Copyright 2001, Regents of University of CaliforniaEECS 42 Intro. electronics for CS Fall 2001 Lecture 14: 10/17/01 A.R. NeureutherVersion Date 10/16/01Lecture 14: October 17, 2001Op-Amp Circuits and CompratorsA)Cascade Op-AmpsB) Integration/Differentiation Op-Amps C)I vs. V of Op-Amps – Source LimitsD)Comparator CircuitsE) D to A ConvertersReading: Schwarz and Oldham 4.3-4.4 (light on non-ideal) and comparator viewgraphsThe following slides were derived from those prepared by Professor Oldham For EE 40 in Fall 01Copyright 2001, Regents of University of CaliforniaEECS 42 Intro. electronics for CS Fall 2001 Lecture 14: 10/17/01 A.R. NeureutherVersion Date 10/16/01CASCADE OP-AMP CIRCUITSV1+−V3V2RFR1R2R3V0+−1K9KIINHow do you get started on finding VO?Hint: IINdoes not affect VO1See the further examples of op-amp circuits in the readerHint: Identify StagesCopyright 2001, Regents of University of CaliforniaEECS 42 Intro. electronics for CS Fall 2001 Lecture 14: 10/17/01 A.R. NeureutherVersion Date 10/16/01INTEGRATING OP-AMPHow do you get started on finding VO?Hint: )(IN0VVand0i=≅≅+−)(IN0VVand0i=≅≅+−Hint: KCL at V-node with IIN-= 0V0+−R1CVINCopyright 2001, Regents of University of CaliforniaEECS 42 Intro. electronics for CS Fall 2001 Lecture 14: 10/17/01 A.R. NeureutherVersion Date 10/16/01Example: Amplifier with gain of 105, with max V0of 3V and min V0of −3V.VIN(µV)123V0 (V)0.10.2−3 −2−1−.2(a)I-V near origin−3(b)I-V over wider rangeVIN(µV)10 2030V0 (V)1−30 −20−10−2−123upper “rail”lower “rail”+−V0+−VIN• Circuit model (ideal op-amp) gives the essential linear part•But V0cannot rise above some physical voltage related to the positive power supply VCC(“ upper rail”) V0 < V+RAIL• And V0cannot go below most negative power supply, VEEi.e., limited by lower “rail” V0> V-RAILOP-AMP I-V CHARACTERISTICS WITH RAILSCopyright 2001, Regents of University of CaliforniaEECS 42 Intro. electronics for CS Fall 2001 Lecture 14: 10/17/01 A.R. NeureutherVersion Date 10/16/01OP-AMP I-V CHARACTERISTICS WITH RAILS (cont.)VIN(V)123V0 (V)12−3 −2−1−2−3−13(c)Same V0vs VINover even wider range−3(b)I-V over wide rangeVIN(µV)10 2030V0 (V)1−30 −20−10−2−123upper “rail”lower “rail”Example: Amplifier with gain of 105, with upper rail of 3V and lower rail of −3V. We plot the V0vs VINcharacteristics on two different scalesCopyright 2001, Regents of University of CaliforniaEECS 42 Intro. electronics for CS Fall 2001 Lecture 14: 10/17/01 A.R. NeureutherVersion Date 10/16/01I-V with equal X and Y axesVIN(V)123V0 (V)12−3 −2−1−2−3−13Note:• (a) displays linear amplifier behavior (|VIN| < 30 µV) and stops at rails• (b) shows comparator decision function (1 bit A/D converter centered at VIN= 0) where lower rail = logic “0” and upper rail = logic “1”SIMPLE A/D CONVERTER+−V0+−VINCopyright 2001, Regents of University of CaliforniaEECS 42 Intro. electronics for CS Fall 2001 Lecture 14: 10/17/01 A.R. NeureutherVersion Date 10/16/01OP-AMP USE AS COMPARATOR (A/D) MODESimple comparator with threshold at 1V. Design lower rail at 0V and upper rail at 2V (logic “1”). A = large (e.g. 102to105 )NOTE: The actual diagram of a comparator would not show an amplifier with “offset” power supply as above. It would be a simple triangle, perhaps with the threshold level (here 1V) specified.If VIN> 1.010 V,V0= 2V = Logic “1”If VIN< 0.99 V,V0= 0V = Logic “0”V0VIN12012+−V0VIN+−1VV0VINComparatorCopyright 2001, Regents of University of CaliforniaEECS 42 Intro. electronics for CS Fall 2001 Lecture 14: 10/17/01 A.R. NeureutherVersion Date 10/16/01ONE-BIT A/D CONVERSIONREQUIRED IN DIGITAL SYSTEMS≈pulses intransmission line→comparatorregenerated pulsespulses outAs we saw, we set comparator threshold at a suitable value (e.g., halfway between rails) and comparator output goes to +rail if VIN> VTHRESHOLDand to −rail if VIN< VTHRESHOLD.The inverse pulse shaped function is generated by applying the input voltage to V- and setting V+ to the threshold voltage. +−V0VIN1V−+What would this circuit do?Copyright 2001, Regents of University of CaliforniaEECS 42 Intro. electronics for CS Fall 2001 Lecture 14: 10/17/01 A.R. NeureutherVersion Date 10/16/01D/A CONVERSIONExample: Digital representation of sound to analog (so you can hear it!) → D/A conversionThe summing junction op-amp provides a simple means of D/A conversion via weighted-adder D/A converter0 0 1 0 10 0 1 10 1 0 01.520 1 0 10 1 1 00 1 1 11 0 0 02.533.541 0 0 11 0 1 01 0 1 11 1 0 01 1 0 11 1 1 01 1 1 14.555.566.577.5BinarynumberAnalogoutput(volts)0 0 0 00 0 0 10.5MSB LSBS1 closed if LSB =1S2 " if next bit = 1S3 " if " " = 1S4 " if MSB = 1Another way (not shown) is to sum charges instead of current with capacitor networks4-Bit D/A8V−+V05K80K40K20K10KS1+-S3S2S4−+Copyright 2001, Regents of University of CaliforniaEECS 42 Intro. electronics for CS Fall 2001 Lecture 14: 10/17/01 A.R. NeureutherVersion Date 10/16/01CHARACTERISTIC OF A 4-BIT DACDigital Input 0123456780246810121416Analog Output
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