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EE 1270: Introduction to Electric Circuits
Lecture 13:
Operational Amplifiers
Part 2
Chapter 5
The Operational Amplifiers
Sections 5.4-5.7
EE 1270 Introduction to Electric Circuits
Suketu Naik
1
Operational Amplifiers
Pin Diagram
Positive
Power
Supply
Negative
Power
Supply
EE 1270 Introduction to Electric Circuits
Suketu Naik
2
OpAmp Characteristics
vp
vo
+
vn
-
An Ideal Op amp has infinite open-loop gain, infinite input
resistance and zero output resistance
When Op amp is connected in the circuit...

v
p
ip
vn in
vo
For an ideal OpAmp
 vp = vn (voltages at positive input
terminal and negative input terminal are
equal to each other)
 ip= in =
0 (currents entering the
positive and negative input terminals are
zero)
EE 1270 Introduction to Electric Circuits
Suketu Naik
3
Op-Amp Configurations
1) Inverting Amplifier
2) Non-inverting Amplifier
3) Voltage Follower (or buffer)
4) Summing Amplifier
5) Difference Amplifier
EE 1270 Introduction to Electric Circuits
Suketu Naik
4
Non-inverting Amplifier
 Rf
vo  1 
 Rs

vg

1) Output and Input signals have same polarity (0 ° phase shift)
2) Non-inverting Amplifier provides positive voltage gain
EE 1270 Introduction to Electric Circuits
Suketu Naik
AP 5.4
5
a) Find the output voltage when Rx=60 kΩ
b) How large can Rx be before saturation
EE 1270 Introduction to Electric Circuits
Suketu Naik
6
Voltage Follower (Buffer)
Suppose
Rg=Rf=0 (short), Rs=Inifinite
(open)
 Circuit becomes voltage follower,
e.g.
vo=vi
Q: Why use voltage follower (Gain=1)?
EE 1270 Introduction to Electric Circuits
Suketu Naik
7
Summing Amplifier
Rf
Rf 
 Rf
vo   Va 
Vb 
Vc 
Rb
Rc 
 Ra
A Summing Amplifier combines several inputs and outputs the
weighted sum of the inputs
EE 1270 Introduction to Electric Circuits
Suketu Naik
Summing Amplifier: Application
8
Digital to Analog Converter (DAC)
Q: Why do we use DAC?
EE 1270 Introduction to Electric Circuits
Suketu Naik
p5.17: Design of a Summing Amp
Va
Ra
Vc
Vb
Rb
Rc
9
Vd
Rd
Design the Summing Amplifier so that
vo  8Va  6Vb  10Vc  6Vd 
EE 1270 Introduction to Electric Circuits
Suketu Naik
Difference (Differential) Amplifier
10
Differential Amp rejects
Common-mode signal (noise or DC)
If Ra/Rb=Rc/Rd (same ratio) then...
Rb
Vb  Va 
vo 
Ra
A Difference (Differential) Amplifier amplifies the difference
between two input signals but rejects signals common to the inputs
EE 1270 Introduction to Electric Circuits
Suketu Naik
Differential Amplifier: Application
11
Instrumentation Amplifer (IA)
An Instrumentation Amplifier (IA) is used for precise measurement and
amplification with noise reduction in sensors and data acquisition systems
EE 1270 Introduction to Electric Circuits
Suketu Naik
12
AP5.5a: Differential Amplifier
Find range of Va that results in linear operation (avoids saturation)
Use Eq (5.22) (because Ra/Rb ≠ Rc/Rd )
Rd ( Ra  Rb )
Rb
vo 
vb 
va
Ra ( Rc  Rd )
Ra
EE 1270 Introduction to Electric Circuits
Suketu Naik