科技英语论文文档格式.docx

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.