Download Operational Amplifiers

Tutorial: Mechanic – electrician
Topic:
Electronics
II. class
Operational Amplifiers - Basics
Prepared by: Ing. Jaroslav Bernkopf
Projekt Anglicky v odborných předmětech, CZ.1.07/1.3.09/04.0002
je spolufinancován Evropským sociálním fondem a státním rozpočtem České republiky.
Definition:
An operational amplifier (OA) is an integrated circuit that amplifies small
voltage signals.
It contains many transistors:
2
Description:
An operational amplifier
• has two inputs of opposite polarity
• amplifies only the voltage difference between the two inputs
• doesn‘t amplify the particular voltages on the inputs
• has a single output
8
Vs+ Positiv e power supply
V+ Input
3
V- Input
1
Output
-
4
2
+
Vs- Negativ e power supply
3
Description:
An operational amplifier has
• a very high voltage gain Av
• a very high input resistance Ri
• a very low input current Ii
• a very low output resistance Ro
4
We want the OAs to be as good as possible:
We want them to be ideal.
An ideal OA would have:
A real OA has:
Infinite voltage gain Av
High voltage gain Av, e.g. 106
Infinite input resistance Ri
High input resistance Ri, e.g. 1 MΩ
Zero input current Ii
Low input current Ii, e.g. 1 nA
Zero output resistance Ro
Low output resistance Ro, e.g. 50 Ω
Real OAs are so good that usually we can assume our real OA to be ideal.
5
If the voltage gain Av is infinite, then with the smallest possible voltage Vi
between the inputs the output voltage Vo must be infinite too:
Vi * Av = Vo
0.001 V * ∞ = ∞ V
0.0001 V * ∞ = ∞ V
0.00001 V * ∞ = ∞ V
0.000001 V * ∞ = ∞ V
0.0000001 V * ∞ = ∞ V
and so on.
But an infinite voltage on the output would be terribly dangerous, and unusable!
Is it really infinite? No, it isn‘t.
Why?
6
Why isn‘t the output voltage infinite?
There are two good reasons:
1) An infinite voltage simply is not physically possible.
2) The negative feedback works for us.
We will explain this in the following slides.
7
Why can‘t the output voltage be infinite?
Your OA can‘t give you more than it gets from you.
And what does it get from you?
It gets the supply voltage of let‘s say +15V and -15V.
That‘s why it can‘t give you more than +15V or less than -15V.
The output voltage can‘t exceed +15V or -15V.
8
Vs+ = +15 V
2
+
1
output
-
4
3
Vs- = -15 V
8
If there is a „reasonable“ voltage on the output, let‘s say 3 V like in the picture,
the only possible explanation is:
The voltage Vi between the inputs must be zero!
R2
R1
1
1
V2 = 3V
Av = infinity
2
Vi = 0.0 V !!!
3
+
2
-
V1 = 1V
9
What forces the OA to behave so „reasonably“?
The negative feedback always sets the output voltage to such a value that the
voltage between the inputs is zero.
This knowledge will help us solve all OA-based circuits in this course.
R2
R1
1
1
V2 = 3V
Av = infinity
2
Vi = 0.0 V !!!
3
+
2
-
V1 = 1V
10
Example:
There is a voltage +5.001 V on the V+ input, and +5.000 V on the V- input.
The operational amplifier amplifies the differential voltage 0.001 V and
ignores the „common“ voltage 5 V.
8
Vs+ = +15 V
V+ = 5.001 V
3
2
1
output
-
4
V- = 5.000 V
+
.
.
Vs- = -15 V
11
Question:
What is the polarity of the output voltage?
8
Vs+ = +15 V
V+ = 5.001 V
3
2
1
output
-
4
V- = 5.000 V
+
.
.
Vs- = -15 V
12
Answer:
The output voltage is positive, because the V+ input is more positive than the
other.
8
Vs+ = +15 V
V+ = 5.001 V
3
2
1
output
-
4
V- = 5.000 V
+
.
.
Vs- = -15 V
13
Question:
What is the output voltage?
8
Vs+ = +15 V
V+ = 5.001 V
3
2
1
output
-
4
V- = 5.000 V
+
.
.
Vs- = -15 V
14
Answer:
The voltage gain is infinite, there is no negative feedback.
The OA tries to produce an infinite positive voltage on its output.
But it can‘t produce more than +15V.
Vs+ = +15 V
8
The output voltage
is close to +15V.
V+ = 5.001 V
3
2
1
output
-
4
V- = 5.000 V
+
.
.
Vs- = -15 V
15
Test: What can be true?
Circle the correct answers!
Property
Ideal OA
Real OA
Voltage gain Av
∞ 106 10 12V 1MΩ 1nA 1kΩ
∞ 106 10 12V 1MΩ 1nA 1kΩ
Input resistance Ri
∞Ω 106 10 12V 1MΩ 1nA 0.0Ω
∞Ω 106 10 12V 1MΩ 1nA 0.0Ω
Output resistance Ro
∞Ω 106 10 12V 50Ω 1nA 0.0Ω
∞Ω 106 10 12V 50Ω 1nA 0.0Ω
Input current Ii
∞A 106 10 12V 50Ω 1nA 0.0nA
∞A 106 10 12V 50Ω 1nA 0.0nA
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http://www.wikipedia.com
http://www.thefreedictionary.com
Operační program Vzdělávání pro konkurenceschopnost, ESF 2007 – 2013.
Link: http://www.msmt.cz/eu/provadeci-dokument-k-op-vzdelavani-prokonkurenceschopnost
Technical Information - National Semiconductor LM741CN Datasheet