Lecture 4: Op-Amps

Lecture 4: Operational

Amplifiers

What can you do with infinite gain?

4/15/2020

Introduction to Engineering Electronics

STOLEN FROM K. A. Connor

1

Some History

Fairchild

1964: The First Linear IC

The µA702 Op-Amp

12 Transistors

Designer: Bob Widlar

0.60 Inches

1965: A Best-Seller

The µA709 Op-Amp

14 Transistors

Designer: Bob Widlar

4/15/2020



2

2 Minute Quiz

Name_______ Section________ Date_________

• What is the voltage measured by the blue probe?

R1

90k

V

V

R2

10k

V1

10V

4/15/2020



0

3

V

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.

4/15/2020



4

Op-Amp

4/15/2020



5

Ideal Op-Amp Model

• Gain is infinite

A



V

OUT

V





V





V

OUT

V

IN

 

• Input resistance is infinite

R

IN

 

• Output resistance is zero

• Input voltage is zero

R

OUT



0

V

IN



0

• Input current is zero

4/15/2020

I





0 I





0



6

Ideal Op-Amp Continued

• Bandwidth is also infinite. Thus, an ideal op-amp works the same at all frequencies.

4/15/2020



7

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

4/15/2020



8

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?

4/15/2020



9

Feedback: Valve Example

• As the water nears the specified level, the valve is closed.

• Negative feedback is most commonly used to control systems.

4/15/2020



10

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.

4/15/2020



11

Positive and Negative

Feedback

• Connecting the output to the positive input is positive feedback

• Connecting the output to the negative input is negative feedback

4/15/2020



12

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

4/15/2020



13


V

2

• Non-Inverting Amplifier

 No voltage difference between inputs

V

1



V

2

 Resistors act like voltage divider

V

2



R

2

R

1



R

2

V

O

4/15/2020



14


• Non-Inverting Amplifier Continued

 Combining the two equations for the voltages gives us the relationship between input and output

V

OUT



V

IN





1



R

1

R

2



Note that this formula uses different variables in the lab write up

4/15/2020



15


V

2

• Inverting Op-Amp

 Current through R

1 through R f equals the current

 No current in the inputs

 The voltage at both inputs is zero

V

2



0

4/15/2020



16


• Inverting Op-Amp Continued

 Current through R

1

I

1



V

1



R

1

 Current through R f

I

2



0

0



V

O



R f

V

1

R

1

 

V

O

R f

4/15/2020



17

2 Minute Quiz

Name______ Section_______ Date________

• What is the voltage measured by the green probe?

4/15/2020

R1

90k

V

R2

10k



0

V

18

V

V1

10V


• Why the minus sign for the current through R

 The convention for Ohm’s Law is that the current flows from the high voltage to the low voltage for a resistor f

?

 Here the current flows from the low voltage

(ground) to the high voltage (V

O

)

4/15/2020



19


• Inverting Op-Amp Continued

 The current through R

1 current through R

2 in the inputs. must equal the since there is no current

 Combining the two equations for the currents

V

OUT

 

V

IN

R f

R

1

4/15/2020



20


• Inverting Summing Amplifier

 Each input resistor contributes to the current.

V

OUT

 







V

1

R

R

1 f 

V

2

R

R

2 f 

V

3

R

R

3 f





4/15/2020



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

CC



V

OUT

 Usually filter capacitors are connected to power to reduce noise

 

V

CC

4/15/2020



22


Note: Literally a

Black Box

• We use real op-amps: 741

• Note the pin connections for the IC

4/15/2020



23


+ 15 V

1.0 uF

Rstability

V1

R1

3

+

2

-

U1

6

LM741

1.0 uF

Vout

-15 V

R2

• Note the pins (not all are used)

• Extra resistor corrects offset problem

R stability



R R

1 2

R

1



R

2

4/15/2020



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

4/15/2020



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

4/15/2020



26

Embedded Control

• Studio Classroom

4/15/2020



27

4/15/2020 http://litec.rpi.edu



28

Feedback and Control Example

• The inverted pendulum is like balancing a baseball bat

4/15/2020



29

Inverted Pendulum

Experiment http://www.univ-valenciennes.fr/LAMIH/pendule/english/index.html

4/15/2020



30

Magnetic Levitation

• Trains can magnetically fly over a roadbed with position sustained by some kind of control system

• Our next lab is on maglev

4/15/2020



31

More Magnetic Levitation

4/15/2020



32

2 Minute Quiz

Name_______ Section________ Date__________

R1

90k

• What is the voltage measured by the red probe?

V

R2

10k

4/15/2020



0

V

33

V

V1

10V

Answers

• The green probe:

10V

• The red probe: 1V

• The blue probe: 0V

• Voltage Divider:

R1

90k

V

R2

10k

V



R

2

R

1



R

2

10 V



1 V

4/15/2020



0

V

34

V

V1

10V

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

4/15/2020



35

Lecture 4: Op-Amps

Related documents

Products

Support

Lecture 4: Op-Amps

Related documents

Add this document to collection(s)

Add this document to saved

Suggest us how to improve StudyLib