Unit - 6 Operational Amplifier: Fig.6.1 Symbol of Op-Amp

Basic Electronics/10ELN15
UNIT -6
OPERATIONAL AMPLIFIER
Op-Amp (operational amplifier) is basically an amplifier available in the IC form. The word
operational is used because the amplifier can be used to perform a variety of mathematical
operations such as addition, subtraction, integration, differentiation etc.
Fig6.1 below shows the symbol of an Op-Amp.
+VCC
V1
Inverting input
V2
Noninverting input
-VEE
Fig.6.1 Symbol of Op-Amp
It has two inputs and one output. The input marked - is known as Inverting input and the input
marked + is known as Non-inverting input.
If a voltage Vi is applied at the inverting input ( keeping the non-inverting input at ground)
as shown below.
Vi
VO
t
t
Vi
VO
Fig.6.2 Op-amp in inverting mode
The output voltage Vo= -AVi is amplified but is out of phase with respect to the input signal by
1800.
SJBIT/ECE Dept
Page 106
If a voltage Vi is fed at the non-inverting input ( Keeping the inverting input at ground) as
shown below.
Vo
VO
t
Vi
Fig.6.3 Op-Amp in Non-inverting mode
The output voltage Vo= AVi is amplified and in-phase with the input signal.
If two different voltages V1 and V2 are applied to an ideal Op-Amp as shown below.
V1
VO
V2
Fig.6.4 Ideal Op-Amp
The output voltage will be Vo = A(V1 V2)

i.e the difference of the tow volatages is amplified. Hence an Op-Amp is also called as a High
gain differential amplifier.
SJBIT/ECE Dept
Page 107
Note: Op-Amp is 8 pin IC ( named as A 741) with pin details as shown.
OFFSET NULL
NO CONNECTION
+VCC
OUTPUT
INVERTING I/P
A 741
NON INVERTING I/P
-VEE
OFFSET NULL
Fig.6. 5 Pin details of Op-Amp
Block Diagram of an Op-AMP

An Op-Amp consists of four blocks cascaded as shown above
Fig. 6.6 Block diagram of an Op-Amp
Input stage: It consists of a dual input, balanced output differential amplifier. Its function is to
amplify the difference between the two input signals. It provides high differential gain, high input
impedance and low output impedance.
Intermediate stage: The overall gain requirement of an Op-Amp is very high. Since the input
stage alone cannot provide such a high gain. Intermediate stage is used to provide the required
additional voltage gain.
It consists of another differential amplifier with dual input, and unbalanced ( single ended) output
SJBIT/ECE Dept
Page 108
Buffer and Level shifting stage

As the Op-Amp amplifies D.C signals also, the small D.C. quiescent voltage level of previous
stages may get amplified and get applied as the input to the next stage causing distortion the final
output.
Hence the level shifting stage is used to bring down the D.C. level to ground potential, when no
signal is applied at the input terminals. Buffer is usually an emitter follower used for impedance
matching.
Output stage- It consists of a push-pull complementary amplifier which provides large A.C.
output voltage swing and high current sourcing and sinking along with low output impedance.
Concept of Virtual ground

We know that , an ideal Op-Amp has perfect balance (ie output will be zero when input voltages
are equal).
Hence when output voltage Vo = 0, we can say that both the input voltages are equal ie V1 = V2.
V1
Vo
Ri
V2
Fig. 6.7(a) Concept of Virtual ground
Since the input impedances of an ideal Op-Amp is infinite ( Ri = ). There is no current flow
between the two terminals.
Hence when one terminal ( say V2 ) is connected to ground (ie V2 = 0) as shown.
VCC
V1 =V2 =0
Ri
VO
V2=0
VEE
Fig. 6.7(b) Concept of Virtual ground
SJBIT/ECE Dept
Page 109
Then because of virtual ground V1 will also be zero.

Applications of Op-Amp
An Op-Amp can be used as
1. Inverting Amplifer
2. Non-Iverting Amplifer
3. Voltage follower
4. Adder ( Summer)
5. Integrator
6. Differentiator
Definitions
1. Slewrate(S): It is defined as The rate of change of output voltage per unit time
s
dVO
dt
volts / sec
Ideally slew rate should be as high as possible.But its typical value is s=0. V/-sec.
2. Common Mode Rejection Ratio(CMRR): It is defined as The ratio of differential
voltage gain to common-mode voltage gain.
CMRR
Ad
ACM
Ideally CMRR is infinite, but its typical value is CMRR = 90 dB

3. Open Loop Voltage Gain (AV): It is the ration of output voltage to input voltage in the
absence of feed back.
Its typical value is AV = 2x105
4. Input Impedance (Ri):It is defined as The impedance seen by the input(source) applied
to one input terminal when the other input terminal is connected to ground.
Ri M
5. Output Impedance (RO): It is defined as The impedance given by the output (load) for
a particular applied input.
Ro 7
SJBIT/ECE Dept
Page 110
Note: Typical values given above are for Op-Amp IC=A741

Characteristics of an Ideal Op-Amp
An ideal Op-Amp has the following characteristics.
1. Infinite voltage gain ( ie AV =)
2. Infinite input impedance (Ri = )
3. Zero output impedance(Ro =0)
4. Infinite Bandwidth (B.W. = C
5. Infinite Common mode rejection ratio (ie CMRR =)
6. Infinite slew rate (ie S=)
7. Zero power supply rejection ratio ( PSRR =0)ie output voltage is zero when power supply
VCC =0
8. Zero offset voltage(ie when the input voltages are zero, the output voltage will also be
zero)
9. Perfect balance (ie the output voltage is zero when the input voltages at the two input
terminals are equal)
10. The characteristics are temperature independent.
Inverting Amplifier
An inverting amplifier is one whose output is amplified and is out of phase by 180 0 with
respect to the input
Rf
i2
R1
V1
i1
G=0
VO
Fig.6.8 Inverting Amplifier
SJBIT/ECE Dept
Page 111
The point G is called virtual ground and is equal to zero.
By KCL we have
i1 i2
Vi 0
R1
Vi
R1
VO
Where
0 Vo
Rf
Vo
Rf
Rf
R1
Rf
R1
Vi
is the gain of the amplifier and negative sign indicates that the output is inverted
with respect to the input.

VO
Vi
Fig.6. 9 Waveforms of Inverting Amplifers
Non- Inverting Amplifier

A non-inverting amplifier is one whose output is amplified and is in-phase with the input.
Rf
i2
R1
V1
i1
G=Vi
VO
Vi
Fig.6.10 Non Inverting Amplifiers
By KCL we have
i1
SJBIT/ECE Dept
i2
Page 112
Vi
Vi
VO
R1
Vi
R1
Rf
VO Vi
Rf
Rf
V0 Vi
Vi
VO
Vi
VO
Vi
Rf
R1
Rf
V0
R1
Ri
Rf
Where 1
R1
Vi
Rf
R1
is the gain of the amplifier and + sign indicates that the output is in-
phase with the input.

Voltage follower
VO
Vi
VO
Vi
Fig. 6.11 Voltage follower
Voltage follower is one whose output is equal to the input.

The voltage follower configuration shown above is obtained by short circuiting R f and open
circuiting R1 connected in the usual non-inverting amplifier.
SJBIT/ECE Dept
Page 113
Thus all the output is fed back to the inverting input of the op-Amp.
Consider the equation for the output of non-inverting amplifer
Rf
V0
Vi
R1
When Rf = 0 short circuiting

R1= open circuiting
VO
VO
Vi
Vi
Therefore the output voltage will be equal and in-phase with the input voltage. Thus voltage
follower is nothing but a non-inverting amplifier with a voltage gain of unity.
Inverting Adder
Inverting adder is one whose output is the inverted sum of the constituent inputs
R1
Rf
V1
i1
If
R2
V2
i2
G=0
VO
V3
R3
i3
Fig.6.12. Inverting Adder
By KCL we have
if
i1
i2
i3
0 VO
Rf
SJBIT/ECE Dept
V1 0
R1
V2 0
R2
V3 0
R3
Page 114
VO
Rf
V1
R1
VO
V2
R2
Rf
V1
R1
V3
R3
V2
R2
V3
R3
If R1 = R2 = R3 =R then
VO
Rf
R
V1 V2 V3
If Rf = R then
VO = -[ V1 + V2 + V3 ]
Hence it can be observed that the output is equal to the inverted sum of the inputs.
Integrator
C
i2
R1
V1
i1
G=0
VO
Fig, 6.13 Integrator
An integrator is one whose output is the integration of the input.,

By KCL we have,
i1 i2
1
SJBIT/ECE Dept
Page 115
From the above figure we have
0
Vi
i1
Vi
R
and similarly we have

0
1
C
VO
1
C
VO
dVO
dt
i 2 dt
i 2 dt
1
i2
C
i.e. i 2
dVO
dt
substituing 2 and 3 in 1 we have

Vi
R
dVO
dt
dVO
dt
1
Vi
RC
VO
1
RC
Vi dt
Differentiator
A differentiator is one whose output is the differentiation of the input
R
i2
V1
i1
G=0
VO
By KCL we have
SJBIT/ECE Dept
Page 116
i1
i2
From the above figure we have

1
i1 dt
C
Vi
dVi
dt
i1
1
i1
C
C.
dVi
dt
and similarly we have
i2
0 VO
R
VO
R
substituting 2 and 3 in 1 we have
dVi
dt
VO
R
VO
RC
SJBIT/ECE Dept
dV
dt
Page 117

Unit - 6 Operational Amplifier: Fig.6.1 Symbol of Op-Amp

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What are the different configurations of an op-amp?

What are the different configurations of an op-amp?

What is the difference between an inverting and non-inverting amplifier?

What is the difference between an inverting and non-inverting amplifier?

Basic Electronics/10ELN15

Fig.6.1 Symbol of Op-Amp

Fig.6.2 Op-amp in inverting mode

Fig.6.3 Op-Amp in Non-inverting mode

Fig.6.4 Ideal Op-Amp

The output voltage will be Vo = A(V1 V2)

NON INVERTING I/P

Fig.6. 5 Pin details of Op-Amp

Block Diagram of an Op-AMP

Fig. 6.6 Block diagram of an Op-Amp

Buffer and Level shifting stage

Concept of Virtual ground

Fig. 6.7(a) Concept of Virtual ground

Then because of virtual ground V1 will also be zero.

Ideally CMRR is infinite, but its typical value is CMRR = 90 dB

Note: Typical values given above are for Op-Amp IC=A741

Fig.6.8 Inverting Amplifier

with respect to the input.

Fig.6. 9 Waveforms of Inverting Amplifers

Non- Inverting Amplifier

Fig.6.10 Non Inverting Amplifiers

phase with the input.

Fig. 6.11 Voltage follower

Voltage follower is one whose output is equal to the input.

When Rf = 0 short circuiting

Fig.6.12. Inverting Adder

Fig, 6.13 Integrator

An integrator is one whose output is the integration of the input.,

and similarly we have

substituing 2 and 3 in 1 we have

From the above figure we have

and similarly we have

substituting 2 and 3 in 1 we have

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