COMPUTER AIDED DESIGN OF OPERATIONALAMPLIFIER CIRCUITSBy David HayesOPERATIONAL AMPLIFIERS / CADXXX This program can aid you in thedesign of most operational amplifier(OP AMP) circuits.  Hence its name,Computer Aided Design (CAD).  Yourcomputer can now remove the burden ofmaking numerous calculations from yourdesign task.  Its spreadsheet-likeoperation allows you to instantly seethe effects of any circuit changes youmake.  If you have an Atari 82580-Column Printer (or just aboutstandard printer Ws attached, you canget a record of your design effort.XXX Although some knowledge of OP AMPcircuit design is required, thisprogram should find wide use amongstudents and hobbyists as well asprofessionals. REQUIRED: Atari BASIC languageand minimum 48K memory. OPTIONAL:PrinterXXXLOADING CAD INTO COMPUTER MEMORY Insert the program disk in drive 1,power up the printer if it will beused, and then boot the computer withAtari BASIC enabled.  The program willrun automaticaly.MAIN MENU SCREEN After the title screen, the programwill automatically proceed to thefirst menu screen, containingfunctions, commands, and menus.XXX Your computer can provide some HELPat any step along the way.  If youwould like some help just press the[H] key during normal programoperation. Press [SELECT] to highlight thedifferent circuits listed on the menu.Press [START] when the circuit youwant is highlighted.XXXDESIGN NOTES Once you make your selection, adesign screen will be displayed.Design notes and general circuitinformation follow. If you are a beginner, you may wishto fist review the design exampledescribed in the section labeledSAMPLE APPLICATIONs.XXX If you have a printer but do not wanta printout of your designs, just turnyour printer off.KEY FUNCTIONS[RESET] - Return control to the mainmenu.[SELECT] - Selects component to bechanged.[OPTION] - Change circuitconfiguration, if supported.XXX[START] or [RETURN] - Enters a newvalue for the selected component.Then the program displays the effectof this change.[H] - Display HELP messages.[0] through [9] number keys - Used toinput a new component value.XXXBelow are notes on the individual types of circuit:INVERTER OP AMPCircuit Characteristics:1)  Gain can be less than, equal to,or greater than one.2)  Input impedance is equal to R1.Calculated Values:1)  R3=R1*R2/(R1+R2)2)  GAIN=-R2/R1XXXNON-INVERTING OP AMPCircuit Characteristics1)  Gain must be greater than one.2)  Input impedance is very large.Calculated Values1)  GAIN=(R1+R2)/R1XXXSCHMITT TRIGGER/COMPARATORCircuit Characteristics1)  Provides rapid output transition.It is most often used as a bufferbetween analog and digital circuits.2)  This circuit will act as acomparator for values of T less thanone.  When T is greater than one thenthe circuit will act as a true Schmitttrigger.XXX3)  T is typically made much greaterthan one to account for largevariations in the open loop gain(AOL).Calculated Values1)  T=R1*(AOL)/(R1+R2)INTEGRATORXXXCircuit Characteristics1)  The output is equal to the gaintimes the integral of the inputvoltage.Calculated Values1)  GAIN=-1/(R1*C1)2)  R2=R1XXXDIFFERENTIAL (DIFFERENCE) AMPLIFIERCircuit Characteristics1)  Input impedance to V1 is R1, V2R2, and V3 R3.2)  R1 must be equal to R3, and R2must be equal to R4 for this circuitto be a true differential amplifier.  Ideally, a differential amplifiershould not amplify signals which arenot common to both inputs.  However, a1% deviation in any one of theresistances will cause a largeXXXreduction in its ability to rejectcommon mode signals.Calculated Values1)  OUTPUT=((R1+R2)/(R3+R4))*(R4/R1)*V2-R2/R1*V1XXXADDER (SUMMING) AMPLIFIERCircuit Characteristics1)  Output is the inverted sum of allinput signals.Calculated Values1)  OUT=-R4*(V1/R1+V2/R2) NOTE: Twoinput option.2)  OUT=-R4*(V1/R1+V2/R2+V3/R3) NOTE:Three input option.XXX3)  R5=1/(1/R1+1/R2+1/R4) NOTE: Twoinput option.4)  R5=1/(1/R1+1/R2+1/R3+1/R4) NOTE:Three input option.SAMPLE APPLICATIONLet's design an inverting op ampamplifier circuit...XXXStep 1: [SELECT] the inverter circuitfrom the main menu.  When the wordINVERTER is highlighted, press[START].  The CAD program will loadfrom the disk and display the selectedcircuit.XXXStep 2: Again, press [SELECT] to seewhich circuit parameters you canmodify.  Pause at some component, thenenter a value for it.  Your new valueis displayed, and all parametersaffected by your change will beupdated.  You may wish to consult yourDesign Notes at this point.XXXStep 3: Repeat Step 2 as often asnecessary to get the right values.Experiment freely.  If you make amistake the computer will recover anddisplay an error message.  It's justthat simple.Remember, press [H] for Help. **XXXADVANCED TECHNICAL INFORMATIONThe integrated circuit OP AMP is theworkhorse of linear amplifiers used inmost audio equipment.  Its low costand high performance has made it theamplifier of choice amongprofessionals, students, and hobbyistsalike.  The generic 741 and improvedversions are typical of this group.XXXMost OP AMPs of 741 quality or bettercan be readily used to construct thecircuits designed with this CADprogram.  I suggest that you consultany of the many good texts andapplication notes for details ofcircuit design and OP AMP selection.Your local electronics shop probablystocks many of these books.XXXYou may wish to pick up the"Engineer's Mini-Notebook, Op Amp ICCircuits" by Forrest M. Mims III.This book is available from RadioShack, catalog number 276-5011.  It isfilled with many excellent examples ofthe 741 and several popular OP AMPS.(Radio Shack also sells the 741 OP AMP.)XXXResistors and capacitors come inindustry standard sizes.  The programwill accept standard and non-standardvalues.  However, you may havedifficulty building your circuit withnon-standard component values.  So,for your reference, the standard(common) values are noted below.XXXSTANDARD COMPONENT VALUESRESISTORSOHMS OHMS OHMS OHMS OHMS OHMS OHMS2.2  20   180  1.6K 15K  130K 1.2M2.4  22   200  1.8K 16K  150K 1.3M2.7  24   220  2.0K 18K  160K 1.5M3.0  27   240  2.2K 20K  180K 1.6M3.3  30   270  2.4K 22K  200K 1.8M3.6  33   300  2.7K 24K  220K 2.0M3.9  36   330  3.0K 27K  240K 2.2M4.3  39   360  3.3K 30K  270K 2.4M4.7  43   390  3.6K 33K  300K 2.7M5.1  47   430  3.9K 36K  330K 3.0MXXX5.6  51   470  4.3K 39K  360K 3.3M6.2  56   510  4.7K 43K  390K 3.6M6.8  62   560  5.1K 47K  430K 3.9M7.5  68   620  5.6K 51K  470K 4.3M8.2  75   680  6.2K 56K  510K 4.7M9.1  82   750  6.8K 62K  560K 5.1M10   91   820  7.5K 68K  620K 5.6M11   100  910  8.2K 75K  680K 6.2M12   110  1.0K 9.1K 82K  750K 6.8M13   120  1.1K 10K  91K  820K 7.5M15   130  1.2K 11K  100K 910K 8.2M16   150  1.3K 12K  110K 1.0M 9.1M18   160  1.5K 13K  120K 1.1M 10MXXXCAPACITORSFARAD    FARAD    FARAD    FARAD10pf     100pf    4.7nf    100nf22pf     220pf    10nf     220nf39pf     330pf    22nf     330nf47pf     470pf    33nf     470nf68pf     1.0nf    47nf