The operational amplifier, cont. Has a multitude of circuit applications Linear applications Inverter, summer, integrator, etc. Non-linear applications Comparator ECE 194 S’01 Introduction to Engineering II Arizona State University 1 Classwork #2 R a i R R f f b R c i n v -V n cc v o + v a + + v + b v v p +V cc c Find vo ECE 194 S’01 Introduction to Engineering II Arizona State University 2 Inverting summing amplifier iA iB iC iF in 0 iA, B, C vA, B, C vn v A, B,C RA, B,C RA,B,C vO vn vo iF RF RF RF RF RF vO v A vB vC RA RB RC ECE 194 S’01 Introduction to Engineering II Arizona State University 3 Op amp non-linear applications The output voltage is a function of the input voltage difference v o V cc v _ _ p v n V cc Op amp can be forced to work in the linear region with feedback Without feedback, the operation is quite different ECE 194 S’01 Introduction to Engineering II Arizona State University 4 The comparator A comparator is a circuit that monitors two input voltages One is called the reference voltage, and the other the input voltage The output voltage depends upon the relation between the reference voltage and the input voltage, i.e., whether Vin is larger or smaller than Vref Op amps operated without feedback can act as comparators ECE 194 S’01 Introduction to Engineering II Arizona State University 5 Comparator Reference voltage to positive input Inverting com parator V out V in A V out V ref + B V ref V in Reference voltage to negative input V ou t Non-i nvertin g compa rator A V ref V in V out B + V ref ECE 194 S’01 Introduction to Engineering II Arizona State University V in 6 Comparator circuits V ref - Electrical sign als V in S’01 Action + Physical signals ECE 194 V out Introduction to Engineering II Physical outputs Arizona State University 7 Generic comparator circuit Physical and electrical input signals R R1 vin VCC R1 RA ECE 194 S’01 V CC Physical and electrical outputs R d R A LED B + R 1 R 2 Introduction to Engineering II vref R2 VCC R2 RB Arizona State University 8 Comparator circuits What are RA and RB? Circuit components that convert physical signals to electrical signals by having electrical properties that change with external physical changes. Examples: Thermistors - temperature sensitive resistors Photoconductors - light sensitive resistors Strain gauges - strain sensitive resistors Microphones - sound sensitive components ECE 194 S’01 Introduction to Engineering II Arizona State University 9 Comparator circuits Output component examples LEDs driven by op amp output Power transistor stages to drive LEDs, buzzers, relays Other amplifier stages Voltage to frequency converters Analog to digital converters Multimeters Motors Timers, clocks, recorders ECE 194 S’01 Introduction to Engineering II Arizona State University 10 Additional circuit components Variable resistor Diode I d + D V d R R 1 _ = V I R d 2 RV = R1 + R2 V d V t ECE 194 S’01 Introduction to Engineering II Arizona State University 11 Additional circuit components Bipolar junction transistor Transistor Field effect transistor FET I M I D C D Q IC ≈ I E I V ID = I S S G + B _ I E I S Large current (VG > 0) Large current (IB > 0) IC ECE 194 Zero (IB = 0) S’01 Introduction to Engineering II ID Zero (VG = 0) Arizona State University 12 Additional circuit components Sensing resistor Relay R R = R(Temperature) R = R(Pressure) R = R(Light) ECE 194 S’01 Introduction to Engineering II Electromechanical device a small current is used to control a large current Arizona State University 13 Example comparator circuit Light activated switch + 9V R 4 470 R 1 100 K _ Q 1 + R 741 OPAMP 3 4.7 K 2N222 2 LED PC 1 LED is on when input is below Vref R 2 1M Resistance of photoconductor decreases when illuminated ECE 194 S’01 Introduction to Engineering II Arizona State University 14 Classwork Building upon ideas from previous circuit, design a Dark Activated Switch ECE 194 S’01 Introduction to Engineering II Arizona State University 15 Classwork Build and test this circuit use a voltmeter to measure voltages in circuit 9V 1K LED 10 0K + 10 K to 100K 10 0K ECE 194 S’01 Introduction to Engineering II Arizona State University 16