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GORESET
TheDigitalLogic"NOT"Gate
Definition
ThedigitalLogicNOTGateisthemostbasicofallthelogicalgatesandissometimesreferredtoasanInverting
BufferorsimplyaDigitalInverter.Itisasingleinputdevicewhichhasanoutputlevelthatisnormallyatlogiclevel"1"
andgoes"LOW"toalogiclevel"0"whenitssingleinputisatlogiclevel"1",inotherwordsit"inverts"(complements)its
inputsignal.TheoutputfromaNOTgateonlyreturns"HIGH"againwhenitsinputisatlogiclevel"0"givingusthe
Booleanexpressionof:A=Q.
ThenwecandefinetheoperationofasingleinputlogicNOTgateasbeing:
"IfAisNOTtrue,thenQistrue"
TransistorNOTGate
Asimple2-inputlogicNOTgatecanbeconstructedusingaRTLResistor-transistorswitchesasshownbelowwiththe
inputconnecteddirectlytothetransistorbase.Thetransistormustbesaturated"ON"foraninversedoutput"OFF"atQ.
LogicNOTGatesareavailableusingdigitalcircuitstoproducethedesiredlogicalfunction.ThestandardNOTgateis
givenasymbolwhoseshapeisofatrianglepointingtotherightwithacircleatitsend.Thiscircleisknownasan
"inversionbubble"andisusedinNOT,NANDandNORsymbolsattheiroutputtorepresentthelogicaloperationof
theNOTfunction.Thisbubbledenotesasignalinversion(complementation)ofthesignalandcanbepresentoneither
orboththeoutputand/ortheinputterminals.
TheDigitalInverterorNOTgate
SymbolTruthTable
InverterorNOTGate
AQ
01
10
BooleanExpressionQ=notAorAReadasinverseofAgivesQ
LogicNOTgatesprovidethecomplementoftheirinputsignalandaresocalledbecausewhentheirinputsignalis
"HIGH"theiroutputstatewillNOTbe"HIGH".Likewise,whentheirinputsignalis"LOW"theiroutputstatewillNOTbe
"LOW".Astheyaresingleinputdevices,logicNOTgatesarenotnormallyclassedas"decision"makingdevicesoreven
asagate,suchastheANDorORgateswhichhavetwoormorelogicinputs.CommercialavailableNOTgatesIC''sare
availableineither4or6individualgateswithinasinglei.c.package.
The"bubble"(o)presentattheendoftheNOTgatesymbolabovedenotesasignalinversion(complimentation)ofthe
outputsignal.Butthisbubblecanalsobepresentatthegatesinputtoindicateanactive-LOWinput.Thisinversionofthe
inputsignalisnotrestrictedtotheNOTgateonlybutcanbeusedonanydigitalcircuitorgateasshownwiththe
operationofinversionbeingexactlythesamewhetherontheinputoroutputterminal.Theeasiestwayistothinkofthe
bubbleassimplyaninverter.
SignalInversionusingActive-lowinputBubble
BubbleNotationforInputInversion
NANDandNORGateEquivalents
AnInverterorlogicNOTgatecanalsobemadeusingstandardNANDandNORgatesbyconnectingtogetherALLtheir
inputstoacommoninputsignalforexample.
Averysimpleinvertercanalsobemadeusingjustasinglestagetransistor
switchingcircuitasshown.Whenthetransistorsbaseinputat"A"ishigh,
thetransistorconductsandcollectorcurrentflowsproducingavoltagedrop
acrosstheresistorRtherebyconnectingtheoutputpointat"Q"toground
thusresultinginazerovoltageoutputat"Q".
Likewise,whenthetransistorsbaseinputat"A"islow(0v),thetransistor
nowswitches"OFF"andnocollectorcurrentflowsthroughtheresistor
resultinginanoutputvoltageat"Q"highatavaluenearto+Vcc.
Then,withaninputvoltageat"A"HIGH,theoutputat"Q"willbeLOWandaninputvoltageat"A"LOWtheresultingoutput
voltageat"Q"isHIGHproducingthecomplementorinversionoftheinputsignal.
HexSchmittInverters
AstandardInverterorLogicNOTGate,isusuallymadeupfromtransistorswitchingcircuitsthatdonotswitchfromone
statetothenextinstantly,thereissomedelay.Alsoasatransistorisabasiccurrentamplifier,itcanalsooperateina
linearmodeandanysmallvariationtoitsinputlevelwillcauseavariationtoitsoutputlevelormayevenswitch"ON"and
"OFF"severaltimesifthereisanynoisepresentinthecircuit.OnewaytoovercometheseproblemsistouseaSchmitt
InverterorHexInverter.
Weknowfromthepreviouspagesthatalldigitalgatesuseonlytwologicvoltagestatesandthatthesearegenerally
referredtoasLogic"1"andLogic"0"anyTTLvoltageinputbetween2.0vand5visrecognisedasalogic"1"andany
voltageinputbelow0.8visrecognisedasalogic"0"respectively.
ASchmittInverterisdesignedtooperateorswitchstatewhenitsinputsignalgoesabovean"UpperThresholdVoltage"
limitinwhichcasetheoutputchangesandgoes"LOW",andwillremaininthatstateuntiltheinputsignalfallsbelowthe
"LowerThresholdVoltage"levelinwhichcasetheoutputsignalgoes"HIGH".InotherwordsaSchmittInverterhassome
formofHysteresisbuiltintoitsswitchingcircuit.
Thisswitchingactionbetweenanupperandlowerthresholdlimitprovidesamuchcleanerandfaster"ON/OFF"
switchingoutputsignalandmakestheSchmittinverteridealforswitchinganyslow-risingorslow-fallinginputsignal
eitherananalogueordigitalsignal.
SchmittInverter
AveryusefulapplicationofSchmittinvertersiswhentheyareusedasoscillatorsorsine-to-squarewaveconvertersfor
useassquarewaveclocksignals.
SchmittInverterOscillator&Converter
ThefirstcircuitshowsaverysimplelowpowerRCtypeoscillatorusingaSchmittinvertertogeneratesquarewaves.
InitiallythecapacitorCisfullydischargedsotheinputtotheinverteris"LOW"resultinginaninvertedoutputwhichis
"HIGH".AstheoutputfromtheinverterisfedbacktoitsinputandthecapacitorviatheresistorRthecapacitorbeginsto
chargeup.
Whenthecapacitorschargingvoltagereachestheupperthresholdlimitoftheinverter,theinverterchangesstate,the
outputbecomes"LOW"andthecapacitorbeginstodischargethroughtheresistoruntilitreachesthelowerthreshold
levelweretheinverterchangesstateagain.Thisswitchingbackandforthbytheinverterproducesasquarewaveoutput
signalwitha33%dutycycleandwhosefrequencyisgivenas:?=680/RC.
Thesecondcircuitconvertsasinewaveinput(oranyoscillatinginputforthatmatter)intoasquarewaveoutput.The
inputtotheinverterisconnectedtothejunctionofthepotentialdividernetworkwhichisusedtosetthequiescentpointof
thecircuit.TheinputcapacitorblocksanyDCcomponentpresentintheinputsignalonlyallowingthesinewavesignalto
pass.
Asthissignalpassestheupperandlowerthresholdpointsoftheinvertertheoutputalsochangesfrom"HIGH"to"LOW"
andsoonproducingasquarewaveoutputwaveform.Thiscircuitproducesanoutputpulseonthepositiverisingedgeof
theinputwaveform,butbyconnectingasecondSchmittinvertertotheoutputofthefirst,thebasiccircuitcanbemodified
toproduceanoutputpulseonthenegativefallingedgeoftheinputsignal.
CommonlyavailablelogicNOTgateandInverterIC''sinclude
TTLLogicTypes
74LS04HexInvertingNOTGate
74LS04HexInvertingNOTGate
74LS14HexSchmittInvertingNOTGate
74LS1004HexInvertingDrivers
CMOSLogicTypes
CD4009HexInvertingNOTGate
CD4069HexInvertingNOTGate
InverterorNOTGate7404
InthenexttutorialaboutDigitalLogicGates,wewilllookatthedigitallogicNANDGatefunctionasusedinbothTTL
andCMOSlogiccircuitsaswellasitsBooleanAlgebradefinitionandtruthtables.
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