The operational amplifier (part 1)

Engr.Tehseen Ahsan
Lecturer, Electrical Engineering Department
EE-307 Electronic Systems Design
HITEC University Taxila Cantt, Pakistan
The Operational Amplifier (Part 1)

12-1 Introduction to Operational Amplifiers
Thedevices,suchasdiodeandthetransistor,areseparatedevicesthatareindividuallypackagedandinterconnectedinacircuitwithotherdevicestoformacomplete,functionalunit-DiscreteComponents.
Linearintegratedcircuits(ICs),wheremanytransistors,diodes, resistorsandcapacitorsarefabricatedonasingletinychipofsemiconductormaterial.
Thismeansthatwewillbeconcernedwithwhatthecircuitdoesfromanexternalviewpointthanfromaninternal, component-levelviewpoint.
2

12-1 Introduction to Operational Amplifiers Continue…
EarlyoperationalAmplifiers(i.e,usedfor+,-,∫andd/dx).
Earlyop-ampwereconstructedwithvaccumtubesandworkedwithhighvoltages.
Today’sop-amparelinearICsthatuserelativelylowdcsupplyvoltagesandarereliableandinexpensive.
3

SymbolandTerminals
Thestandardop-ampsymbolisshowninfigure12-1(a).
Ithastwoterminalsi.e,inverting(-)andnon-inverting(+).
Ithasoneoutputterminal.
Thetypicalop-ampoperateswithtwodcsupplyvoltages,one+veandtheother–veasshowninfigure12-2(b).
Usuallythesedcvoltageterminalsareleftofftheschematicsymbolforsimplicitybutareunderstoodtobethere. 4

TheIdealOp-Amp
Ithasinfinitegainandinfinitebandwidth.
Ithasinfiniteinputimpedance(open)sothatitdoesnotloadthedrivingsource.
Ithaszerooutputimpedance.
Thesecharacteristicsareillustratedinfigure12-2(a).Theinputvoltage,Vin,appearsb/wthetwoterminals,andtheoutputvoltageisAvVin,asindicatedbytheinternalvoltagesourcesymbol.
5

ThePracticalOp-Amp
AnydevicehaslimitationsandICop-ampisnoexception.
Op-Amphascertainlimitationsbutthetwomentionedbelowaremajorones.
i.SlewRate(willbediscussedin12-2Section).
ii.Outputcurrentisalsolimitedbyinternalrestrictionssuchaspowerdissipationandcomponentratings.
6

InternalBlockDiagramofanOp-Amp
Atypicalop-ampismadeupofthreetypesofamplifiercircuits.
ADifferentialAmplifieri.e,itistheinputstagefortheop-amp.Itprovidestheamplificationofthedifferencevoltageb/wthetwoinputs.
AVoltageAmplifier(s)i.e,itisthesecondstageofop-ampandisusuallyaclassAamplifierthatprovidesadditionalgain.
APush-PullClassBAmplifieri.e,itistypicallyusedforoutputstage.
7

TheDifferentialAmplifierInputStage
Thedifferentialamplifierformstheinputstageofoperationalamplifiers.
Thetermdifferencecomesfromtheamplifier’sabilitytoamplifythedifferenceoftwoinputsignalsappliedtoinputs.
Onlythedifferenceinthetwosignalsisamplified;ifthereisnodifference,thentheoutputiszero.
8

TheDifferentialAmplifierInputStageContinue…
Thetransistors(Q1andQ2)andthecollectorresistors(RC1andRC2)arecarefullymatchedtohaveidenticalcharacteristics(Refertofigureinpreviousslide).
NoticethatthetwotransistorsshareasingleRE.
Letussupposebothbasesareconnectedtoground.Theemittervoltagewillbe-0.7V.
Theemittercurrentsareequal(IE1=IE2).
Thecollectorcurrentsarebothequalandareapproximatelyequaltotheemittercurrents.
Sincecollectorcurrentsarethesame,thecollectorvoltagesarealsothesame.
Thereisazerodifferenceintheinputvoltages(bothbasesareat0V).9

TheDifferentialAmplifierInputStageContinue…
IfthebaseofQ1isdisconnectedfromgroundandconnectedtoasmallpositivevoltage,Q1willconductmorecurrentbecausethepositivevoltageonitsbasecausestheemittervoltagetoincreaseslightly.
TheemittercurrentisnowdividedsothatmoreofitisinQ1andlessinQ2.
AsaresultthecollectorvoltageofQ1willdecreaseandthecollectorvoltageofQ2willincrease,reflectingthedifferenceintheinputvoltages(oneis0Vandtheotheratasmallpositivevalue).
Thisconditionisillustratedinfigurenextslide.
10

TheDifferentialAmplifierInputStageContinue… 11

12-2 OP-AMP Input Modes and Parameters
Thedifferentialamplifierexhibitsthreemodesofoperationbasedonthetypeofinputsignals.Thesemodesaresingle-ended,differentialandcommon.Sincethedifferentialamplifieristheinputstageoftheop- amp,theop-ampexhibitsthesamemodes. 12

12-2 OP-AMP Input Modes and Parameters Continue…
Single-EndedMode
Oneinputisgroundedandasignalvoltageisappliedonlytotheotherinputasshowninfigure12-4.
Whenthesignalisappliedtotheinvertinginputasinpart(a), aninverted,amplifiedsignalvoltageappearsattheoutput.
Whenthesignalisappliedtothenon-invertinginputasinpart(b),annon-inverted,amplifiedsignalvoltageappearsattheoutput. 13

DifferentialMode
Twoopposite-polarity(out-of-phase)signalsareappliedtotheinputsasshowninfigure12-5.
Thistypeofoperationisalsoreferredtoasdouble-ended.
Theamplifieddifferencebetweenthetwoinputsappearsontheoutput. 14

CommonMode
Twosignalvoltagesofthesamephase,frequencyandamplitudeareappliedtothetwoinputsasshownisfigure12-6.
Thisresultsinazerooutputvoltage(asdifferenceis0V).
Thisactioniscalledcommon-moderejection. 15

CommonModeContinue…
It’simportanceliesinthesituationwhereanunwantedsignalappearscommonlyonbothop-ampinputs.
Common-moderejectionmeansthatthisunwantedsignalwillnotappearontheoutputanddistortthedesiredsignal.
CommonModeRejectionRatio
Desiredsignalscanappearononlyoneinputorwithoppositepolaritiesonbothinputlines.Thesedesiredsignalsareamplifiedandappearontheoutput.
Unwantedsignals(noise)appearingwiththesamepolarityonbothinputlinesareessentiallycancelledbytheop-ampanddon’tappearontheoutput.Themeasureofanamplifier’sabilitytorejectcommon-modesignalsisaparametercalledtheCMRR(Common-ModeRejectionRatio). 16

CommonModeRejectionRatioContinue...
Ideally,anop-ampprovidesaveryhighgainfordesiredsignals(single-endedordifferential)andzerogainforcommon-modesignals.
Howeverpracticalop-ampsdoexhibitaverysmallcommon- modegain(usuallymuchlessthan1),whileprovidingahighopen-loopvoltagegain(usuallyseveralthousands).
Thehighertheopen-loopgainw.r.tthecommon-modegain, thebettertheperformanceofop-ampintermsofrejectionofcommon-modesignals.
Thissuggests:
17

CommonModeRejectionRatioContinue...
ThehighertheCMRR,thebetter.AveryhighvalueofCMRRmeansthattheopen-loopgain,Aolishighandcommon-modegain,Acmislow.
TheCMRRisoftenexpressesindecibels(dB)as
Theopen-loopvoltagegainofanop-ampistheinternalvoltagegainofthedeviceandrepresentstheratiooftheoutputvoltagetotheinputvoltagewhentherearenoexternalcomponents(nofeedbackcircuit).
Open-loopvoltagegaincanrangeupto200,000andisnotawelltoleratedparameter.Datasheetsoftenrefertotheopen- loopvoltagegainasthelarge-signalvoltagegain.
18

InputOffsetVoltage
Idealop-ampproduceszerovoltsoutforzerovoltsin.
However,inpracticalop-ampasmalldcvoltage,VOUT(error), appearsattheoutputwhennodifferentialinputvoltageisapplied.Itsprimarycauseisaslightmismatchofthebase- emittervoltagesofthedifferentialamplifierinputstageofanop-amp.
Inputoffsetvoltage,VOS,isthedifferentialdcvoltagerequiredbetweentheinputstoforcetheoutputtozerovolts.
Typicalvaluesofinputoffsetvoltageareintherangeof2mVorless.Itis0Vinidealcase.
Inputoffsetvoltagedriftisaparameterrelatedto,VOS,thatspecifieshowmuchchangeoccursintheinputoffsetvoltageforeachdegreechangeintemperature.20

InputBiasCurrent
Inputterminalsofadifferentialamplifierarethetransistorbasesandtherefore,theinputcurrentsarethebasecurrents.
Theinputbiascurrentisthedccurrentrequiredbytheinputsofanamplifiertoproperlyoperatethefirststage.Bydefinitiontheinputbiascurrentistheaverageofbothinputcurrentsasfollows: 21

InputImpedance
Twobasicwaysofspecifyingtheinputimpedance
1.TheDifferentialinputimpedanceisthetotalresistancebetweentheinvertingandnon-invertingterminalsasillustrated12-8(a)nextslide.Differentialimpedanceismeasuredbydeterminingthechangeinbiascurrentforagivenchangeindifferentialinputvoltage.
2.Thecommon-modeinputimpedanceistheresistancebetweeneachinputandgroundandismeasuredbydeterminingthechangeinbiascurrentforagivenchangeincommon-modeinputvoltageshowninfigure12-8(b)nextslide.
22

InputImpedanceContinue…
23

InputOffsetCurrent
Ideally,thetwoinputbiascurrentsareequalandthustheirdifferenceiszero.
However,inapracticalop-amp,thebiascurrentsarenotexactlyequal.
Theinputoffsetcurrent,IOSisthedifferenceoftheinputbiascurrentsexpressedasanabsolutevalue. 24

InputOffsetCurrent
Theactualmagnitudesofoffsetcurrentareusuallyatleasttentimeslessthanthebiascurrent.
Inmanyapplicationsoffsetcurrentcanbeneglected.
25

OutputImpedance
Theoutputimpedanceistheresistanceviewedfromtheoutputterminaloftheop-ampasshowninfigurebelow
26

SlewRate
Theslewrate(SR)ofanop-ampisthemaximumrateatwhichtheoutputvoltagecanchangeinresponsetoinputvoltage.
WhentheSRistooslowfortheinput,distortionresults.
Whenainputsinewaveisappliedtoavoltagefolloweritproducesatriangularoutputwaveform.
Thetriangularwaveformresultsbecausetheop-ampsimplycannotmovefastenoughtofollowthesinewaveinput.
Thishappensbecausevoltagechangeinthesecondstage( VoltageAmplifier(s))islimitedbythecharginganddischargingofcapacitor.
Theslewrateisexpressedas:SR=ΔVO/Δt
TheunitofSRisvoltspermicroseconds.
27

12-3 Negative Feedback
Negativefeedbackistheprocesswherebyaportionoftheoutputvoltageofanamplifierisreturnedtotheinputwithaphaseanglethatopposestheinputsignal.
Negativefeedbackisillustratedinfigure12-14.Theinverting(-)inputeffectivelymakesthefeedbacksignal180˚outofphasewiththeinputsignal. 29

WhyUseNegativeFeedback?
Theinherentopen-loopvoltagegainofatypicalop-ampisveryhigh(usuallygreaterthan100,000).Therefore,anextremelysmallinputvoltagedrivestheop-ampintoitssaturatedoutputstates.Infact,eventheinputoffsetvoltageofanop-ampcandriveitintosaturation.Forexample,assumeVIN=1mVandAol=100,000.Then,Vout=VINAol=100V
Sincetheoutputlevelofanop-ampcanneverreach100V,itisdrivendeepintosaturationandtheoutputislimitedtoitsmaximumoutputlevelsasillustratedinfigure12-15nextslideforbothapositiveandnegativeinputvoltageof1mV.
30

WhyUseNegativeFeedback?(continue…)
Theusefulnessofanop-ampoperatedwithoutnegativefeedbackisseverelyrestrictedandisgenerallylimitedtocomparatorandotherapplications.
Withnegativefeedback,theclosed-loopvoltagegain(Acl)canbereducedandcontrolledsothatop-ampcanfunctionasalinearamplifier.
31

12-4 OP-AMPS With Negative Feedback
Theextremelyhighopen-loopgainofanop-ampcreatesanunstablesituationbecauseasmallnoisevoltageontheinputcanbeamplifiedtoapointwheretheamplifierisdrivenoutofitslinearregion.
Negativefeedbacktakesaportionoftheoutputandappliesitbackoutofphasewiththeinput,creatinganeffectivereductioningain.
Thisclosed-loopgainisusuallymuchlessthantheopen-loopgain.
32

12-4 OP-AMPS With Negative Feedback Continue…
Closed-LoopVoltageGain,Acl
Theclosed-loopgainisthevoltagegainofanop-ampwithexternalfeedback.Theamplifierconfigurationconsistsoftheop-ampandanexternalfeedbackcircuitthatconnectstheoutputtoinvertinginput.
Theclosed-loopvoltagegainisdeterminedbytheexternalcomponentvaluesandcanbepreciselycontrolledbythem. 33

Non-invertingAmplifier
Anop-ampinaclosed-loopconfigurationasanon-invertingamplifierwithacontrolledamountofvoltagegainisshowninfigure12-16nextslide.
Theinputsignalisappliedtothenon-inverting(+)input.
Theoutputisappliedbacktotheinverting(-)inputthroughthefeedbackcircuit(closedloop)formedbytheinputresistorRiandthefeedbackresistorRf.
ResistorsRiandRfformavoltage-dividercircuitwhichreducesVoutandconnectsthereducedvoltageVftotheinvertinginput.
Thefeedbackvoltageisexpressedas34

Non-invertingAmplifierContinue…
ThedifferenceoftheinputvoltageVin,andthefeedbackvoltageVf,isthedifferentialinputtotheop-ampasshowninfigure12-17nextslide.Thisdifferentialvoltageisamplifiedbytheopen-loopvoltagegainoftheop-amp(Aol)andproducesanoutputvoltageexpressedas
35

Non-invertingAmplifierContinue…
36

Non-invertingAmplifierContinue… 37

Non-invertingAmplifierContinue… 38

VoltageFollower
Itisaspecialcaseofthenon-invertingamplifierwherealloftheoutputvoltageisfedbacktotheinverting(-)inputbyastraightconnectionasshowninfigure12-19.
Thestraightfeedbackconnectionhasavoltagegainof1.
Theclosed-loopvoltagegainofanon-invertingamplifieris1/B.
SinceB=1foravoltagefollowercase,theclosed-loopvoltagegainofthevoltagefollowerisAcl(VF)=1/1=1
40

InvertingAmplifier
Anop-ampconnectedasaninvertingamplifierwithacontrolledamountofvoltagegainasshowninfigure12-20.
TheinputsignalisappliedthroughaseriesinputresistorRitotheinverting(-)input.
TheoutputisfedbackthroughRftothesameinput.Thenon- invertinginputisgrounded.
41

InvertingAmplifierContinue…
Atthispoint,theidealop-ampparametersmentionedearlierareusefulinsimplifyingtheanalysisofthiscircuit.
Particularlytheconceptofinfiniteinputimpedanceisofgreatvalue.
Aninfiniteinputimpedanceimplieszerocurrentattheinvertinginput.
Ifthereiszerocurrentthroughtheinputimpedance,thentheremustbenovoltagedropbetweentheinvertingandnon- invertingterminals.
Thismeansthatthevoltageattheinverting(-)inputiszero. Thiszerovoltageattheinvertinginputterminalisreferredtoasvirtualground.Thisconditionisillustratedinfigure12-21(a) nextslide.42

InvertingAmplifierContinue…
Sincethereisnocurrentattheinvertinginput,thecurrentthroughRiandthecurrentthroughRfareequalasshowninfigure12-21(b).i.e,Iin=If43

InvertingAmplifierContinue… 44

The operational amplifier (part 1)

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The operational amplifier (part 1)