21EES101T-ELECTRICAL AND

ELECTRONICSENGINEERING
EEE-UNIT 2

Unit-2-Electronics
Overview of Semiconductors, Diodes and Transistors, Introduction to JFET
and MOSFET. Construction and working of power devices-SCR, BJT,
MOSFET, IGBT -Switching Characteristics of SCR- Types of power
converters- Natural and force commutation, Linear voltage Regulator,
SMPS
Realize the logic expression using basic logic gates, Combinational logic
design-Sum of Product form (SOP) and Product of Sum (POS) form,
Minterm and Maxterm, Karnaugh Map (K-Map) representation of logical
functions, Two variables K-Map, Three variables K-Map, Four variables K-
Map. Introduction to FPGA.
Practice on realization of logical expression, combinational circuits, PCB
design, soldering and testing
1
OVERVIEW OF SEMICONDUCTORS
• Depending on their conductivity, materials can be
classified into three types as conductors,
semiconductors and insulators. Conductor is a
good conductor of electricity. Insulator is a poor
conductor of electricity. Semiconductor has its
conductivity lying between these two extremes.
Energy Band of Semiconductor
In terms of energy band shown in Fig., the valence
band is almost filled (partially filled) and
conduction band is almost empty.

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A comparatively smaller electric field (smaller than required for
insulator) is required to push the electrons from the valence band
to conduction band. At low temperatures, the valence band is
completely filled and the conduction band is completely empty.
Therefore a semiconductor virtually behaves as an insulator at
low temperature. However even at room temperature some
electrons crossover to the conduction band giving conductivity to
the semiconductor. As temperature increases, the number of
electrons crossing over to the conduction band increases and
hence electrical conductivity increases. Hence a semiconductor
has negative temperature coefficient of resistance.
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Classifications of Semiconductors
Intrinsic Semiconductor: A pure
semiconductor is called intrinsic
semiconductor.
Extrinsic Semiconductor: Due to the poor
conduction at room temperature, the intrinsic
semiconductor, as such, is not useful in the
electronic devices. Hence the current
conduction capability of the intrinsic
semiconductor should be increased. This can
be achieved by adding a small amount of
impurity to the intrinsic semiconductor, so that
it becomes impurity semiconductor or extrinsic
semiconductor. This process of adding impurity
is known as doping.
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N-type Semiconductor: A small amount of pentavalent
impurities such as arsenic, antimony or phosphorus is
added to the pure semiconductor (germanium or silicon
crystal) to get N-type semiconductor. Thus, the addition
of pentavalent impurity (antimony) increases the number
of electrons in the conduction band thereby increasing
the conductivity of N-type semiconductor. As a result of
doping, the number of free electrons far exceeds the
number of holes in an N-type semiconductor. So electrons
are called majority carriers and holes are called minority
carriers
P-type Semiconductor: A small amount of trivalent
impurities such as aluminium or boron is added to the
pure semiconductor to get the P-type semiconductor. The
number of holes is very much greater than the number of
free electrons in a P-type material, holes are termed as
majority carriers and electrons as minority carriers.
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 THEORY OF PN JUNCTION DIODE

In a piece of semiconductor material, if one half is doped by P-

type impurity and the other half is doped by N-type impurity,
a PN junction is formed. The plane dividing the two halves or
zones is called PN junction. As shown in Fig., the N-type
material has high concentration of free electrons while P-type
material has high concentration of holes. Therefore at the
junction there is a tendency for the free electrons to diffuse
over to the P-side and holes to the N-side. This process is
called diffusion.

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As the free electrons move across the junction from N-type to P-
type, the donor ions become positively charged. Hence a positive
charge is built. on the N-side of the junction. The free electrons
that cross the junction uncover the negative acceptor ions by filling
in the holes. Therefore a net negative charge is established on the
P-side of the junction. This net negative charge on the P-side
prevents further diffusion of electrons into the P-side. Similarly,
the net positive charge on the N-side repels the holes crossing from
P-side to N-side. Thus a barrier is set up near the junction which
prevents further movement of charge carriers, i.e. electrons and
holes. This is called potential barrier or junction barrier V0. V0 is
0.3 V for germanium and 0.72 V for silicon. The electrostatic field
across the junction caused by the positively charged N-type region
tends to drive the holes away from the junction and negatively
charged P-type region tends to drive the electrons away from the
junction. Thus the junction region is depleted to mobile charge
carriers. Hence it is called depletion layer.

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Under Forward Bias Condition
When positive terminal of the battery is connected to
the P-type and negative terminal to the N-type of the
PN junction diode, the bias applied is known as
forward bias. Under the forward bias condition, the
applied positive potential repels the holes in P-type
region so that the holes move towards the junction and
the applied negative potential repels the electrons in
the N-type region and the electrons move towards the
junction. Eventually when the applied potential is more
than the internal barrier potential, the depletion region
and internal potential barrier disappear.

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V–I Characteristics of a Diode under Forward Bias
For VF > V0, the potential barrier at the junction
completely disappears and hence, the holes cross the
junction from P-type to N-type and the electrons cross
the junction in the opposite direction, resulting in
relatively large current flow in the external circuit.

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Under Reverse Bias Condition
When the negative terminal of the battery is connected to
the P-type and positive terminal of the battery is connected
to the N-type of the PN junction, the bias applied is known
as reverse bias. Under applied reverse bias, holes which
form the majority carriers of the P-side move towards the
negative terminal of the battery and electrons which form
the majority carrier of the N-side are attracted towards the
positive terminal of the battery. Hence the width of the
depletion region which is depleted of mobile charge carriers
increases. Thus the electric field produced by applied
reverse bias, is in the same direction as the electric field of
the potential barrier. Hence, the resultant potential barrier
is increased, which prevents the flow of majority carriers in
both directions. Therefore, theoretically no current should
flow in the external circuit. But in practice, a very small
current of the order of a few microamperes flows under
reverse bias.
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V–I Characteristics of a Diode under Reverse Bias
For large applied reverse bias, the free electrons from the N-type
moving towards the positive terminal of the battery acquire
sufficient energy to move with high velocity to dislodge valence
electrons from semiconductor atoms in the crystal. These newly
liberated electrons, in turn, acquire sufficient energy to dislodge
other parent electrons. Thus, a large number of free electrons are
formed which is commonly called as an avalanche of free
electrons. This leads to the breakdown of the junction leading to
very large reverse current. The reverse voltage at which the
junction breakdown occurs is known as breakdown voltage.

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APPLICATIONS OF PN JUNCTION DIODE
RECTIFIERS, CLIPPERS, CLAMPERS ect..
RECTIFIERS-Rectifier is defined as an electronic device used for
converting ac voltage into dc voltage
Half-wave Rectifier
It converts an ac voltage into a pulsating dc voltage using only one half
of the applied ac voltage. The rectifying diode conducts during one half
of the ac cycle only. Figure shows the basic circuit and waveforms of a
half wave rectifier.

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 BIPOLAR JUNCTION TRANSISTOR [BJT]
A Bipolar Junction Transistor (BJT) is a three terminal
semiconductor device in which the operation depends on the
interaction of both majority and minority carriers and hence the
name Bipolar. It is used in amplifier and oscillator circuits, and as
a switch in digital circuits. It has wide applications in computers,
satellites and other modern communication systems.

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TRANSISTOR BIASING
Usually the emitter-base junction is forward biased and collector-base
junction is reverse biased. Due to the forward bias on the emitter-
base junction an emitter current flows through the base into the
collector. Though, the collector-base junction is reverse biased, almost
the entire emitter current flows through the collector circuit.

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 Difference Between NPN and
PNP Transistor
Basis For
NPN PNP
Comparison
Direction of The current flows from collector The current flows from
Current terminal to emitter terminal. emitter to collector
terminal.
Construction One P-type semiconductor is It is made of up two P-type
sandwiched between the two N- material layers with N-type
type semiconductors. sandwiched between them.
Turn-on When electrons enters into the When holes enter into the
base. base.
Turn-off When the current is reduced in When a current is present
the base, the transistor doesn’t at the base of a PNP
function across the collector transistor, then the
terminal and switches OFF transistor switches OFF.

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 Difference Between NPN and
PNP Transistor
Basis For Comparison NPN PNP
Inside Current Develop because of Originate because of
varying position of varying position of holes.
electrons.
Outside Current Current develop because Current develop because
of the flow of holes. of the flow of electrons.
Majority Charge Carrier Electron Hole
Switching Time Faster Slower
Positive Voltage Collector Terminal Emitter Terminal
Forward Biased Emitter Base Junction Emitter Base Junction
Reverse Biased Collector Base Junction Collector Base Junction
Small current Flows from emitter-to- Base to emitter
base
Ground Signal Low High

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MOSFET

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 IGBT
Insulated Gate Bipolar Transistor

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SCR – Transistor model

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VI characteristics of SCR

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The three states of a thyristor
• Forward blocking
• Reverse blocking
• Forward conducting

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How a thyristor latches on

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 How a thyristor latches on

1.With no current flowing

into the gate, the
thyristor is switched off
and no current flows
between the anode and
the cathode.

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How a thyristor latches on

2. When a current flows

into the gate, it
effectively flows into
the base (input) of the
lower (n-p-n)
transistor, turning it
on.

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 How a thyristor latches on

3. Once the lower

transistor is switched
on, current can flow
through it, activating
the base (input) of the
upper (p-n-p)
transistor, turning that
on as well.

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How a thyristor latches on

4. Once both transistors

are turned on completely
("saturated"), current
can flow all the way
through both of them—
through the entire
thyristor from the anode
to the cathode.

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How a thyristor latches on

5.Since the two

transistors keep one
another switched on,
the thyristor stays on—
"latches"—even if the
gate current is removed.

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Switching Characteristics of
thyristor

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60
 Commutation

• Commutation is the process of turning

off a conducting thyristor. There are
two methods for commutation namely,
natural commutation and forced
commutation.

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 Natural Commutation
• In natural commutation, the source
of commutation voltage is the supply
source itself. If the SCR is connected
to an AC supply, at every end of the
positive half cycle, the anode current
naturally becomes zero (due to the
alternating nature of the AC Supply).
As the current in the circuit goes
through the natural zero, a reverse
voltage is applied immediately across
the SCR (due to the negative half
cycle). These conditions turn OFF the
SCR.
• This method of commutation is also
called as Source Commutation or AC
Line Commutation or Class F
Commutation. This commutation is
possible with line commutated
inverters, controlled rectifiers, cyclo
converters and AC voltage regulators
because the supply is the AC source
in all these converters. 62
 Forced Commutation
• In case of DC circuits, there is no
natural current zero to turn OFF the
SCR. In such circuits, forward current
must be forced to zero with an
external circuit (known as
Commutating Circuit) to commutate
the SCR. Hence the name, Forced
Commutation.

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 Forced Commutation
• This commutating circuit consist of
components like inductors and
capacitors and they are called
Commutating Components. These
commutating components cause to
apply a reverse voltage across the SCR
that immediately bring the current in
the SCR to zero.

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 Forced Commutation
• Depending on the process for achieving zero
current in the SCR and the arrangement of the
commutating components, Forced Commutation
is classified into different types. They are:
– Class A – Self Commutation by Resonating the Load
– Class B – Self Commutation by Resonating the Load
– Class C – Complementary Commutation
– Class D – Auxiliary Commutation
– Class E – Pulse Commutation

• This commutation is mainly used in chopper

and inverter circuits.

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1-AC to DC Converters
1A-Diode Rectifiers: This rectifier circuit changes applied ac input voltage into a
fixed dc voltage. Either a single-phase or three-phase ac signal is applied at the
input. These are mainly used in electric traction and in electrochemical processes
like electroplating along with in battery charging and power supply. These are also
used in welding and UPS related services.
1B-Phase Controlled Rectifiers: Unlike diode rectifiers, phase-controlled
rectifiers are designed to convert a fixed value of ac signal voltage into a variable
dc voltage. Here line voltage operates the rectifier hence these are sometimes
known as line commutated ac to dc converters. Similar to diode rectifiers, here
also the applied ac signal can be a single-phase or three-phase ac signal. Its major
applications are in dc drives, HVDC systems, compensators, metallurgical and
chemical industries as well as in excitation systems for synchronous machines.
2-DC to DC Converters
The converters that convert the dc signal of fixed frequency present at the input
into a variable dc signal at the output are also known as choppers. Here the
achieved output dc voltage may have a different amplitude than the source
voltage. Generally, power transistors, MOSFETs, and thyristors are the
semiconductor devices used for their fabrication. The output is controlled by a low
power signal that controls these semiconductor devices from a control unit.

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Here forced commutation is required to turn off the semiconductor device. Generally, in low
power circuits power transistors are used while in high power circuits thyristors are used.
Choppers are classified on the basis of the type of commutation applied to them and on the
basis of the direction of power flow. Some major uses of choppers are in dc drives, SMPS,
subway cars, electric traction, trolley trucks, vehicles powered by battery, etc.
3-DC to AC Converters
The devices that are designed to convert the dc signal into ac signal are known as inverters.
The applied input is a fixed dc voltage that can be obtained from batteries but the output
obtained is variable ac voltage. The voltage and frequency of the signal obtained are of
variable nature. Here the semiconductor device i.e., the thyristor is turned off by using either
line, load, or forced commutation.
Thus, it can be said that by the use of inverters, a fixed dc voltage is changed into an ac
voltage of variable frequency. Generally, the semiconductor devices used for its fabrication
are power transistors, MOSFETs, IGBT, GTO, thyristors, ect
Inverters mainly find applications in induction motor and synchronous motor drives along with
UPS, aircraft, and space power supplies. In high voltage dc transmission system, induction
heating supplies as well as low power systems of mobile nature like flashlight discharge
system in photography camera to very high power industrial system.
Like choppers, in inverters also conventional thyristors are used in high power applications
and power transistors are used in low power applications

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4-AC to AC Converters
An ac to ac converter is designed to change the ac signal of fixed frequency into a variable
ac output voltage.

There are two classifications of ac to ac converters which are as follows:

4A-Cycloconverters: A cycloconverter is a device used for changing ac supply of fixed
voltage and single frequency into an ac output voltage of variable voltage as well as different
frequency. However, here the obtained variable ac signal frequency is lower than the
frequency of the applied ac input signal. It adopts single-stage conversion. Generally, line
commutation is mostly used in cycloconverters however forced or load commutated
cycloconverters are also used in various applications.
These mainly find applications in slow-speed large AC traction drives such as a rotary kiln,
multi MW ac motor drives, etc.
4B-AC Voltage Controllers (AC voltage regulators): The converters designed to change
the applied ac signal of fixed voltage into a variable ac voltage signal of the same frequency
as that of input. For the operation of these controllers, two thyristors in an antiparallel
arrangement are used. Line commutation is used for turning off both the devices. It offers the
controlling of the output voltage by changing the firing angle delay.
The major applications of ac voltage controllers are in lighting control, electronic tap
changers, speed control of large fans and pumps as well.
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 Linear Voltage Regulator
• Electronic systems usually receive a
power-supply voltage that is higher than
the voltage required by the system’s
circuitry. For example, a 9 V battery
might be used to power an amplifier that
needs an input range of 0 to 5 V, In such
case, we need to regulate the input
power using a component that accepts a
higher voltage and produces a lower
voltage. One very common way to
achieve this type of regulation is to
incorporate a linear voltage regulator.
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 Linear Voltage Regulator
• The simplest regulators are called 3-pin regulators, which output a
stable fixed voltage just by inserting an input capacitor (CIN)
between the VIN and the GND pins, and an output capacitor (COUT)
between the VOUT and the GND pins.

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 Linear Voltage Regulator
• The figure illustrates that the controlling circuit
supervises the output voltage and regulates the
resistance value of the variable resistor so that the IC
can output the set fixed voltage. For instance, if the
input voltage (VIN) is fixed, a linear regulator can
maintain a stable output voltage by keeping the ratio
between the variable resistance value and the load
resistance value fixed according to the changing rate of
the load resistance value. The input voltage is divided
by the two resistors, so linear regulators generate a
lower output voltage than their input voltage. The
difference between the higher input voltage and lower
output voltage will generate heat which is called waste
heat. The current flowing inside the load resistor goes
on to flow to the variable resistor, where the electricity
is consumed with some heat generated.
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 SMPS-Switched Mode Power Supply
SMPS is an electronic power supply system that makes use of a switching
regulator to transfer electrical power effectively. It is a PSU (power supply
unit) and is usually used in computers to change the voltage to the
appropriate range for the computer. The figure below is the buck Switch
Mode Power Supply.

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FUNCTIONAL BLOCK
DIAGRAM

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 SMPS
• The SMPS is mostly used where switching of
voltages is not at all a problem and where
efficiency of the system really matters. There
are few points which are to be noted regarding
SMPS. They are
• SMPS circuit is operated by switching and hence the
voltages vary continuously.
• The switching device is operated in saturation or cut
off mode.
• The output voltage is controlled by the switching
time of the feedback circuitry.
• Switching time is adjusted by adjusting the duty
cycle.
• The efficiency of SMPS is high because, instead of
dissipating excess power as heat, it continuously
switches its input to control the output.
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 Advantages of SMPS
• The efficiency is as high as 80 to 90%
• Less heat generation; less power wastage.
• Reduced harmonic feedback into the supply
mains.
• The device is compact and small in size.
• The manufacturing cost is reduced.
• Provision for providing the required number
of voltages.

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 Disadvantages of SMPS
• The noise is present due to high frequency
switching.
• The circuit is complex.
• It produces electromagnetic interference.

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Digital Electronics

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Realization [Implementation] of logic expression [Boolean Function ] using basic logic
gates

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SUM OF PRODUCTS [SOP] AND PRODUCT OF SUMS
[POS]
Logical functions (Boolean expression) are generally
expressed in terms of logical variables (inputs) in
following forms. (Each input variable can have the
value, either 0 or 1 only)
•SUM OF PRODUCTS [SOP] Ex: AB’+ BC+C’D
•PRODUCT OF SUMS [POS] Ex: (A’+B’) (B’+C) (C’+D)
MINTERMS
A product term containing all the inputs of the
functions in either complemented or uncomplemented
form is called MINTERMS.
Let us consider 3 variable (input) function. It has
23 all possible combinations. [A ‘n’ variable (input)
function has 2n all possible combinations]. Let the
inputs are A, B, C and output is Y.
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TRUTH TABLE-Example

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•In minterms, 0 are assigned with bar letter and
1 are assigned with unbar letter.
•Within the row, all are multiplied (Product)
•Choose only the output 1.
•Add the minterms which having 1 output.
•In this example, we get Y= A’BC’ + A’BC+ AB’C’+
ABC’. This expression is called canonical SOP
form. [Standard SOP form]
•Each input is assigned with it equivalent
decimal value. In the truth table, only the output
Y= 1 is chosen, it corresponding input’s decimal
values are stated as below.
Y= ∑m (2,3,4,6)

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MAXTERMS
A sum term containing all the inputs of the functions in
either complemented or uncomplemented form is called
MAXTERMS. Let us consider the same truth table.

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•In maxterms, 1 are assigned with bar letter and
0 are assigned with unbar letter.
•Within the row, all are summed (Added)
•Choose only the output 0.
•Product the maxterms which having 0 output.
•In this example, we get Y= (A+B+C) (A+B+C’)
(A’+B+C’) (A’+B’+C’). This expression is called
canonical POS form. [Standard POS form]
•Each input is assigned with it equivalent decimal
value. In the truth table, only the output Y= 0 is
chosen, it corresponding input’s decimal values
are stated as below.
Y= ∏M (0,1,5,7)
Note: Minterms and Maxterms are complement
with each other.
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 1. For the Boolean function given below, obtain the (i)
canonical SOP form (ii) canonical POS form.
Y(A,B,C)= A+B’C
= AXX+ XB’C
= AB’C’+ AB’C+ ABC’+ ABC+A’B’C+ AB’C
[Remove the common term; Since A+A=A ]
Y= AB’C’+ AB’C+ ABC’+ ABC+A’B’C [Canonical
SOP form]
100 101 110 111 001
(m4 m5 m6 m7 m1)

Y= ∑m (1,4,5,6,7)

Y= ∏M (0,2,3) [ Minterms and Maxterms are

complement with each other]
M0 M2 M3
000 010 011
Y= (A+B+C) (A+B’+C) (A+B’+C’) [Canonical POS form]
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Karnaugh maps/ K-map
If the number of input variables is more than 2,
its very difficult to minimize the Boolean function by
Boolean algebra. Karnaugh maps/ K map overcomes
this difficulty.
Karnaugh maps/ K map
•A visual way to simplify logic expressions
•It gives the most simplified form of the expression
•K-Maps are a graphical technique used to simplify a
logic equation.
•K-Maps can be used for any number of input
variables, BUT are only practical for two, three, and
four variables

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90
Procedure to minimize Boolean expression by K-map:
1. We have to cheek, number of variables (Inputs).
(i)If the maximum number in the Boolean expression is ≤3, it
is 2 variable function.
(ii)If the maximum number in the Boolean expression is ≤7,
it is 3 variable function.
(iii)If the maximum number in the Boolean expression is ≤15,
it is 4 variable function.
Note: Some times, in the question itself, inputs will be given.
Ex: Y(A,B,C)=∑(0,4,5,7)
2. Check the given question is Minterms or Maxterms. If ∑ is
given, it is Minterms. In K-map, for the given decimal
location, we have to enter 1. In remaining location, we have
to enter 0.
If ∏ is given, it is Maxterms. In K-map, for the given decimal
location, we have to enter 0. In remaining location, we have
to enter 1.
3. Draw the K-map and fill it. (use step 1 & 2)
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4 (a) Solution Procedure for SOP method
(i) We have to box ALL the 1’s.
(ii) Larger the box, smaller the equation. Since all are
minimization problem, we have to choose larger box.
(iii) The number of 1’s inside the box must be 2n. [ie we have
to try boxing 16 , if not possible we have to try boxing 8, if not
possible we have to try boxing 4, if not possible we have to try
boxing 2, if not possible we have to box 1]
(iv) The shape of the box must
be square or rectangular. ie
(v) For each box, we have to find unchanged input.
For that, we have see K-map from right to left, then bottom to
top. The unchanged input within the box should be product.
The product of one box should be sum with next box.[In
input, 0 are assigned with bar letter and 1 are assigned with
unbar letter]
(vi) Overlapping is allowed to make larger box.

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4 (b) Solution Procedure for POS method
(i) We have to box ALL the 0’s.
(ii) Larger the box, smaller the equation. Since all are
minimization problem, we have to chose larger box.
(iii) The number of 0’s inside the box must be 2n. [ie we have
to try boxing 16 , if not possible we have to try boxing 8, if not
possible we have to try boxing 4, if not possible we have to try
boxing 2, if not possible we have to box 1]
(iv) The shape of the box must
be square or rectangular. ie
(v) For each box, we have to find unchanged inputs.
For that, we have see K-map from right to left, then bottom to
top. The unchanged input within the box should be summed.
The sum of one box should be product with next box.[In
input, 1 are assigned with bar letter and 0 are assigned with
unbar letter]
(vi) Overlapping is allowed to make larger box.

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 K-MAP-SOP METHOD
1

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1

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2

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Three-Variable K-Map : Examples

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Four-Variable K-Maps Examples

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Four-Variable K-Maps Examples

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3,4-Variable K-Maps Examples

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K-MAP-POS METHOD

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3,4-Variable K-Maps Examples [POS]

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