The document provides an introduction to operational amplifiers (op-amps), discussing their structure, function, and key characteristics including ideal versus practical limitations. It explores different modes of operation for differential amplifiers, input parameters, and the significance of negative feedback in controlling gain and stability. Additionally, the document details various parameters such as input impedance, common-mode rejection, and the concept of closed-loop voltage gain.

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

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

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

4. 12-1 Introduction to Operational Amplifiers Continue…
SymbolandTerminals
Thestandardop-ampsymbolisshowninfigure12-1(a).
Ithastwoterminalsi.e,inverting(-)andnon-inverting(+).
Ithasoneoutputterminal.
Thetypicalop-ampoperateswithtwodcsupplyvoltages,one+veandtheother–veasshowninfigure12-2(b).
Usuallythesedcvoltageterminalsareleftofftheschematicsymbolforsimplicitybutareunderstoodtobethere. 4

5. 12-1 Introduction to Operational Amplifiers Continue…
TheIdealOp-Amp
Ithasinfinitegainandinfinitebandwidth.
Ithasinfiniteinputimpedance(open)sothatitdoesnotloadthedrivingsource.
Ithaszerooutputimpedance.
Thesecharacteristicsareillustratedinfigure12-2(a).Theinputvoltage,Vin,appearsb/wthetwoterminals,andtheoutputvoltageisAvVin,asindicatedbytheinternalvoltagesourcesymbol.
5

6. 12-1 Introduction to Operational Amplifiers Continue…
ThePracticalOp-Amp
AnydevicehaslimitationsandICop-ampisnoexception.
Op-Amphascertainlimitationsbutthetwomentionedbelowaremajorones.
i.SlewRate(willbediscussedin12-2Section).
ii.Outputcurrentisalsolimitedbyinternalrestrictionssuchaspowerdissipationandcomponentratings.
6

7. 12-1 Introduction to Operational Amplifiers Continue…
InternalBlockDiagramofanOp-Amp
Atypicalop-ampismadeupofthreetypesofamplifiercircuits.
ADifferentialAmplifieri.e,itistheinputstagefortheop-amp.Itprovidestheamplificationofthedifferencevoltageb/wthetwoinputs.
AVoltageAmplifier(s)i.e,itisthesecondstageofop-ampandisusuallyaclassAamplifierthatprovidesadditionalgain.
APush-PullClassBAmplifieri.e,itistypicallyusedforoutputstage.
7

8. 12-1 Introduction to Operational Amplifiers Continue…
TheDifferentialAmplifierInputStage
Thedifferentialamplifierformstheinputstageofoperationalamplifiers.
Thetermdifferencecomesfromtheamplifier’sabilitytoamplifythedifferenceoftwoinputsignalsappliedtoinputs.
Onlythedifferenceinthetwosignalsisamplified;ifthereisnodifference,thentheoutputiszero.
8

9. 12-1 Introduction to Operational Amplifiers Continue…
TheDifferentialAmplifierInputStageContinue…
Thetransistors(Q1andQ2)andthecollectorresistors(RC1andRC2)arecarefullymatchedtohaveidenticalcharacteristics(Refertofigureinpreviousslide).
NoticethatthetwotransistorsshareasingleRE.
Letussupposebothbasesareconnectedtoground.Theemittervoltagewillbe-0.7V.
Theemittercurrentsareequal(IE1=IE2).
Thecollectorcurrentsarebothequalandareapproximatelyequaltotheemittercurrents.
Sincecollectorcurrentsarethesame,thecollectorvoltagesarealsothesame.
Thereisazerodifferenceintheinputvoltages(bothbasesareat0V).9

10. 12-1 Introduction to Operational Amplifiers Continue…
TheDifferentialAmplifierInputStageContinue…
IfthebaseofQ1isdisconnectedfromgroundandconnectedtoasmallpositivevoltage,Q1willconductmorecurrentbecausethepositivevoltageonitsbasecausestheemittervoltagetoincreaseslightly.
TheemittercurrentisnowdividedsothatmoreofitisinQ1andlessinQ2.
AsaresultthecollectorvoltageofQ1willdecreaseandthecollectorvoltageofQ2willincrease,reflectingthedifferenceintheinputvoltages(oneis0Vandtheotheratasmallpositivevalue).
Thisconditionisillustratedinfigurenextslide.
10

11. 12-1 Introduction to Operational Amplifiers Continue…
TheDifferentialAmplifierInputStageContinue… 11

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

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

14. 12-2 OP-AMP Input Modes and Parameters Continue…
DifferentialMode
Twoopposite-polarity(out-of-phase)signalsareappliedtotheinputsasshowninfigure12-5.
Thistypeofoperationisalsoreferredtoasdouble-ended.
Theamplifieddifferencebetweenthetwoinputsappearsontheoutput. 14

15. 12-2 OP-AMP Input Modes and Parameters Continue…
CommonMode
Twosignalvoltagesofthesamephase,frequencyandamplitudeareappliedtothetwoinputsasshownisfigure12-6.
Thisresultsinazerooutputvoltage(asdifferenceis0V).
Thisactioniscalledcommon-moderejection. 15

16. 12-2 OP-AMP Input Modes and Parameters Continue…
CommonModeContinue…
It’simportanceliesinthesituationwhereanunwantedsignalappearscommonlyonbothop-ampinputs.
Common-moderejectionmeansthatthisunwantedsignalwillnotappearontheoutputanddistortthedesiredsignal.
CommonModeRejectionRatio
Desiredsignalscanappearononlyoneinputorwithoppositepolaritiesonbothinputlines.Thesedesiredsignalsareamplifiedandappearontheoutput.
Unwantedsignals(noise)appearingwiththesamepolarityonbothinputlinesareessentiallycancelledbytheop-ampanddon’tappearontheoutput.Themeasureofanamplifier’sabilitytorejectcommon-modesignalsisaparametercalledtheCMRR(Common-ModeRejectionRatio). 16

17. 12-2 OP-AMP Input Modes and Parameters Continue…
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

18. 12-2 OP-AMP Input Modes and Parameters Continue…
CommonModeRejectionRatioContinue...
ThehighertheCMRR,thebetter.AveryhighvalueofCMRRmeansthattheopen-loopgain,Aolishighandcommon-modegain,Acmislow.
TheCMRRisoftenexpressesindecibels(dB)as
Theopen-loopvoltagegainofanop-ampistheinternalvoltagegainofthedeviceandrepresentstheratiooftheoutputvoltagetotheinputvoltagewhentherearenoexternalcomponents(nofeedbackcircuit).
Open-loopvoltagegaincanrangeupto200,000andisnotawelltoleratedparameter.Datasheetsoftenrefertotheopen- loopvoltagegainasthelarge-signalvoltagegain.
18

19. 19

20. 12-2 OP-AMP Input Modes and Parameters Continue…
InputOffsetVoltage
Idealop-ampproduceszerovoltsoutforzerovoltsin.
However,inpracticalop-ampasmalldcvoltage,VOUT(error), appearsattheoutputwhennodifferentialinputvoltageisapplied.Itsprimarycauseisaslightmismatchofthebase- emittervoltagesofthedifferentialamplifierinputstageofanop-amp.
Inputoffsetvoltage,VOS,isthedifferentialdcvoltagerequiredbetweentheinputstoforcetheoutputtozerovolts.
Typicalvaluesofinputoffsetvoltageareintherangeof2mVorless.Itis0Vinidealcase.
Inputoffsetvoltagedriftisaparameterrelatedto,VOS,thatspecifieshowmuchchangeoccursintheinputoffsetvoltageforeachdegreechangeintemperature.20

21. 12-2 OP-AMP Input Modes and Parameters Continue…
InputBiasCurrent
Inputterminalsofadifferentialamplifierarethetransistorbasesandtherefore,theinputcurrentsarethebasecurrents.
Theinputbiascurrentisthedccurrentrequiredbytheinputsofanamplifiertoproperlyoperatethefirststage.Bydefinitiontheinputbiascurrentistheaverageofbothinputcurrentsasfollows: 21

22. 12-2 OP-AMP Input Modes and Parameters Continue…
InputImpedance
Twobasicwaysofspecifyingtheinputimpedance
1.TheDifferentialinputimpedanceisthetotalresistancebetweentheinvertingandnon-invertingterminalsasillustrated12-8(a)nextslide.Differentialimpedanceismeasuredbydeterminingthechangeinbiascurrentforagivenchangeindifferentialinputvoltage.
2.Thecommon-modeinputimpedanceistheresistancebetweeneachinputandgroundandismeasuredbydeterminingthechangeinbiascurrentforagivenchangeincommon-modeinputvoltageshowninfigure12-8(b)nextslide.
22

23. 12-2 OP-AMP Input Modes and Parameters Continue…
InputImpedanceContinue…
23

24. 12-2 OP-AMP Input Modes and Parameters Continue…
InputOffsetCurrent
Ideally,thetwoinputbiascurrentsareequalandthustheirdifferenceiszero.
However,inapracticalop-amp,thebiascurrentsarenotexactlyequal.
Theinputoffsetcurrent,IOSisthedifferenceoftheinputbiascurrentsexpressedasanabsolutevalue. 24

25. 12-2 OP-AMP Input Modes and Parameters Continue…
InputOffsetCurrent
Theactualmagnitudesofoffsetcurrentareusuallyatleasttentimeslessthanthebiascurrent.
Inmanyapplicationsoffsetcurrentcanbeneglected.
25

26. 12-2 OP-AMP Input Modes and Parameters Continue…
OutputImpedance
Theoutputimpedanceistheresistanceviewedfromtheoutputterminaloftheop-ampasshowninfigurebelow
26

27. 12-2 OP-AMP Input Modes and Parameters Continue…
SlewRate
Theslewrate(SR)ofanop-ampisthemaximumrateatwhichtheoutputvoltagecanchangeinresponsetoinputvoltage.
WhentheSRistooslowfortheinput,distortionresults.
Whenainputsinewaveisappliedtoavoltagefolloweritproducesatriangularoutputwaveform.
Thetriangularwaveformresultsbecausetheop-ampsimplycannotmovefastenoughtofollowthesinewaveinput.
Thishappensbecausevoltagechangeinthesecondstage( VoltageAmplifier(s))islimitedbythecharginganddischargingofcapacitor.
Theslewrateisexpressedas:SR=ΔVO/Δt
TheunitofSRisvoltspermicroseconds.
27

28. 28

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

30. 12-3 Negative Feedback
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

31. 12-3 Negative Feedback
WhyUseNegativeFeedback?(continue…)
Theusefulnessofanop-ampoperatedwithoutnegativefeedbackisseverelyrestrictedandisgenerallylimitedtocomparatorandotherapplications.
Withnegativefeedback,theclosed-loopvoltagegain(Acl)canbereducedandcontrolledsothatop-ampcanfunctionasalinearamplifier.
31

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

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

34. 12-4 OP-AMPS With Negative Feedback Continue…
Non-invertingAmplifier
Anop-ampinaclosed-loopconfigurationasanon-invertingamplifierwithacontrolledamountofvoltagegainisshowninfigure12-16nextslide.
Theinputsignalisappliedtothenon-inverting(+)input.
Theoutputisappliedbacktotheinverting(-)inputthroughthefeedbackcircuit(closedloop)formedbytheinputresistorRiandthefeedbackresistorRf.
ResistorsRiandRfformavoltage-dividercircuitwhichreducesVoutandconnectsthereducedvoltageVftotheinvertinginput.
Thefeedbackvoltageisexpressedas34

35. 12-4 OP-AMPS With Negative Feedback Continue…
Non-invertingAmplifierContinue…
ThedifferenceoftheinputvoltageVin,andthefeedbackvoltageVf,isthedifferentialinputtotheop-ampasshowninfigure12-17nextslide.Thisdifferentialvoltageisamplifiedbytheopen-loopvoltagegainoftheop-amp(Aol)andproducesanoutputvoltageexpressedas
35

36. 12-4 OP-AMPS With Negative Feedback Continue…
Non-invertingAmplifierContinue…
36

37. 12-4 OP-AMPS With Negative Feedback Continue…
Non-invertingAmplifierContinue… 37

38. 12-4 OP-AMPS With Negative Feedback Continue…
Non-invertingAmplifierContinue… 38

39. 39

40. 12-4 OP-AMPS With Negative Feedback Continue…
VoltageFollower
Itisaspecialcaseofthenon-invertingamplifierwherealloftheoutputvoltageisfedbacktotheinverting(-)inputbyastraightconnectionasshowninfigure12-19.
Thestraightfeedbackconnectionhasavoltagegainof1.
Theclosed-loopvoltagegainofanon-invertingamplifieris1/B.
SinceB=1foravoltagefollowercase,theclosed-loopvoltagegainofthevoltagefollowerisAcl(VF)=1/1=1
40

41. 12-4 OP-AMPS With Negative Feedback Continue…
InvertingAmplifier
Anop-ampconnectedasaninvertingamplifierwithacontrolledamountofvoltagegainasshowninfigure12-20.
TheinputsignalisappliedthroughaseriesinputresistorRitotheinverting(-)input.
TheoutputisfedbackthroughRftothesameinput.Thenon- invertinginputisgrounded.
41

42. 12-4 OP-AMPS With Negative Feedback Continue…
InvertingAmplifierContinue…
Atthispoint,theidealop-ampparametersmentionedearlierareusefulinsimplifyingtheanalysisofthiscircuit.
Particularlytheconceptofinfiniteinputimpedanceisofgreatvalue.
Aninfiniteinputimpedanceimplieszerocurrentattheinvertinginput.
Ifthereiszerocurrentthroughtheinputimpedance,thentheremustbenovoltagedropbetweentheinvertingandnon- invertingterminals.
Thismeansthatthevoltageattheinverting(-)inputiszero. Thiszerovoltageattheinvertinginputterminalisreferredtoasvirtualground.Thisconditionisillustratedinfigure12-21(a) nextslide.42

43. 12-4 OP-AMPS With Negative Feedback Continue…
InvertingAmplifierContinue…
Sincethereisnocurrentattheinvertinginput,thecurrentthroughRiandthecurrentthroughRfareequalasshowninfigure12-21(b).i.e,Iin=If43

44. 12-4 OP-AMPS With Negative Feedback Continue…
InvertingAmplifierContinue… 44

45. 45