科技英语论文文档格式.docx
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科技英语论文文档格式.docx
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030941103
Summary:
Thecircuitisgenerallyanoverkillinmanyrespects,butwheninitiallydesignedIdidnotknowwhatrangeoffrequenciesmighteventuallyberequired.SoIaddedtheextratransistorhereandtherebecauseitimprovedpotentialperformance.Asithappens,muchlesserstabilityandrangewouldhavebeenperfectlysatisfactory.
Theconnectionmarked'
gang'
wasprovidedsothattwosuchoscillatorscouldbeconnectedtogether.Thefasteroscillatorwilldominateandbothwillworkinsynchronism.
Muchofthecircuitisinfactacomparatormadewithdiscretetransistors.Indeed,thereisnogoodreasonwhyacomparatorshouldnotbeused.Howeverwiththedarlingtonarrangementshown,basecurrentisquitetiny.Forevensmallerbasecurrent,thelong-tailedpaircouldbeexchangedforapairofJFETs.Modernsilicontransistorscanworkdowntoanewnanoamps,sothechargecurrentcanbequitetiny.HowaboutincreasingC1andmakinganoscillatorwhichpulsesonceaweek?
Keywords:
oscillators,comparator,basecurrent
Multivibrators:
anold,wellknownandboringsubject.No-notatall:
therearelotsofdifferentwaysoflookingathemandtheyarenotatallwhattheymayseem.
Whatexactlyisamultivibrator?
Isupposeonedefinitionwouldbe'
acircuitwhichhasseveralstates'
.Thiswilldofornow,it'
squiteloosesoleavesplentytotheimagination.Conventionalmultivibratorshaveonlytwostagesandcomeinthreeflavours:
astable(whereneitherstateisstableandthecircuitoscillatesbetweenthetwostates);
monostable(whereonestateisstable,theothertransient).Andbistable,wherethecircuitcanbeflippedfromstateonetostatetwo.Thefactthatitcanbe'
flopped'
backagainleadstoanotherterm'
flip-flop'
-butyoucancallanastableanastableflip-flop!
Thenyoucangettristables-andonecircuitIshallintroduceisa'
donkeysimulator'
where3transistorsarecombinedtomake3separateastablemulivibrators!
Andwhatisarelaxationoscillator?
Acircuitwhichchanges'
slowly'
fromoneleveltoanotheratwhichitresetstothefirststateandstartschangingagain.
Thetwodescriptionsmayseemtotallydifferent-butareinfactthesamething-oratleastthereisadistinctlygreyareawheretheyoverlap,asIwillshow.Therearealsoalotofotheroscillatorcircuitswhicharen'
tquiteconventionalmultivibsorrelaxationoscillators..
1:
The'
common'
Astablemultivibrator.
AcircuitwhichIfindprettyboringbut,ifIdon'
tintroduceitfirst,howamItoelaborateonallitsmoreinterestingrelatives?
Operation
AssumethattheTr1isturnedhardon(ifyou'
veindexedthissiteby'
hardon'
sorry-it'
snotforyou!
)andTr2isnotconducting,sotherightendofC2isatVcc:
currentisflowingintothebaseofTr1viaR3causingittoconduct,sotheleftendofC1isat0v.
AssumeC1isnotcharged:
itsleftendisclampedto0vbyTr1butitsrightendisbeingfedfromVccviaR1,soitwillstarttochargeupthroughR1,itsrightendmovingpositivetowardsVcc.AtacertainpointtherewillbeenoughvoltageontherightendofC1tocauseTr2tostartconducting.
AsTr2startstoturnon,Tr2'
scollectorwillstarttofallandwillpulldowntherighthandendofC2.SinceC2isacapacitor,thevoltageacrossitcannotsuddenlychangesothelefthandendofC2willalsostarttofall.ThiswillrobcurrentfromTr1whichwillstarttoturnoffanditscollectorvoltagewillstarttorise.Asitdoesso,C1willfeedthisrisingvoltageintoTr2'
sbase,helpingittoturnon-sothecircuitwill'
collapse'
intoastatewhereTr2isfullyonandTr1isfullyoff.
AsTr2quicklyswitchesfromofftoon,itscollectorfallsfromVccto0vand(bycapacitoraction)thebaseofTr1willbereversedbiasedfromabout0.6vto(Vcc-0.6v)belowearthastheleftendofC2followstherightenddownwards.
NowR3startstochargeC2'
sleftendpositivelyfromnear-Vcc(belowthe0vline)towards0vandpast0vtowardsVcc.RememberthatTr2isturnedon,sotherightendofC2isclampedto0v(viaTr2).
AsthelefthandendofC2getstoaround+0.6v,Tr1startstoconductandthesequencerepeats.
Sothecircuitswitchesbetweentwostates,Tr1on(Tr2off)andTr2on(Tr1off).ThetimeineachstateisdeterminedbyR1C1andR3C2andis0.69CR(butreadon).Sothecircuitgivesa50:
50markspaceandisa'
fairly'
reliableandpredictableoscillator.Thevaluesshownwillgiveafrequencyaround1kHz(0.5mSecon,0.5mSoff).
Isay'
reliablebecausethereareacoupleofpoints...ImaginethatTr1andTr2arebothconductingandbothcapacitorsare'
discharged'
:
R1'
scurrentflowsintoTr2andR3'
scurrentflowsintoTr1.Thecapacitorsare'
sonocurrentisflowinginthem:
thecircuitisinastablestateandisnotoscillating.Thisconditionhappensandisprobablythebiggestsingleproblemwiththecommonmultivibrator-andwithseveralofitsrelatives.
The'
astablemultivibrator'
(togivethiscircuitits'
proper'
namewasinventedinthedaysofvalvesandwascommonwithearlygermaniumtransistors.Thesehasagainofmaybe20(gainistheratioofcollectorcurrenttobasecurrent:
feed1microampintothebaseofatransistorand,withagainof20,youwillget20microampscollectorcurrent).WiththesetypicallyR2andR4mightbe1K,R1andR310K.Thensilicontransistorscameout:
thesehadhighergains.This'
failuretooscillate'
becamecommon:
siliconshadmoregainandthecircuitoscillatesmorereliablyifthetransistorsarenotturnedtoohardon:
I'
vechosenR1:
R2of100,whichwouldsuitatransistorhavingagainofatleast100.
Nevertheless:
Iwouldnotusesuchasimplemultivibinacriticalpartofacommercialcircuittoday,becauseitcanfailtostart.Butitremainsaninterestingcircuitandagoodlearningexercise!
AsIhavesaid,theperiodofthemultivibis0.69CR.Butthisonlyappliesforlowvoltages.Whenonetransistorturnson,thebaseoftheothertransistorgetspulledveryhardnegative(wellbelowthe0vline)andthebaseisturnedoff.Buttransistorbasejunctionscannotstandalotofreversevoltage:
modernonesseemtobearound10v.Ifyoutryandputalargerreversevoltageonthebase-thetransistorbaseemitterjunctionconductslikeazenerdiode.Thisinterfereswiththeformula-andyougetashortertimethanyouexpect.
2:
Asymmetricalmultivibrator.
Thesecondproblemwiththesimplecircuit,above,isthatitsedgesaren'
tverysharp.JustafterTr1turnsoff,fullVccispresentacrossC1sothereisasmallvoltagedropacrossR2fromthis:
thechargingofC2tendstoroundtheedgesandmakethemlessthanideallysharp.
Thesecondcircuitshowsonewayroundthis.Notethatthevaluesinonehalfare10timesthoseintheotherhalf:
thetimesofbothhalvesremainthesamebutthe'
rounding'
effectonTr1isreducedbythesmallerC1.Thetrade-offisthatTr2'
scollectorwaveformismademuchworsebythelargerC2reactingwiththeincreasedR4.
Theasymmetricalcircuitalsoismorereliable:
sincethetwohalvesarenotequalthechanceofthembothswitchingontogetherisreduced.
3:
Amultivibindisguise.
Ifyouwanttodrawamultivibratorsoitconfusesanengineer,trythiscircuit.Ifyoucompareitwiththebasicmultivib-itisactuallythesame.Butitisdrawnlikeatwostagea.c.coupledamplifier,withoveralla.c.feedback.Itdoesn'
tlooksymmetrical,doesn'
tappeartohavetwoidenticalhalves.Thewayyoudrawacircuitisimportant.Abadlydrawncircuitcanhidethecircuit'
sfunctionandconfusethereader!
4:
ComplementaryMultivibs.
Anobviousfact:
foralltransistorcircuitsthereexistsacomplement,replaceallPNPtransistorsbyNPNandviceversa,reversealldiodesandother2terminalpolarisedcomponents.Anidenticalbutcomplementarycircuitresults.Thisisoneofthenicethingsyoucandowithtransistors-butnotwithICs.VirtuallyallICshaveanegativeearthanddonotworkinacomplementaryversion.Thereasonsforthisaremainlyhistorical-theearlysilicontransistorfabricationmadebetterNPNtransistorsthanPNP.Althoughmodernfabricationmakesboth,thereareverysmallintrinsicperformancedifferences(duetodifferencesinelectronandholeflow)andNPNisthereforethemorecommonmodel.
Butinmosttwotransistormultivibcircuitsitispossibletoreplaceoneofthetransistorsbyitscomplement(i.e.turnhalfthecircuit'
upside-down'
andthecircuitstillworks.Theoperationischanged,andsometimessomeheadscratchingisrequiredtoseetheresult!
Neverthelessitremainsafascinatingandinstructiveexercise!
Thediagramaboveshowsthestandardmultivibwithonestagemadeintoitscomplement.Thecircuitmaylookincrediblysimilar-buttheoperationisdramaticallychanged!
Insteadofthetransistorsalternatingnowtheybothswitchontogether.Thecircuit,withanalmosttrivialchange,hasstopsbeingamultivibratorandhasbecomearelaxationoscillator!
WhichshouldexplainwhyIdonotconsiderthesetobetwotobedistincttypesofcircuit!
Namesarelanguagethings:
therealworldisacontinuumanddoesn'
tneatlyfitthepigeonholesoflanguage!
Sincebothtransistorsconductsimultaneously,theonperiodisashortpulsefollowedbyalongrecoverysymptomaticofarelaxationoscillator.Toincreasetheontime,resistorscanbeputinserieswiththecapacitorsasisdoneinthecircuitbelow.
Yes-thisisaslightlydifferentcircuittotheabove:
oneofthemanycircuitsI'
vecollectedovertheyears.
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