Table Of ContentColin May
Passive Circuit
Analysis
with LTspice®
An Interactive Approach
1
Passive Circuit Analysis with LTspice
Colin May (retired)
Passive Circuit Analysis
1
with LTspice
An Interactive Approach
ColinMay(retired)
UniversityofWestminster
London,UK
Simulationfilescanbefoundat:https://www.springer.com/us/book/9783030383039.Inorder
to use the simulation files with the extension ‘.asc’ referenced in the text it is necessary to
downloadthefreeprogrammeLTspice.Thesimulationfilesaregroupedinfoldersforeach
chapter.Thesecanbedownloadedintoanappropriateenclosingfolder.Theessentialfilesin
eacharetheschematic‘.ASC’fileswhichcanberunbyLTspice. Someschematicsrelyon
symbols and sub-circuits created in the text. Most (if not all) can be found in the folders
‘Mysym’and‘Mysubs’.ThesecanbedownloadedandincludedintheLTspicesearchpaths.
Additionally,someschematics,especiallychapter2,usecommercialsub-circuitsandsymbols
providedbydevicemanufacturers.Thesetoocanbedownloadedandmustbeincludedinthe
searchpaths.Someadditionalfilesarealsoincluded,butnotforeveryschematic.The‘.NET’
filesarenetlistfilesthatcanberunbyLTspiceinsteadofthe‘.ASC’ filebutthesedonotshow
acircuitschematic.The‘.PLT’filesarepre-determinedsettingsforhowthetracesshouldbe
drawnandalsoannotationstothetrace.Theyarenotexecutablefiles.
ISBN978-3-030-38303-9 ISBN978-3-030-38304-6 (eBook)
https://doi.org/10.1007/978-3-030-38304-6
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In Memoriam
Eλἐνη Αθανασoπoύλoυ
(1979–2019)
‘Σε ἐναν ‘Αγγελo...’
Eleni Athanasopoulou
She was taken too early
Foreword
Thisbookisidiosyncraticinthatitincludesodditiesnotincludedinthegeneralrun
ofthings,suchastheHamonvoltagedivider,theMurraylooptest,theElmoredelay
andtheThiele-Small loudspeakermodel.Theseareapplicationsofpassivecircuits
which were encountered during countless trawls through the Internet and seemed
both ingenious and interesting. But it is hoped that the text also encompasses the
usualrangeofcircuits.
SPICEisananalytical tool,justasmanualanalysisis,averypowerfultoolthat
canreturnfarmoreaccurateresultsthanaregenerallypossiblewithpencil-and-paper
analysis because it can include nonlinearities and temperature effects, but still it is
just an analytical tool. At present, it is not possible to insert the required system
performanceandforSPICEtocreateanappropriatecircuit:this,perhaps,isaproject
for Mike Engelhardt. So LTspice does not absolve the circuit designer from the
oneroustaskofdesigningthecircuit–itwillonlyreporthowthecircuitperforms.
We must also answer the question “Why bother using SPICE?” To this are our
severalanswers.Grantedthatittakesalittletimetolearnthebasics–andabitmore
tobecomeproficient–itwillbefoundafterwardsthatcircuitscanbesimulatedand
tested faster than is possible with manual calculations. But does this mean that
prototypingontheworkbenchisredundant?Farfromit.LTspicewillonlyanalyse
thedataitisgiven:sostraycapacitanceandinductancethatbedevilpracticalcircuits
willonlycometolightontherealphysicalcircuit.Ontheotherhand,theexigences
of health and safety considerations, as well as the time needed to construct pro-
totypes,makeLTspiceanattractiveadjunctwherecomponentvaluescanbechanged
andtestedinseconds.Anditisalsoinvaluableinoneotherimportantareawhichis
oftenoverlooked–productionyield.Componentsarenotexactbuthavetolerances.
Itisclearlyunreasonabletobuildafewthousandactualcircuitstoseewhichareout
ofspecifications,butnoproblemwithLTspice–evenifitmeansleavingittorunall
night.
Intheearlydays,that is,pre-millenium,some versions ofSPICEwereproneto
unfortunate errors; and although it is not something germane in this text, in one
versionatleast,itwaspossibletomistakenlyconnectanopampintendedasaunity
vii
viii Foreword
gainbuffer,withthefeedbacktakentothepositiveinputinsteadofthenegative,and
to find that the circuit still worked as a buffer. Nowadays, such errors have been
fixed,butstilltheusermustbewareoffalseresults.
These still occur and originate from two sources; one is the default settings of
LTspice.TheseareaccessedthroughTools->ControlPanel,andtherearethreeof
importance:thefirstisthatbydefaulttheresultsarecompressed;inmostcases,this
isveryusefulsinceitgreatlyreducesthesizeofthesimulationfile–therawfile–by
afactorofatleastten.However,itdoesmeanthatthereisimprecisioninmeasuring
sharpresonantpeaksortransitions,andthenitmustbeturnedoff.Thesecondisthat
bydefault,inductorshaveaparalleldampingresistorandasmallseriesresistance:it
isthislatterthatcanleadtofalsereductionsintheoutputofatunedcircuit.Thethird
areaofconcernistheriseandfalltimesofpulseinputwaveformswherethedefault
ofzerohassignificantslopes;settingeachtosomethinglike1e-15resultsinvertical
edges, but it is always worth checking. Otherwise, the default settings have been
used throughout this book, only needing the Gear method to resolve a rather
awkward example of the definition of a capacitor by its charge in Chap. 7, nor
generally adjusting the settings in Tools ! Control Panel ! SPICE, although
tolerancechangesweretriedforoneortwosimulationsthatranslowlyorstopped,
buttheredidnotseemtobeanyimprovement.Thisleadstoasecondconsideration,
andthatalthoughitisveryunlikelythatLTspicemiscalculatestheperformanceofa
circuit,itisalwaysagoodideatostartwithasimplecircuit,withoutthecomplica-
tions of nonlinearities or temperature effects, and check that the results agree with
calculation.
Thesecondsourceoferrorsisinmodelsandsubcircuits.Modelsarepredefinedin
SPICE and it is not within our power to add extra parameters. Therefore any
shortcomings, such as the failure of bipolar transistor models to include the base-
emitterbreakdown,canonlybedealtwithbycreatingasubcircuit.Theseareanother
matter and are essential where the performance of a device such as a thermistor
cannotadequatelybereplicatedbyanexistingmodel,anditisthereforenecessaryto
constructacircuittocreatethecorrectcharacteristics.Insomecases,thereareagreed
subcircuitstructures,butnotalways,andtheperformanceofthesubcircuitmayfall
shortoftheactualdevicewhichweshallexplore,especiallyinChap.3.
The structure of thisbook, asfar as possible, istostartwith simple circuits and
basic analytical tools and then progress to more complex circuits and advanced
analyticalmethodsonthebasisthatitismoreproductivetomovefromtheknownto
theunknownratherthantheotherwayround.Also,anexhaustivelistingof,say,the
.measstatementisapttoberatherindigestible,andsoitsfeaturesareintroducedas
needed, scattered over several chapters. On the other hand, there seemed no alter-
native but to list, with illustrations, the enormous capabilities of the voltage and
current sources as Chap. 5. Additionally, some topics such as RCL circuits are
coveredtwice,butindifferentfashions.
Perforce, the text is essentially mathematical because LTspice makes a
no-compromise analysis of the circuit, returning results correct to six or seven
figures, including aspects of the circuit’s performance that are not always covered
in text books. This sometimes results in puzzling results prompting a “What the
Foreword ix
(insert expletive) is happening here?” These can often be the most fruitful simula-
tionsbringingtolightsideeffectsthatwereoverlookedinasimplemanualanalysis
(orwereomittedduetoaninadequategraspofthetheory).
Worked examples with accompanying simulations pervade the book, but there
arenoend-of-chapterlistsofquestions,justafewsuggestedexplorationsattheend
eachsection.Thehopeisthatthetheoryandexampleswillstimulatemoreinvesti-
gations,whichshouldbeseenasachallenge,notanonerousburden.Indeed,ifthe
thought of having to simulate a circuit brings a deep sigh and a heavy heart, it is
respectfullyaskedifelectronicsistherightcareer–lifeistooshorttospendone’s
workingdaysdoingsomethingthatdoesnotbringdeepsatisfaction.
LTspice is under continuous development, so some simulations that caused
problemsnowrunsmoothly,andforothersthecommentsmaynotnowberelevant.
Also,afewcircuitsymbolshavechanged.Andsourcesnowrequirefiveratherthan
fourparameters.
Inevitably, there are mistakes, for which I crave the readers’ indulgence and a
gentlesmileratherthanschadenfreude.Toerrishuman,butforareallygoodfoulup
you need a computer. I hope I have avoided the opacity variously attributed to
Robert Browning at https://www.google.com/search?client=firefox-b-d&q=only
+god+and+browning.
Patra,Greece ColinMay
Contents
1 LTspiceEssentials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 RepresentingtheCircuit. . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.3 DrawingConventions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3.1 ComponentSymbols. . . . . . . . . . . .. . . . . . . . . . . . 5
1.4 DrawingtheCircuitandOhm’sLaw. . . . . . . . . . . . . . . . . . . 6
1.4.1 DrawingtheCircuit. . . . . . . . . . . . . . . . . . . . . . . . 6
1.4.2 PlacingComponents. . . . . . . . . . . . . . . . . . . . . . . . 7
1.4.3 ConnectingtheCircuit. . . . . . . . . . . . . . . . . . . . . . 10
1.4.4 AddingValues. . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
1.4.5 EditingtheCircuit. . . . . . . . . . . . . . . . . . . . . . . . . 12
1.4.6 Annotations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
1.5 RunningtheSimulationandthe.opCommand. . . . . . . . . . . . 15
1.5.1 SimulationResults. . . . . . . . . . . . . . . . . . . . . . . . . 16
1.6 SweepingVoltageandCurrentSources. . . . . . . . . . . . . . . . . 19
1.6.1 DCSweepCommand. . . . . . . . . . . . . . . . . . . . . . . 19
1.6.2 TheTraceWindow. . . . . . . . . . . . . . . . . . . . . . . . . 20
1.6.3 TheControlPanel. . . . .. . . . .. . . . .. . . . .. . . . .. 24
1.7 ChangingtheValueofaComponentDuringAnalysis. . . . . . . 26
1.7.1 UsingParameters‘.param’. . . . . . . . . . . . . . . . . . . 27
1.7.2 StepCommand‘.step’. . . . . . . . . . . . . . . . . . . . . . . 28
1.7.3 ProductionYields. . . . . . . . . . . . . . . . . . . . . . . . . . 29
1.8 SPICE. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
1.8.1 SchematicCapture. . . . . . . . . . . . . . . . . . . . . . . . . 32
1.8.2 SPICEAnalysis. . . . . . . . . . . . . . . . . . . . . . . . . . . 33
1.8.3 Performance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
1.9 Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
xi
xii Contents
2 DCCircuits. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
2.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
2.2 Kirchhoff’sLaws. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
2.2.1 ResistorsinParallelandKirchhoff’s
CurrentLaw. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
2.2.2 ResistorsinSeriesandKirchhoff’s
VoltageLaw. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
2.3 SomeUsefulCircuits.. . . . . .. . . . . .. . . . .. . . . . .. . . . . .. 48
2.3.1 ThePotentialDivider. . . . . . . . . . . . . . . . . . . . . . . 49
2.3.2 The‘.measure’(.meas)Directive. . . . . . . . . . . . . . . 52
2.3.3 MaximumPowerTransfer. . . . . . . . . . . . . . . . . . . . 54
2.3.4 TheWheatstoneBridge. . . . . . . . . . . . . . . . . . . . . . 59
2.4 MoreAnalysisMethods. . . . . . . . .. . . . . . . . . . . . . . . . . . . . 67
2.4.1 Superposition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
2.4.2 TheTheveninModel. . . . . . . . . . . . . . . . . . . . . . . 69
2.4.3 TheNortonModel. . . . . . . . . . . . . . . . . . . . . . . . . 71
2.5 Attenuators. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
2.5.1 TheL-Attenuator. . . . . . . . . . . . . . . . . . . . . . . . . . 75
2.5.2 The‘T’-Attenuator. . . . . . . . . . . . . . . . . . . . . . . . . 77
2.5.3 ThePi-Attenuator. . . . . . . . . . . . . . . . . . . . . . . . . . 81
2.6 Delta-StarConversion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
2.6.1 Delta-StarConversion. . . . . . . . . . . . . . . . . . . . . . . 83
2.6.2 Star-DeltaConversion. . . . . . . . . . . . . . . . . . . . . . . 85
2.7 TheThermocouple. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
2.8 Metrology. . . .. . . . .. . . . .. . . . .. . . .. . . . .. . . . .. . . . .. 90
2.8.1 VoltageStandard. . . . . . . . . . . . . . . . . . . . . . . . . . 90
2.8.2 Resistance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
2.9 PracticalConsiderations. . .. . . . . . . . . . . .. . . . . . . . . . .. . . 96
2.9.1 FixedResistors. . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
2.9.2 VariableResistors. . . . . . . . . . . . . . . . . . . . . . . . . . 99
2.10 Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
3 Non-linearResistors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
3.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
3.2 TheLTspiceResistor.. . . . . .. . . . . .. . . . .. . . . . .. . . . . .. 101
3.2.1 TheComponentAttributeEditor. . . . . . . . . . . . . . . 102
3.3 VariableResistors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
3.3.1 Potentiometers. . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
3.4 ResistorTemperatureEffects. . . . . . . . . . . . . . . . . . . . . . . . . 106
3.4.1 AddingTemperatureCoefficients. . . . .. . . . . . . . .. 106
3.4.2 TemperatureAnalysis. . . . . . . . . . . . . . . . . . . . . . . 108
3.5 ThePlatinumResistanceThermometer. . . . . . . . . . . . . . . . . . 110
3.5.1 ArbitraryTemperatureCoefficient. . . . . . . . . . . . . . 111
3.5.2 TheCubicEquation. . . . . . . . . . . . . . . . . . . . . . . . 112
