Lecture 4: Operational Amplifiers What can you do with infinite gain?

Lecture 4: Operational Amplifiers What can you do with infinite gain? July 13, 2023 Introduction to Engineering Electronics 1

Some History Fairchil d 0.60 Inches 1964: The First Linear 1965: A Best-Seller IC The µA709 Op-Amp The µA702 Op-Amp 14 Transistors 12 Transistors Designer: Bob Widlar Designer: July 13, 2023 Bob Widlar Introduction to Engineering 2 Electronics

2 Minute Quiz Name Section 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? July 13, 2023 R1 V 90k V1 V R2 10V 10k Introduction to Engineering Electronics 0 V 3

Answers The green probe: 10V The red probe: 1V The blue probe: 0V VoltageR Divider: 2 V R1 R R1 V 90k V1 V R2 10V 10k 10V 2 0 July 13, 2023 Introduction to Engineering Electronics 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. July 13, 2023 Introduction to Engineering Electronics 5

Op-Amp July 13, 2023 Introduction to Engineering Electronics 6

Ideal Op-Amp Model Gain is infinite A V V O U T V V O U T V IN Input resistance is infinite R IN Output resistance is zero R O U T Input voltage is zero Input current is zero I 0 July 13, 2023 Introduction to Engineering Electronics V IN 0 0 I 0 7

Ideal Op-Amp Continued Bandwidth is also infinite. Thus, an ideal op-amp works the same at all frequencies. July 13, 2023 Introduction to Engineering Electronics 8

Feedback Like most engineered systems, the op-amp 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 July 13, 2023 Introduction to Engineering Electronics 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? July 13, 2023 Introduction to Engineering Electronics 10

Feedback: Valve Example As the water nears the specified level, the valve is closed. Negative feedback is most commonly used to control systems. July 13, 2023 Introduction to Engineering Electronics 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. July 13, 2023 Introduction to Engineering Electronics 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 July 13, 2023 Introduction to Engineering Electronics 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 July 13, 2023 Introduction to Engineering Electronics 14

Op-Amp Configurations V2 Non-Inverting Amplifier No voltage difference between inputs V1 V 2 Resistors act like voltage divider V July 13, 2023 2 R2 V R1 R2 Introduction to Engineering Electronics O 15

Op-Amp Configurations Non-Inverting Amplifier Continued Combining the two equations for the voltages gives us the relationship between input and output V up July 13, 2023 O U T V IN R1 1 R2 Note that this formula is different in the lab write Introduction to Engineering Electronics 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 V July 13, 2023 2 0 Introduction to Engineering Electronics 17

Op-Amp Configurations Inverting Op-Amp Continued Current through R1 V1 0 V1 I1 R1 R1 Current through Rf I2 July 13, 2023 Introduction to Engineering Electronics V 0 O R f VO R f 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) July 13, 2023 Introduction to Engineering Electronics 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 R V July 13, 2023 O U T V Introduction to Engineering Electronics f IN R1 20

Op-Amp Configurations Inverting Summing Amplifier Each input resistor contributes to the current. R f R f R f V July 13, 2023 O U T V 1 R1 V Introduction to Engineering Electronics 2 R V 2 3 R 3 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 V C C V O U T V Usually filter capacitors are connected to power to reduce noise July 13, 2023 Introduction to Engineering Electronics 22 CC

Op-Amps: Practical Issues Note: Literally a Black Box We use real op-amps: 741 Note the pin connections for the IC July 13, 2023 Introduction to Engineering Electronics 23

Op-Amps: Practical Issues 15 V 1.0 uF 7 1 U1 3 2 6 Vout LM741 4 5 R1 Rstability V1 1.0 uF -15 V R2 Note the pins (not all are used) Extra resistor corrects offset R1R 2 problem R s ta b ility R1 R July 13, 2023 Introduction to Engineering Electronics 2 24

ICs come in many types of packages. We will use the 8-pin, dual-in-line or DIP package Note the other offset nulling circuit July 13, 2023 Introduction to Engineering Electronics 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 July 13, 2023 Introduction to Engineering Electronics 26

Embedded Control Studio Classroom July 13, 2023 Introduction to Engineering Electronics 27

July 13, 2023 http://litec.rpi.e du Introduction to Engineering Electronics 28

Feedback and Control Example The inverted pendulum is like balancing a baseball bat July 13, 2023 Introduction to Engineering Electronics 29

Inverted Pendulum Experiment http://www.univ-valenciennes.fr/LAMIH/pendule/english/index.html July 13, 2023 Introduction to Engineering Electronics 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 July 13, 2023 Introduction to Engineering Electronics 31

More Magnetic Levitation July 13, 2023 Introduction to Engineering Electronics 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 July 13, 2023 Introduction to Engineering Electronics 33