UNIT II

Applications of OP- AMP

 Linear Applications of Op-Amp:
 Inverting, non-inverting, Differential amplifiers,
adder, subtractor, Instrumentation amplifier,
AC amplifier, V to I and I to V converters,
Integrator and differentiator.
 Non-Linear Applications of Op-Amp:
 Sample and Hold circuit, Log and Antilog
amplifier, multiplier and divider, Comparators,
Schmitt trigger, Multi vibrators, Triangular and
Square waveform generators, Oscillators.
CONFIGURATIONS OF AN OP-AMP

 - In general the op-amp can be configured into two

ways they are:
 1. Open loop configuration
 2. Closed loop configuration

 => Open loop Indicates there is no connection,
either direct or via another network exist between
output and input terminals i.e the output signal is not
fed back for any form as part of input signal.

 => Closed loop indicates there is a connection,
either direct or via another network existing between
output and input terminals.


(i) Open loop op-amp configurations:
- When an op-amp is connected in open loop
configuration. It simply functions as high gain
amplifier. These are three open loop op-amp
configurations.
(1) Differential Amplifier
(2) Inverting Amplifier
(3) non-Inverting Amplifier

(ii)closed loop configuration:

closed loop: In this configuration the feedback signal
is used to connect o/p to i/p
1)Inverting
2)Non-Inverting
By proper selection of external components, we can
implement so many linear and non-linear
applications.
Gain (output / input) is under designer control.
(1) Differential Amplifier:
 The below figure shows the open loop
differential amplifier
In which V1 and V2 are inputs of the op-amp applied on the IN
and NI terminals of an op-amp. Since the amplifier amplifies
the difference b/w the input signal hence it is called as
Differential amplifier.

From above Fig the output voltage Vo = A(V2-V1) i.e The

output is open loop gain times of difference b/w input voltages.
As we know the op-amp is a versatile device hence it will
amplify both AC and Dc input signals.
2) Inverting Amplifier :

 In the Inverting Amplifier only one input is

applied and that is to the inverting input
terminal and non inverting terminal is
grounded.
In the circuit the output voltage Vo = -AVi, the negative sign
indicates the output and input voltages are in 180° phase shift.
(or) the output voltage is out of phase w.r.t input voltage 180°
(or) opposite polarity.

3) The non-inverting Amplifier :

The non-inverting amplifier only one input is applied that is to
the non-inverting terminal of an OP-AMP and inverting terminal
should be ground. Below figure shows the open loop non-
inverting Amplifier
In these the output is in-phase with the input voltage which
indicates the both input and output are having same polarity

Importance of op-amp :

The Proper Selection of external components) the

op-amp configured as .amplifier, oscillator, compara-
tor, [Adder , Subtractor , Differential s , multiplier]
[Integrator , log and , Antilog amp] , mathematical
operators.

Fundamentals of -Amplifier :
loading effect :
Apply voltage divide rule at input
port:

Vi = [Ri /(Ri + Rs)] · Vs……………..(1)

Apply voltage divide rule at output port:

Vo = [RL /RL + Ro ]· Aoc· Vi...........(2)

Substitute 1 in 2
1) considers voltage amplifier parameters Vs=10v , Rs=25kΩ
Ri=10kΩ ; Aoc=10 V/V , Ro=5Ω , RL=10Ω Find the following
i) Input attenuation ii) output attenuation iii) Vo iv) Vo / Vs v)
Vo ideal

2) Repeat above problem for Rs=10k Ω Ro= 3Ω

3) Repeat above problem for Rs=2k Ω Ro= 1Ω

Craw backs of open-loop
configuration:
 In this configuration output always either
+Vcc or –Vcc irrespective of gain.
 Output voltage is not under designer

control.
 Used for limited applications like

comparators.
Closed loop configuration:
 In this configuration the feedback signal is
used to connect output to input.
 1. Inverting
 2. Non-inverting
 1. Inverting configuration: in this the output

Vo is fed back to the

Inverting configuration circuit:
Circuit analysis:

Negative sign indicates there is 180º phase shift

between Vi and Vo
Non-inverting
configuration:
Circuit Analysis:
Applications of OP-AMP
Since no current
flows
Into op-amp,the
current
I flowing (upwards)
in R is I=(V1-V2)/R
AC Voltage follower:
Current to Voltage Converter:
Sample and Hold circuit:
Log amplifier:
Anti log amplifier:
1. Frequency doubling
2. Divider
The operation of this circuit is given in
following table.

Input voltage Difference Output voltage

Voltage Vd VO
Vin > Vref Vd positive VO = + Vsat
Vin < Vref Vd negative VO = - Vsat

sat

sat

A. A. Lande, E & TC Dept

Inverting Comparator
 In inverting comparator, ac input voltage is
applied to inverting terminal while dc
reference voltage Vref is applied to non
inverting terminal.
 Here, differential input voltage V is given
d
by,
Vd = V1 – V2 = Vref - Vin
in
 The operation of this circuit is given in
following table.
Input voltage Difference Output voltage
Voltage Vd VO
Vin < Vref Vd positive VO = + Vsat
Vin > Vref Vd negative VO = - Vsat
A. A. Lande, E & TC Dept
MULTIVIBRATORS
OSCILLATOR
S

A. A. Lande, E & TC Dept

Introduction
 Feedback is defined as the process in which
a part of output signal is returned back to
the input.
 Types of feedback:

1. Positive feedback 2. Negative feedback

 If the original input signal and feedback
signal are in phase, the feedback is called
as “positive feedback”.
 Positive feedback is used in oscillators.

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 Oscillators
 Oscillators are basically ac signal
generators of desired shape (sine, square,
triangular etc.) at desired frequency.
 The output voltage and frequency of an

oscillator can be variable.

+
V
Output VO
voltage
control
Output
Oscillator
frequency
control
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Block Diagram of an
Oscillator
1800 phase
shift

Vi
No external
ac input
total phase shift
= 00

Vf

1800 phase
shift

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 An oscillator basically consists of an amplifier
and phase shifting network. The amplifier
receives the output of the phase shifting
network.
 The amplifier then amplifies it, phase shifts it
through 1800 and applies it to the input of
the phase shifting network.
 The phase shifting network shifts the
amplifier output through another 1800 and
attenuates it before applying it back to the
amplifier input.

A. A. Lande, E & TC Dept

Expression for the gain with
positive feedback
 Let “A” be the amplifier gain without
feedback and “β” be the feedback factor.
Then the gain of the amplifier with positive
feedback is given by,
Af = A/ (1 - Aβ)

A. A. Lande, E & TC Dept

Barkhausen Criteria
 An amplifier will work as an oscillator if and
only if it satisfies as set of conditions called
the “Barkhausen Criterion.”
 Statements of Barkhausen criteria:
The Barkhausen criterion states that:
1. An oscillator will operate frequency for which
the total phase shift introduced, as the signal
proceeds from the input terminals, through
the amplifier and feedback network and back
again to the input is precisely 00 or 3600 or
integral multiple of 3600.

A. A. Lande, E & TC Dept

2. At the oscillator frequency, the magnitude
of the product of open loop gain of the
amplifier A and the feedback factor β is
equal to or greater then unity.
∴ |Aβ| > 1
The product Aβ is called as the “loop gain”.

A. A. Lande, E & TC Dept

Types of Oscillators
 On the basis of the type of components
used, for the feedback network the
oscillators are classified into following three
categories.
1. RC oscillators
2. LC oscillators
3. Crystal oscillator

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 Comparison of RC, LC and
Crystal oscillator
Sr. RC oscillator LC oscillator Crystal oscillator
No.
1. Frequency of Frequency of Frequency of
oscillations is oscillations is oscillations
dependent of values dependent on values depends on the
of R and C. of L and C. dimensions of
crystal.
2. These are used at These are preferred These are
low and medium at high frequencies. preferred at high
frequencies. frequencies.
3. Example of RC Examples of LC Examples of
oscillators: Phase oscillators: Hartley, crystal oscillator:
shift and wein Colpitt’s and clapp Miller crystal
bridge oscillators oscillators. oscillator and
pierce crystal
oscillator.
4. Poor frequency Poor frequency Very high
stability. stability except for frequency stability.
the clapp oscillator
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Applications of oscillators
1. As a local oscillator in radio receivers
2. In TV receiver
3. In signal generators
4. As clock generation for logic circuits
5. AM and FM transmitters
6. In phase lock loops.