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1
Unit 3: Other Circuit Analysis
Techniques
Unit 3.3 – The Operational Amplifier
2
The Operational Amplifier
f
In this lecture, you will learn:
±
The characteristics of an ideal operational
amplifier
±
The application of the operational amplifier to
five circuit configurations:
• The comparator
• The inverting amplifier
• The noninverting amplifier
• The summing amplifier
• The difference amplifier
3
The Operational Amplfier
f
The term is rooted in the
days of the analog computer
±
A unit used to perform
addition, subtraction,
multiplication, division,
differentiation or integration
f
Today, the term “op amp”
usually refers to a highly
integrated circuit capable of
performing these same
operations by adding
external components
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The 741 Op Amp
f
One of the first
solidstate
integrated opamps
(1966)
f
Still widely used
today
Inverting input
Noninverting input
Output
V
+
V

2
3
7
4
6
15
Of set nul

+

+
Inverting input 2
Noninverting input 3
V

4
Offset null 1
7 V
+
6 Output
5 Offset Null
8 NC
5
The 741 OpAmp
You get all of this for around 20 cents.
Inverting input
Noninverting input
Output
V
+
V

2
3
7
4
6
Of set nul

+
6
Real World (Nonideal) Op Amp
f
All real op amps have:
±
A finite input
resistance R
i
±
A finite (but very
large) gain called the
openloop gain (A)
±
A nonzero output
resistance R
o
±
Current drive
limitations
±
Power dissipation
limitations
±
Input offsets that
must be dealt with in
some circuits
v
o
R
i
Av
d
R
o

+

v
d
+
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7
The Ideal Operational Amplifier
f
We will make some assumptions about the
op amp characteristics that simplify
circuit analysis
f
For many circuits, these simplifications
are justified
f
The ideal operational amplifier can be
characterized as follows:
±
Infinite open loop gain, A
≅
∞
±
Infinite input resistance, R
i
≅
∞
±
Zero output resistance, R
o
≅
0
v
o
R
i
Av
d
R
o

+

v
d
+
8
Real vs. Ideal Specs.
f
The ideal op amp:
±
Infinite open loop gain, A
≅
∞
±
Infinite input resistance, R
i
≅
∞
±
Zero output resistance, R
o
≅
0
f
Real opamp specs
(LM741):
±
A = 200,000 (typical)
±
R
i
= 6 M
Ω
(typical)
±
R
o
≈
300
Ω
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 Fall '08
 WilliamNeal
 Amplifier, Operational Amplifier

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