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Lecture 4: Operational Amplifiers What can you do with infinite gain? 17 July 2015 Introduction to Engineering Electronics K. A. Connor 1 Some History Fairchild 0.60 Inches 1964: The First Linear IC 1965: A Best-Seller The µA702 Op-Amp The µA709 Op-Amp 12 Transistors 14 Transistors Designer: Bob Widlar Designer: Bob Widlar 17 July 2015 Introduction to Engineering Electronics K. A. Connor 2 2 Minute Quiz Name_____________ • What is the voltage measured by the blue probe? • What is the voltage measured by the green probe? • What is the voltage measured by the red probe? 17 July 2015 Section _____ R1 V 90k V1 V R2 10V 10k Introduction to Engineering Electronics K. A. Connor 0 V 3 Answers • The green probe: 10V • The red probe: 1V • The blue probe: 0V • Voltage Divider: R2 V 10V R1 R2 R1 V 90k V1 V R2 10V 10k 0 17 July 2015 Introduction to Engineering Electronics K. A. Connor V 4 What can you do with infinite gain? • The goal of amplifier designers: huge gain. • What are the problems and opportunities associated with infinite gain? • First, we need a model. 17 July 2015 Introduction to Engineering Electronics K. A. Connor 5 Op-Amp 17 July 2015 Introduction to Engineering Electronics K. A. Connor 6 Ideal Op-Amp Model • Gain is infinite VOUT VOUT A V V VIN • Input resistance is infinite RIN • Output resistance is zero ROUT 0 • Input voltage is zero VIN 0 • Input current is zero 17 July 2015 I 0 Introduction to Engineering Electronics K. A. Connor I 0 7 Ideal Op-Amp Continued • Bandwidth is also infinite. Thus, an ideal op-amp works the same at all frequencies. 17 July 2015 Introduction to Engineering Electronics K. A. Connor 8 Feedback • Like most engineered systems, the opamp uses feedback to realize its potential value. • Feedback comes in two forms Positive Feedback Negative Feedback • It seems like positive feedback might be best, but negative feedback makes the op-amp work 17 July 2015 Introduction to Engineering Electronics K. A. Connor 9 Feedback Examples From a Zoology Course • You just ate a Krispy Kreme donut and your blood glucose levels are on the rise. In response to this rise, the pancreas is releasing insulin into the blood stream stimulating storage of glucose. As a result, blood glucose levels begin to drop. Is this an example of positive or negative feedback? • A woman is in labor, pressure receptors in the birth canal send messages to her brain that result in increased contraction of the uterus and increased pressure in the birth canal. Is this positive or negative feedback? 17 July 2015 Introduction to Engineering Electronics K. A. Connor 10 Feedback: Valve Example • As the water nears the specified level, the valve is closed. • Negative feedback is most commonly used to control systems. 17 July 2015 Introduction to Engineering Electronics K. A. Connor 11 Golden Rules for Op-Amps • The output attempts to do whatever is necessary to make the voltage difference between the two inputs zero. (Negative Feedback is Required) • The inputs draw no current. 17 July 2015 Introduction to Engineering Electronics K. A. Connor 12 Positive and Negative Feedback • Connecting the output to the positive input is positive feedback • Connecting the output to the negative input is negative feedback 17 July 2015 Introduction to Engineering Electronics K. A. Connor 13 Op-Amp Configurations • Buffer or Voltage Follower No voltage difference between the output and the input Draws no current, so it puts no load on the source Used to isolate sources from loads 17 July 2015 Introduction to Engineering Electronics K. A. Connor 14 Op-Amp Configurations V2 • Non-Inverting Amplifier No voltage difference between inputs V1 V2 Resistors act like voltage divider R2 V2 VO R1 R2 17 July 2015 Introduction to Engineering Electronics K. A. Connor 15 Op-Amp Configurations • Non-Inverting Amplifier Continued Combining the two equations for the voltages gives us the relationship between input and output VOUT R1 V IN 1 R2 Note that this formula is different in the lab write up 17 July 2015 Introduction to Engineering Electronics K. A. Connor 16 Op-Amp Configurations V2 • Inverting Op-Amp Current through R1 equals the current through Rf No current in the inputs The voltage at both inputs is zero V2 0 17 July 2015 Introduction to Engineering Electronics K. A. Connor 17 Op-Amp Configurations • Inverting Op-Amp Continued Current through R1 Current through Rf 17 July 2015 V1 0 V1 I1 R1 R1 VO 0 VO I2 Rf Rf Introduction to Engineering Electronics K. A. Connor 18 Op-Amp Configurations • Why the minus sign for the current through Rf? The convention for Ohm’s Law is that the current flows from the high voltage to the low voltage for a resistor Here the current flows from the low voltage (ground) to the high voltage (VO) 17 July 2015 Introduction to Engineering Electronics K. A. Connor 19 Op-Amp Configurations • Inverting Op-Amp Continued The current through R1 must equal the current through R2 since there is no current in the inputs. Combining the two equations for the currents VOUT VIN 17 July 2015 Introduction to Engineering Electronics K. A. Connor Rf R1 20 Op-Amp Configurations • Inverting Summing Amplifier Each input resistor contributes to the current. R R R VOUT V1 17 July 2015 f R1 V2 Introduction to Engineering Electronics K. A. Connor f R2 V3 f R3 21 Op-Amps: Practical Issues • Op-Amps require power 741 requires plus and minus 15V Others may require only positive or both positive and negative voltages Output voltage is limited to VCC VOUT VCC Usually filter capacitors are connected to power to reduce noise 17 July 2015 Introduction to Engineering Electronics K. A. Connor 22 Op-Amps: Practical Issues Note: Literally a Black Box • We use real op-amps: 741 • Note the pin connections for the IC 17 July 2015 Introduction to Engineering Electronics K. A. Connor 23 Op-Amps: Practical Issues + 15 V + 1.0 uF 7 1 U1 + 2 - 6 Vout LM741 4 5 R1 3 Rstability V1 1.0 uF + -15 V R2 • Note the pins (not all are used) • Extra resistor corrects offset problem Rstability 17 July 2015 R1 R2 R1 R2 Introduction to Engineering Electronics K. A. Connor 24 • ICs come in many types of packages. We will use the 8pin, dual-in-line or DIP package • Note the other offset nulling circuit 17 July 2015 Introduction to Engineering Electronics K. A. Connor 25 Where Will You See This Information Next? • Op-amps: Many Courses Including ECSE-2010 Electric Circuits ECSE-2050 Analog Electronics • Feedback and Control ENGR-2350 Embedded Control ECSE-4440 Control Systems Engineering ECSE-496x Control Systems Design 17 July 2015 Introduction to Engineering Electronics K. A. Connor 26 Embedded Control • Studio Classroom 17 July 2015 Introduction to Engineering Electronics K. A. Connor 27 http://litec.rpi.edu 17 July 2015 Introduction to Engineering Electronics K. A. Connor 28 Feedback and Control Example • The inverted pendulum is like balancing a baseball bat 17 July 2015 Introduction to Engineering Electronics K. A. Connor 29 Inverted Pendulum Experiment http://www.univ-valenciennes.fr/LAMIH/pendule/english/index.html 17 July 2015 Introduction to Engineering Electronics K. A. Connor 30 Magnetic Levitation • Trains can magnetically fly over a roadbed with position sustained by some kind of control system • Our Lab 10 is on maglev 17 July 2015 Introduction to Engineering Electronics K. A. Connor 31 More Magnetic Levitation 17 July 2015 Introduction to Engineering Electronics K. A. Connor 32 Engineering Ethics • Electrical and Computer Engineers do not usually face immediate ethical issues involving public health and safety • System control is one of many exceptions • From the IEEE Code of Ethics We agree to accept responsibility in making engineering decisions consistent with the safety, health and welfare of the public, and to disclose promptly factors that might endanger the public or the environment; • http://www.iit.edu/departments/csep/eac/post _workshop.html 17 July 2015 Introduction to Engineering Electronics K. A. Connor 33