Applications of Op-Amp

1. Applications
The
Operational
Amplifier
Dr. Vaishali Deshmukh
Department of Physics
Shri Shivaji Science College, Amravati (MS)

2. Op-Amp Applications
Linear
A circuit is said to be linear, if
there exists a linear relationship
between its input and the output.
A circuit is said to be linear, if
there exists a linear relationship
between its input and the output.
Voltage follower, Dfferential
amplifier, Instrumentation amp,
Inv amp, Non-Inv amp etc.
Voltage follower, Dfferential
amplifier, Instrumentation amp,
Inv amp, Non-Inv amp etc.
Nonlinear
A circuit is said to be non-
linear, if there exists a non-
linear relationship between its
input and output..
A circuit is said to be non-
linear, if there exists a non-
linear relationship between its
input and output..
Precision rectifiers, Comparators,
Clampers, Limiters, Schmitt trigger
etc.
Precision rectifiers, Comparators,
Clampers, Limiters, Schmitt trigger
etc.
While deriving the expressions and analysing such circuits, the realistic assumptions can be
conveniently used. The op-amp input current is zero while the potential difference between inverting and
non inverting terminals is zero. Thus if one terminal is grounded, then potential of other terminal can be
assumed zero i.e. it is also at ground potential. This is the concept of virtual ground, which plays an
important role in analysing the Op Amp Applications circuits.

3. Inverting Amplifier
Ib = 0
R1
Rf
Vo
I
I
Vin
+VCC
‒VEE
•Let us derive the expression for its
closed loop gain which is Vo / Vi.
•As node B is grounded, node A is also
at ground potential, from the concept of
virtual ground, so VA = 0.
• I = (Vin ‒ VA) / R1
• I = Vin / R1
• Now from the output side, considering
the direction of current I we can write,
• I = (VA ‒ Vo) / Rf
• I = ‒Vo / Rf
• Entire current I passes through R f as
op-amp input current is zero.
Equating (1) and (2) we get,
AVF = Vo / Vin = ‒ Rf / R1
•The Rf /R1 is the gain of the amplifier
while negative sign indicates that the
polarity of output is opposite to that of
input.
A
B
An amplifier which provides a phase
shift of 180⁰ between input and output
is called inverting amplifier.

4. Non-Inverting Amplifier
Vo
Rf
I
Vin
R1
A
B
+VCC
‒VEE
I
An amplifier which amplifies the
input without producing any phase
shift between input and output is
called Noninverting Amplifier.
•Let us derive the expression for its closed
loop voltage gain.
•The node B is at potential Vin, hence the
potential of point A is same as B which is
Vin, from the concept of virtual share.
•VA = VB = Vin
•From the output side we can write,
• I = = ………. (1)
•At the inverting terminal,
• I = = ………. (2)
•Entire current passes through R1 as input
current of op-amp is zero.
Equating equations (1) and (2),
AVF = = 1 +
Vo ‒ VA
Rf
Vo ‒ Vin
Rf
VimVA ‒ 0
R1 R1
Vo
Vin
Rf
R1

5. Adder/ Summing Amplifier
Rf
R2
R1
I1
I2
Vo
V1
V2
A
B
I
As the input impedance of an op-amp is
extremely large, more than one input
signal can be applied to the inverting
amplifier. Such circuit gives the addition
of the applied signals at the output. Hence
it is called Summer or adder circuit.
•In this circuit, all the input signals to
be added are applied to the inverting
input terminal, of the op-amp.
•As point B is grounded, due to virtual
ground concept the node A is also at
virtual ground potential. VA = VB = 0
•Now from input side,
•I = = …… (1)
•I = = …… (2)
•Applying at node A and as input op-amp
current is zero, I = I1 + I2 …… (3)
•From output side,
•I = = …… (4)
•Substituting (4), (1) and (2) in (3),
Vo = ‒ ( V1+V2)
•In this circuit, all the input signals to
be added are applied to the inverting
input terminal, of the op-amp.
•As point B is grounded, due to virtual
ground concept the node A is also at
virtual ground potential. VA = VB = 0
•Now from input side,
•I = = …… (1)
•I = = …… (2)
•Applying at node A and as input op-amp
current is zero, I = I1 + I2 …… (3)
•From output side,
•I = = …… (4)
•Substituting (4), (1) and (2) in (3),
Vo = ‒ ( V1+V2)
V1 ‒ VA
R1
V1
R1
V2 ‒ VA
R2
V2
R2
VA ‒ Vo
Rf
‒Vo
Rf

6. Subtractor
Vo
Rf
R2
R1
I1
I2 I2
A
B
V1
V2
Similar to the summing circuit, the
subtraction of 2 input voltages is possible
with the help of op-amp circuit called
subtractor or difference amplifier circuit.
•To find the relation between the inputs and
output let us use Superposition principle.
•Let Vo1 be the output, with input V1 acting,
assuming V2 to be zero. And Vo2 be the
output, with input V2 acting, assuming V1 to
be zero.
•With V2 zero, the circuit acts as an
inverting amplifier. Hence we can write,
• Vo1 = – V1 …… (1)
Rf
R1
Rf
I1

7. Vo2
Rf
I
A
R1
I
V1 = 0
B
R2
V2
Rf
I
•While with V1 as zero, the circuit reduces to as
shown in the Fig.
•Let potential of node B be VB. The potential of
node A is same as B i.e. VA = VB. Applying
voltage divider rule to the input V2 loop,
• VB = V2 …… (2)
• I = = = 0 …… (3)
• I = = = 0 …… (4)
•Equating the equations (3) and (4),
•Vo2 = 1 + VB …… (5)
•Substituting VB from (2) in (5) we get,
•Vo2 = 1 + V2 .…… (6)
•Hence using Superposition principle,
• Vo = Vo2 + Vo2 …… (7)
Rf
R2 + Rf
VA
R1
VB
R1
Vo2 – VA
Rf
Vo2 – VB
Rf
Rf
R1
Rf
R1
Rf
R2 + Rf
•Now if the resistances are selected as
R1 = R2,
Vo = + (V2 – V1) ……...(8)
Thus the output voltage is
proportional to the difference between
the two input voltages.
Vo = (V2 – V1)
Rf
R1

8. Further Presentation
 Nonlinear applications of Op-Amp