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Module 4

Operational Amplifier
By

H V Balachandra Achar

Contents











Introduction
OPAMP Symbol
Internal block diagram
Open-loop configuration
Common mode rejection ratio
OPAMP equivalent circuit
OPAMP characteristics
Closed-loop configuration

Contents








Inverting Amplifier
Non-inverting Amplifier
Summing Amplifier Difference Amplifier
Differentiator
Integrator
Tutorials

Reference book


OPAMPS and Linear Integrated Circuits
by Ramakanth Gayakwad

Introduction


Operational Amplifier (OPAMP) is a very
high gain amplifier fabricated on Integrated
Circuit (IC)
 Finds application in
 Audio amplifier
 Signal generator
 Signal filters
 Biomedical Instrumentation
 And

numerous other applications

Introduction


Advantages of OPAMP over transistor
amplifier
 Less power consumption
 Costs less
 More compact
 More reliable
 Higher gain can be obtained
 Easy design

OPAMP terminals
+VC
C

Non inverting
input

+

Inverting input

–

Output

–VEE

OPAMP terminals

OPAMP terminals


If input is applied to non inverting input
terminal, then output will be in-phase with
input
 If input is applied to inverting input
terminal, then output will be 180 degrees
out of phase with input
 If inputs are applied to both terminals, then
output will be proportional to difference
between the two inputs

OPAMP terminals


Two DC power supplies (dual) are required
 Magnitudes of both may be same
 The other terminal of both power supplies
are connected to common ground
 All input and output voltages are measured
with reference to the common ground

OPAMP terminals

Integrated Circuit

Internal Block Diagram
Differential
Amplifier
Stage

Intermediate
Stage

Level
Shifter
Stage

Output
Stage

Four stages can be identified –
 Input stage or differential amplifier stage
can amplify difference between two input
signals; Input resistance is very high;
Draws zero current from the input sources


Internal Block Diagram


Intermediate stage (or stages) use direct
coupling; provide very high gain
 Level shifter stage shifts the dc level of
output voltage to zero (can be adjusted
manually using two additional terminals)
 Output stage is a power amplifier stage; has
very small output resistance; so output
voltage is the same, no matter what is the
value of load resistance connected to the
output terminal

Open-loop configuration

If v1 = 0, then vo = –AOLv2 Inverting amplifier

If v2 = 0, then vo = AOLv1

Non inverting amp

Open-loop configuration


AOL is the open-loop voltage gain of OPAMP
Its value is very high
Typical value is 0
...
If non inverting input voltage
is 150 μV and inverting input voltage is 140 μV,
calculate the output voltage of OPAMP
Ans: 1
...
5 mV and v2 is –0
...
For the same OPAMP,
when v1 = v2 = 1 mV, output voltage is 12 mV
...
48 dB

OPAMP equivalent circuit
Practical OPAMP

Ri
AOLvid

vi1

vi2

+
–

Ro

vo

OPAMP equivalent circuit
Ideal OPAMP

+
–

AOLvid

vi1

vi2

vo

OPAMP Characteristics


Ideal OPAMP











Infinite differential mode gain
Zero common mode gain
Infinite CMRR
Infinite input resistance
Zero output resistance
Infinite bandwidth
Infinite slew rate
Zero input offset voltage
Zero input offset current
Zero output offset voltage

OPAMP Characteristics


Differential mode gain Ad




Common mode gain Acm




It is the factor by which the difference between
the two input signals is amplified by the OPAMP

It is the factor by which the common mode input
voltage is amplified by the OPAMP

Common mode rejection ratio CMRR


Is the ratio of Ad to Acm expressed in decibels

OPAMP Characteristics


Input resistance Ri




Output resistance Ro




It is the equivalent resistance measured between
the two input terminals of OPAMP

It is equivalent resistance measured between
output terminal and ground

Bandwidth


It is the range of frequency over which the gain
of OPAMP is almost constant

OPAMP Characteristics


Output offset voltage Voo





It is the output voltage when both input voltages
are zero
Denoted as Voo

Input offset voltage Vio




It is the differential input voltage that must be
applied at the input terminals in order to make
output voltage equal to zero
Vio = |v1 – v2|
for vo = 0

OPAMP Characteristics


Input offset current Iio


It is the difference between the currents in the
input terminals when both input voltages are
zero
 Iio = | I1 – I2 |
when v1 = v2 = 0


Input bias current Iib



It is the average of the currents in the input
terminals when both input voltages are zero
Iib = (I1 + I2) / 2
when v1 = v2 = 0

OPAMP Characteristics


Slew rate SR





It is the maximum rate of change of output
voltage with respect to time
Slew rate has to be very high if OPAMP has to
operate efficiently at high frequencies

Supply voltage rejection ratio SVRR


It is the maximum rate at which input offset
voltage of OPAMP changes with change in
supply voltage

OPAMP Characteristics


Practical characteristics of 741C OPAMP










Differential mode gain is 200,000
CMRR is 90 dB
Input resistance is 2 MΩ
Output resistance is 75 Ω
Unity-gain Bandwidth is 1 MHz
Slew rate is 0
...
1sin(ωt)
...

Ans: –10sin(ωt)



For a non inverting amplifier, R1=10K, RF=100K
...

Ans: 275 mV dc



An ac signal of rms value 2 mV needs to be
amplified to 1
...

Design a suitable amplifier choosing R1=1
...
4K

Voltage Follower



Special case of non inverting amplifier where RF=0
 Voltage gain is unity
...
5v1+0
...
Choose RF=10K
 Design an OPAMP subtractor to have output
given by vo  2 v1  v2 Choose RF=R2=1K
3
 Design an OPAMP adder/subtractor to get
output voltage vo   1 v1  2 v2  v3


2

3

Integrator

Integrator


Integrator is a circuit whose output is
proportional to (negative) integral of the
input signal with respect to time
 Feedback is given through capacitor to
inverting terminal


Since same current flows through R and C,
vin
dvo
 C
R
dt

1
vo 
 vindt
RC 0
t

Integrator

Differentiator

Differentiator


Differentiator is circuit whose output is
proportional to (negative) differential of
input voltage with respect to time
 Input is given through capacitor, feedback
given through resistor to inv terminal


Since current through R and C are same,
dvin
vo
C

dt
R

dvin
vo   RC
dt

End of Module 4



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