11Op Amp Basicsop amp = operational amplifierPrepared by Scott RobertsonFall 2007Diefenderfer and Holton (D&H), Ch. 9Horowitz and Hill (H&H), Ch. 4,5,6,72What are they good for?• Amplifiers: sum and difference of voltages• Differentiators• Integrators• Buffers or follower (cable drivers)• Filters: high pass, low pass, bandpass, band reject, notch• Comparators• Limitations of op amps (nonideal behavior)• Oscillators (another lecture)23Theorists view: 3 pins:Experimentalists view:7 pins + ground:+−VoutVBVAVoutVBVAV+V−Offset adjust(optional)Vout= Gain × (VB–VA)Voutfrom Golden RulesInverting inputNon-inverting input+−Positive power supplyNegative power supplyGround is sometimes not connected to the op amp. 4Manufacturer’s viewsDual-in-line (DIP) PackageThe socket also has a notch at pin 1 so you don’t put the op amp in backwards. Notch35What’s inside? National’s LF3566“Bypass” the impedance of the power supply wires using small capacitorsVoutVBVAV+V−+−0.01 μF, >15 VFailure to do this can cause oscillation and “cross talk”between circuits because the power supply voltage will otherwise vary when the load varies. To power supply To power supply A screwdriver-adjust potentiometeris used for “one time” offset adjustmentSmall disc ceramic capacitors are used for “bypass.” They MUST be located near (a few cm) to the chip and attached to a “good” ground.47Characteristics of the ideal op amp1. Input impedance is infiniteinputs draw no current2. Gain G is infinitein the math, terms with 1/G →03. Negative feedback determines the function performed Negative feedback is a connection from the output to the inverting input4. Output impedance is zero From D & H ch 9-18Inverting x1 amplifier (G not ∞)+−VoutVBVARRI1I2VinBGAGVABinGGVinoutAinoutoutAAinAoutBVVVVVVVVVVRVVRVVVGVVoutout⎯⎯→⎯=−−⎯⎯→⎯−−=+−=−=−=−×==∞→∞→ ;unknowns 2 equations, 22(2) II(1) )0( thus0221Negativefeedback59Golden rules of ideal op amps1. The output does what is necessary to make VB= VA.2. The inputs draw no current. With these rules, knowledge of G is not needed as long as G>>1. Reference: H&H 4.0310Inverting amplifier from Golden Rules+−VoutVBVAR2R1I1I2VininoutoutinABVRRVRVRVVVII122121unknown 1 equation, 1000 and :rulesGolden −=−=−===X10 inverting amplifier if R2= 100KΩ and R1= 10 kΩ.611Summing amplifier+−VoutVBVARV1V2V3VA= 0Vout= -(V1+ V2+ V3)R,R,R“intuitively obvious”12Current to voltage converteror electrometer+VoutVBVARI1-I1Vout= −I1R + VoffsetUsed to measure small currents. Offset error: Voffset= -IleakR,where Ileakis the nonideal input offset current (also called Ibias). → Ileak-713Non-inverting amplifier with gain >1+−VoutVAR2R1I1I2Vin121212111:divider voltagesimple isFeedback rule)(Golden RRVVRRRVVRRRVVVVinoutinoutoutAAin+=⎟⎟⎠⎞⎜⎜⎝⎛+=⎟⎟⎠⎞⎜⎜⎝⎛+==Answer is NOT R2/R1.Can’t make a x1 or x0.5 amplifier this way!VB14Non-inverting amplifier with gain = 1 or <1+−VoutVAVinx1 amplifier is a buffer or followerIt is used to drive long lengths of cable, otherwise signal is decreased.High current buffer amplifiers with no “-B”input are available.VB+−VoutVAVinVBVolume control, gain <1BUF634 can put out 250 mA !815Difference or “B-A” amplifier+−VoutV1VAR2R1V2R1R2VBThe mathematical analysis is in D&H, ch. 9-5, (3 eqns., 2 unknowns) 1212)(RRVVVout−=Gain can be 1, >1 or <1. 16Differentiator from Golden RulesinoutoutinoutinABVdtdRCVRCVVdtdRVVdtdCVVII−=−=−====unknown 1 equation, 1or 0 and :rulesGolden 21+−VoutVBVARCI1I2VinThe derivative of a step fcn. is a delta fcn. What determines the width of the delta fcn. in this case?What determines the height? VinVoutHere is what it will do to a square pulse: oscilloscope917Simple integrator is “ruined” by nonideal Ibias+→ IleakVoutVBVACRI1I2Vin stops.n integratio andcurrent leakage by the voltagesupply the tocharged iscapacitor the,Eventually)0(11unknown 1 equation, 1or 0 and :rulesGolden nintegratioofconstant 00214342143421=+′−′−=+−=−=−===∞→∫∫tVtdICtdVRCVCIRCVVdtdVdtdCIRVVVIIouttleaktinoutleakinoutoutleakinABIleakis the tiny input current to terminal VA, often ignored. Ileakis also called Ibias. 18Practical op amp integrator with extra resistor+VoutVBVACR1I1I2Vin22202merror ter0201212121 :is thisofcost The big. R make so1or 11if small is error term11or 0 and :rulesGolden RIVCRtVtdVCRtdVCRtdVCRVCRVCRVVdtd RVVdtdCRVVVIIleakoffsetouttouttouttinoutoutinoutoutoutinAB−=<<<<′′−′−=−−=−−====∫∫∫4434421Ileakis the tiny input current to terminal VA, often ignored.R2R2tends to discharge C→ Ileak1019Error of practical integrator is “droop”Vint’ → tVoutTrue integralOutput of practical integrator “droops” with time constant R2CNot significant if R2C >> tInput signaloscilloscope20The comparatora special kind of op amp1121Comparatoroutput is either high (V+) or low (V-)+−VoutVAV+VBVrefVtestVtest> VrefVoutis V+Vtest< VrefVoutis V-Op amps designed to be comparators are not damaged if VA≠ VB.22Comparator as temperature controller+−VoutV+VrefVtestV+HeaterRTThermistor RTis heated and gets more resistive, driving Vtestdown below Vref, and the heater turns off until the comparator changes state again. BufferRRRNote that V-can be ground.“Thermostat”ColderHotter1223Some comparators only “sink” currentthe output is an “open collector”+−VoutVAV+VBVrefVtestRloadLF311Wait a bit and we will talk about transistors and what this means.V+24Schmitt triggerDefinition: A comparator that goes highat a higher reference voltage than the reference voltage for going low. If off, and thenVtest> Vref- ΔV then output goes high (furnace turns on at 68 F)If on, and thenVtest< Vref+ ΔV then output goes low (furnace turns off at 70 F)Why do this? So that your furnace (for example) will stay on until the house heats up a few degrees, then turn off. The furnace should run 5 minutes per hour rather than 5 seconds per minute. ΔV determines how much the parameter being controlled is allowed to vary.1325Comparator made into Schmitt trigger+−VoutV+Vref“thermostat”VtestV+HeaterRTBufferPositive feedback (100RT) raises Vtestrelative to Vrefwhen the heater is on, which has the same effect as lowering Vref(by about 1% in this case). This circuit provides control to order 1%. 100 RTRRRRT≅ R26Schmitt trigger for digital