DATA ACQUISTION AND SIGNAL
PROCESSING
Lecture
Introduction to Opamps & Multisim
Dr. Tayab Din Memon
Objectives


To introduce the amplifier and its Transfer
Characteristics.
Opamp




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Ideal Characteristics of Opamp.
Differential Mode of Opamp.
Power of Negative Feedback
Opamp Modes of Operation
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Definition, Internal Architecture, Notation, and its Package.
Inverting
Non-Inverting
Differential
Session-II Lab Work
Simulation of Modes of Opamp.
Amplifier?
Why do we need to amplify
signals?
PHYSICAL
PHENOMENA
SENSOR
SIGNAL
CONDITIONING
ANALOG TO
DIGITAL
CONVERSION
DIGITAL
OUTPUT
To increase level
of signals.
Amplifier-Transfer Characteristics

Transfer Characteristic is the
relationship between input
and output voltages
The level of signal amplification is
called the Gain, Av, and is the
slope of the amplifier transfer
characteristics.
Input voltage is Vi
Output voltage is Vo
How do we build an amplifier?
Discrete Devices, Transistors, Resistors,
Capacitors.
Attractive
Solution
Operational
Amplifiers (Op-Amp)
Use of the Integrated Circuits –
Inexpensive, Easy to use, and
readily available.
How Opamp can be Defined?
The op-amp is a high gain dc
amplifier that has high input
impedance and low output
impedance.
Note: Op-amps are analog,
rather than digital devices,
although they are used in
many digital instrument.
DC Amplifier – An amplifier that is capable of amplifying dc voltages and
slowly varying voltages.
Op-Amp Definition

An operational amplifier (op-amp) is
a DC-coupled (?)high-gain electronic
voltage amplifier with a differential
input and, usually, a single-ended
output. An op-amp produces an output
voltage that is typically hundreds of
thousands of times larger than the
voltage difference between its input
terminals [source: Wikipedia].
DC Coupled


Direct coupled refers to coupling between stages of an amplifier
and coupling of the output stage to the load. The reason for coupling
devices between stages is to accommodate the different DC biases
needed by each stage. Normally you can't just connect the collector
output of one stage to the base input of another identical stage, since
collector and base operate at very different voltages. So an interstage
transformer or capacitor provides DC isolation between stages while
passing the audio.
Direct-coupled interstage designs often use complementary stages; a
PNP transistor drives an NPN transistor, etc. This makes it much easier
to obtain bias compatibility between stages. See ref, figure 2.5.
Direct output coupling is used to eliminate the nonlinearity, phase-shift
and limited frequency response of a transformer or capacitor. The
drawback is the possibility of applying a DC bias to the load. Direct
output coupled amplifiers usually have a bias nulling adjustment which
may need occasional adjustment.
Internal Structure of Opamp.
Op-amp Notation
+Vcc
v-
v+
-
vVO
+
v0
v+
+
-Vcc
The Opamp has minimum five terminals:
Two Signal Input Terminals: +v and –v
One Output terminal vo
Two dc Power Supply Terminals +Vcc and –Vcc
Opamp Packages
IC Package
Pin Configuration
Opamp uA741 present in 8pin Dip IC.
Also available in different size pins
i.e. 16 also.
Opamp Transfer Characteristics…..
For example……
For different inputs
and respective
outputs the table is
provided:
S.No.
+Vin
-Vin
Vd
Output
1
5
3
2
+
2
-5
-7
2
+
3
-3
-2
-1
-
4
3
5
-2
-
n first and second +Vin is more positive
then –Vin so Output will be + saturated.
In third and fourth –Vin is more positive
then +Vin so Output will be - saturated.
Opamp Saturation ………..
Input Signal
30v pk-pk.
Output Signal
30V pk-pk Gain
1,000,000 OpAmp Saturates
at 10V
Power of Negative Feedback
Vo
Vi
Advantages and
disadvantages of
Negative Feedback:
Less Noise,
Less Disturbance,
High Bandwidth
Gain is decreased
Feedback
Figure: Negative Feedback
Opamp Modes of Operation
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Inverting Mode
Noninverting Mode
Differential Mode
Inverting Mode
Rf
+Vcc
R
vv+
AC
-
v0
+
-Vcc
Non-Inverting Mode
Rf
+Vcc
R
vv+
-
v0
+
-Vcc
AC
Differential Mode
+Vcc
vv+
-
v0
+
-Vcc
AC
AC
Voltage Follower
+Vcc
vv+
-
v0
+
-Vcc
AC
SESSION II
LAB WORK
All tasks given in the Handout
should be completed
Introduction to MultiSim
Multisim is fully featured SPICE schematic
capture and simulation tool used for circuit
design, simulation, and in combination with
Ultiboard for layout of custom PCBs.
Design of an amplifier with a gain 10.
TASK-I
Design an amplifier with gain of 10?
Mode choice is open!!
Ok! Design using Inverting mode. In inverting
mode Voltage gain is ratio of Feedback
Resistance to Input Resistance. Vo/Vi= -(Rf/Ri).
So Select Rf=10KΩ and Ri=1KΩ, then voltage
gain will be 10.
HERE IS THE
SOLUTION!!
Inverting Amplifier with Gain 10.
Rf=10K
+Vcc
R=1K v
v+
AC
-
v0
+
-Vcc
1V/1KHz
WHAT DIFFERENCE DID YOU
OBSERVE IN TWO OUTPUTS?
ANSWER?
Saturation Due to input voltage increase.
Repeat same Task with Non-Inverting
Amplifier.
In Non-inverting mode Voltage gain is:
1+Rf/Ri.
So if we select Rf=4.7KΩ then Ri=0.52KΩ
~0.51K Ω (because 0.51K Ω is a standard
Resistor).
If we place these values then Output of
the Circuit should be 10.
HERE IS SOLUTION
Voltage Follower Simulated Result
Summary
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Note that in Inverting and Non-Inverting
Opamp modes, feedback is – ve.
The only difference is that; input is
applied at different terminals.
Output is 1800 out of phase with input in
Inverting whereas in Non-Inverting
Output is in phase with Input.