound
ductors at radio frequencies (including hookup wire, capacitor resistorslooklikeinductors,theirimpedanceswillfirstincrease
leads,etc.)tendtoexhibitthepropertyofinductance.Inductors as the frequency increases. At some frequency (Fr), however,
willbediscussedingreaterdetaillaterinthischapter. theinductance(L)willresonatewiththeshuntcapacitance(C),
producinganimpedancepeak.Anyfurtherincreaseinfrequency
will cause the resistor’s impedance to decrease as shown in
RESISTORS Fig.1-3.
Resistanceisthepropertyofamaterialthatdeterminestherateat A metal-film resistor seems to exhibit the best characteris-
whichelectricalenergyisconvertedintoheatenergyforagiven tics over frequency. Its equivalent circuit is the same as the
Resistors 3
F
r
EXAMPLE1-3
InFig.1-2,theleadlengthsonthemetal-filmresistorare
) 1.27cm(0.5inch),andaremadeupofNo.14wire.The
Z
e ( totalstrayshuntcapacitance(C)is0.3pF.Iftheresistor
c
n
a valueis10,000ohms,whatisitsequivalentRFimpedance
d
e
p at200MHz?
m
I
Solution
FromTable1-1,thediameterofNo.14AWGwireis64.1
mils(0.1628cm).Therefore,usingEquation1-1:
(cid:1) (cid:2) (cid:3)(cid:4)
4(1.27)
L=(0.002)(1.27) 2.3log −0.75
Frequency (F) 0.1628
=8.7nanohenries
FIG.1-3. Impedancecharacteristicofawirewoundresistor. Thispresentsanequivalentreactanceat200MHzof:
X =ωL
L
carbon-compositionandwirewoundresistor,butthevaluesofthe
=2π(200×106)(8.7×10−9)
individualparasiticelementsintheequivalentcircuitdecrease.
=10.93ohms
The impedance of a metal-film resistor tends to decrease with
frequency above about 10MHz, as shown in Fig. 1-4. This is Thecapacitor(C)presentsanequivalentreactanceof:
due to the shunt capacitance in the equivalent circuit. At very 1
highfrequencies,andwithlow-valueresistors(under50(cid:1)),lead Xc= ωC
inductance and skin effect may become noticeable. The lead
1
inductance produces a resonance peak, as shown for the 5(cid:1) =
2π(200×106)(0.3×10−12)
resistanceinFig.1-4,andskineffectdecreasestheslopeofthe
curveasitfallsoffwithfrequency. =2653
120 Thecombinedequivalentcircuitforthisresistor,at200
5Ω MHz,isshowninFig.1-5.
ce) 100 100Ω j10.93Ω j10.93Ω
n
esista 80 1KΩ 10K
dc r 10KΩ
% 60
e ( Carbon Composition (cid:2) j2653Ω
nc 40
a
d
pe 100 KΩ FIG.1-5. EquivalentcircuitvaluesforExample1-3.
m 20
I
1 MΩ Fromthissketch,wecanseethat,inthiscase,thelead
0
inductanceisinsignificantwhencomparedwiththe10K
1.0 10 100 1000
seriesresistanceanditmaybeneglected.Theparasitic
Frequency (MHz)
capacitance,ontheotherhand,cannotbeneglected.
Whatwenowhave,ineffect,isa2653(cid:1)reactancein
parallelwitha10,000(cid:1)resistance.Themagnitudeofthe
FIG.1-4. Frequencycharacteristicsofmetal-filmvs.carbon-composition
combinedimpedanceis:
resistors.(AdaptedfromHandbookofComponentsforElectronics,
McGraw-Hill) Z= (cid:5) RXe
Many manufacturers will supply data on resistor behavior at R2+X2
e
radiofrequenciesbutitcanoftenbemisleading.Onceyouunder-
(10K)(2653)
standthemechanismsinvolvedinresistorbehavior,however,it = (cid:5)
willnotmatterinwhatformthedataissupplied. Example1-3 (10K)2+(2653)2
illustratesthatfact.
=2564.3ohms
Therecenttrendinresistortechnologyhasbeentoeliminateor
Thus,our10K resistorlookslike2564ohmsat200MHz.
greatlyreducethestrayreactancesassociatedwithresistors.This
has led to the development of thin-film chip resistors, such as
4 RF CIRCUIT DESIGN
thoseshowninFig.1-6.Theyaretypicallyproducedonalumina However, the farad is much too impractical to work with, so
orberylliasubstratesandofferverylittleparasiticreactanceat smallerunitsweredevised.
frequenciesfromDCto2GHz.
1microfarad=1µF=1×10−6farad
1picofarad=1pF=1×10−12 farad
Asstatedpreviously,acapacitorinitsfundamentalformconsists
of two metal plates separated by a dielectric material of some
sort. If we know the area (A) of each metal plate, the distance
(d)betweentheplate(ininches),andthepermittivity(ε)ofthe
dielectric material in farads/meter (f/m), the capacitance of a
parallel-platecapacitorcanbefoundby:
0.2249εA
C = picofarads (Eq.1-2)
dε
0
where
ε =free-spacepermittivity=8.854×10−12 f/m.
0
In Equation 1-2, the area (A) must be large with respect to the
distance(d).Theratioofεtoε isknownasthedielectriccon-
0
stant(k)ofthematerial.Thedielectricconstantisanumberthat
providesacomparisonofthegivendielectricwithair(seeFig.
FIG.1-6. Thin-filmresistors.(CourtesyofVishayIntertechnology) 1-7). The ratio of ε/ε0 for air is, of course, 1. If the dielectric
constantofamaterialisgreaterthan1,itsuseinacapacitoras
CAPACITORS adielectricwillpermitagreateramountofcapacitanceforthe
Capacitors are used extensively in RF applications, such as same dielectric thickness as air. Thus, if a material’s dielectric
bypassing,interstagecoupling,andinresonantcircuitsandfil- constantis3,itwillproduceacapacitorhavingthreetimesthe
ters.Itisimportanttoremember,however,thatnotallcapacitors capacitanceofonethathasairasitsdielectric.Foragivenvalue
lend themselves equally well to each of the above-mentioned of capacitance, then, higher dielectric-constant materials will
applications. TheprimarytaskoftheRFcircuitdesigner, with producephysicallysmallercapacitors.But,becausethedielec-
regardtocapacitors,istochoosethebestcapacitorforhispar- tric plays such a major role in determining the capacitance of
ticularapplication. Costeffectivenessisusuallyamajorfactor a capacitor, it follows that the influence of a dielectric on
intheselectionprocessand,thus,manytrade-offsoccur.Inthis capacitor operation, over frequency and temperature, is often
section, we’ll take a look at the capacitor’s equivalent circuit important.
and we will examine a few of the various types of capacitors
usedatradiofrequenciestoseewhicharebestsuitedforcertain Dielectric K
Air 1
applications.Butfirst,alittlereview.
Polystrene 2.5
Paper 4
Parallel-PlateCapacitor Mica 5
Ceramic (low K) 10
A capacitor is any device which consists of two conducting Ceramic (high K) 100(cid:2)10,000
surfaces separated by an insulating material or dielectric. The
dielectricisusuallyceramic,air,paper,mica,plastic,film,glass,
oroil.Thecapacitanceofacapacitoristhatpropertywhichper-
FIG.1-7. Dielectricconstantsofsomecommonmaterials.
mits the storage of a charge when a potential difference exists
between the conductors. Capacitance is measured in units of
farads.A1-faradcapacitor’spotentialisraisedby1voltwhenit Real-WorldCapacitors
receivesachargeof1coulomb. Theusageofacapacitorisprimarilydependentuponthechar-
Q acteristics of its dielectric. The dielectric’s characteristics also
C =
V determine the voltage levels and the temperature extremes at
whichthedevicemaybeused.Thus,anylossesorimperfections
where,
inthedielectrichaveanenormouseffectoncircuitoperation.
C=capacitanceinfarads,
TheequivalentcircuitofacapacitorisshowninFig.1-8,whereC
Q=chargeincoulombs,
equalsthecapacitance,R istheheat-dissipationlossexpressed
s
V=voltageinvolts. eitherasapowerfactor(PF)orasadissipationfactor(DF),R
p