ANALOGUE AND DIGITAL ELECTRONICS

CERTIFICATE IN ELECTRICAL POWER
CEV 20042 DIGITAL AND ANALOGE

DIGITAL ANG ANALOGE

Course Rationale
This course is offered to expose students to the fundamental skills in construction
equipment and methods for their preparation to face future career.

Course Synopsis
This subject provides strong knowledge on construction equipment and methods
relating to electronic basic construction.

Learning Outcomes

After completing the course, students should be able to:
1. Apply the various knowledge in selecting construction equipment and methods

for electronic method.
2. Identify in details the construction equipment, methods and plants activities

according to its specific application.
3. Relate an understanding knowledge of function and safety requirement to well-

defined practices in construction.
4. Organize case study of electronic equipment and methods using best practices

at construction.

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Original Writers
Roslee Bin Abu Bakar

Panel of Editor
-

Cover Design & Developed by

Bahagian Kemahiran dan Teknikal
Cawangan Pengajian
Majlis Amanah Rakyat

First Edition, JUL 2018
This module is permitted for internal circulation (IKM/KKTM) only

Unit Pembangunan Modul Pembelajaran
Cawangan Pengajian
Bahagian Kemahiran dan Teknikal
Tingkat 20, Ibu Pejabat MARA
Jalan MARA
50609 Kuala Lumpur
Web Page: http://www.mara.gov.my

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Module Advisor
Prof. Madya Dr. Hj. Jamaludin bin Hj. Ahmad
Fakulti Pengajian Pendidikan
Universiti Putra Malaysia
43400 Serdang, Selangor

INDEX

UNIT CONTENTS PAGES
Synopsis i
Credit page ii
Index iii

1 Semiconductor Diodes 4-10
2 Diode Application 14-31
3 Zener’s And Other Two Terminal Devices 32-42
4 Bipolar Junction Transistor 43-60
5 PNP – NPN And Other Devise 61-91
6 Numbering System 92-110
7 Logic Gate
110-147

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MODULE NO 1: SEMICONDUCTOR DIODES
(GENERAL CHARACTERISTICS)

INTRODUCTION

 All materials are made up of atoms. These atoms contribute to the electrical
properties of a material, including its ability to conduct electrical current.

 For purposes of discussing electrical properties, an atom can be represented by
the valence shell and a core that consists of all the inner shells and the nucleus.

 This concept is illustrated in figure 1-3 for carbon atom. Carbon is used in many
types of electrical resistors. Notice that the carbon atom has four electrons in the
valence shell and two electrons in the inner shell (K).

LEARNING OBJECTIVES

The objectives of this module are to:
1. Define the core of atom.
2. Describe the atomic structure of copper, silicon, germanium and carbon.
3. List the four best conductor and semiconductor.
4. Discuss the difference between conductor and semiconductor.
5. Discuss the difference between silicon and germanium semiconductor.
6. Explain why silicon is much more widely used than germanium.

LEARNING OUTCOMES

After completing the module, students should be able to:
1. Define the core of atom.
2. Describe the atomic structure of copper, silicon, germanium and carbon.

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3. List the four best conductor and semiconductor.
4. Discuss the difference between conductor and semiconductor.
5. Discuss the difference between silicon and germanium semiconductor.

6. Explain why silicon is much more widely used than germanium.

1.1 GENERAL CHARACTERISTICS

All materials are made up of atoms. These atoms contribute to the electrical
properties of a material, including its ability to conduct electrical current.For purposes of
discussing electrical properties, an atom can be represented by the valence shell and a
core that consists of all the inner shells and the nucleus. This concept is illustrated in
figure 1-3 for carbon atom. Carbon is used in many types of electrical resistors. Notice
that the carbon atom has four electrons in the valence shell and two electrons in the inner
shell (K).The nucleus consists of six protons and six neutrons so the +6 indicates the
positive charge of the six protons. The core has a net charge of +4 (+6 for the nucleus and
–2 for two inner-shell electrons)

1.1.1 Conductors

A conductor is a material that easily conducts electrical current. The best
conductors are single-element materials, such as copper, silver, gold and aluminum,
which are characterized by atoms with only one valence electron very loosely bound to

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the atom. These loosely bound valence electrons can easily break away from their atoms
and become free electrons. Therefore, a conductive material has many free electrons that,
when moving in the same direction, make up the current.

1.1.2 Insulators

An insulator is a material that does not conduct electrical current under normal
conditions. Most good insulator are compounds rather than single-element materials.
Valence electrons are tightly bound to the atoms; therefore, there are few free electrons
in an insulator.

1.1.3 Semiconductors

A semiconductor is a material that is between conductors and insulators in its ability
to conduct electrical current. A semiconductor in its pure (intrinsic) state is neither a good
conductor nor a good insulator. The most common single element semiconductor are
silicon, germanium and carbon. Compound semiconductors such as gallium arsenide
are also commonly used. The single element semiconductors are characterized by atoms
with four valence electrons.

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1.1.4 Energy bands

Recall that the valence shell of an atom represents a band of energy levels and the
valence electrons are confined to that band. When an electron acquires enough additional
energy from an external source, it can leave the valence shell and become a free electron
and exist in what is known as the conduction band. The difference in energy between the
valence band and the conduction band is called an energy gap. This is the amount of
energy that a valence electron must have in order to jump from the valence band to the
conduction band. Once in the conduction band, the electron is free to move throughout
the material and is not tied to any given atom.

Figure 1-4 shows energy diagrams for insulators, semiconductors and conductor.
Notice in part (a) that insulator have a very wide energy gap. Valence electrons do not
jump into the conduction band except under breakdown conditions where extremely high
voltages are applied across the material. As you can see in part (b), semiconductors have
a much narrower energy gap. This gap permits some valence electrons to jump into the
conduction band and become free electrons. By contrast, as part (c) illustrates, the energy
bands in conductors overlap. In a conductive material there is always a large number of
free electrons.

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1.1.5 Comparison of a Semiconductor Atom to a Conductor Atom

Now let’s examine some basic reasons why silicon is a semiconductor and copper is
a conductor. Diagrams of the silicon atom and the copper atom are shown in figure 1-5.

Notice that the core of the silicon atom has a net charge of +4 (14 protons – 10
electrons) and the core of the copper atom has a net charge of +1 (29 protons – 28
electrons). The valence electron in the copper in the copper atom “feels” an attractive force
of +1 compared to a valence electron in the silicon atom which “feels” an attractive force
of +4. So, there is four times more force trying to hold a valence electron to the atom in
silicon than in copper.

The copper’s valence electron is in the fourth shell, which is a greater distance from
its nucleus than the silicon’s valence electron in the third shell. Recall that electrons
farthest from the nucleus have the most energy. Therefore, in copper, the valence electron
has less force holding it to the atom than does the valence electron in silicon. Also, the
valence electron in copper has more energy than the valence electron in silicon. This
means that it is easier for valence electrons in copper to acquire enough additional energy
to escape from their atoms and become free electrons in the condition band than it is in
silicon.

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1.1.6 Silicon and Germanium

The atomic structures of silicon and germanium are shown in figure 1-6.

Silicon is the most widely used material in diodes, transistor, integrated circuits and
other semiconductor devices. Notice that both silicon and germanium have the
characteristics four valence electrons. The valence electrons in germanium are in the
fourth shell while those in silicon are in third shell, closer to nucleus. This means that the
germanium valence electrons are at higher energy levels than those in silicon and
therefore require a smaller amount of additional energy to escape from the atom. The
property makes germanium more unstable at high temperatures, and this is a basic reason
why silicon is the most widely used semi conductive material.

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1.2 N-TYPE AND P-TYPE SEMICONDUCTORS
1.2.1 Doping

The conductivity of silicon and germanium can be drastically by the controlled
addition of impurities to the intrinsic (pure) semiconductive material. This process , called
doping, increases the number of current carriers (electrons or holes). The two categories
of impurities are n-type and p-type.

To increase the number of conduction-band electrons in intrinsic silicon,
pentavalent impurity atoms are added. These are atoms with five valence electrons such
as arsenic (As), phosphorus (P), bismuth (Bi) and antimony (Sb).

1.2.2 N-Type Semiconductor

As illustrated in figure 1-15, each pentavalent atom (antimony, in this case) forms
covalent bonds with four adjacent silicon atoms.

Four of the antimony atom’s valence electrons are used to form the covalent bonds
with silicon atoms, leaving one extra electron. This extra electron becomes a conduction
electron because it is not attached to any atom. Because the pentavalent atom gives up
an electron, it is often called a donor atom. The number of conduction electrons can be
carefully controlled by the number of impurity atoms added to the silicon. A conduction
electron created by this doping process does not leave a hole in the valence band because
it is excess of the number required to fill the valence band.

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1.2.3 Majority and Minority

Since most of the current carriers are electrons, silicon (or germanium) doped with
pentavalent atoms is an n-type semiconductive material (the n stands for the negative
charge on an electron). The electrons are called the majority carriers in n-type material.
Although the majority of current carriers in n-type material are electrons, there are also a
few holes that are created when electron-hole pairs are thermally generated.
These holes are not produced by the addition of the pentavalent impurity atoms. Holes in
an n-type material are called minority carriers.

1.2.4 P-Type Semiconductor

To increase the number of holes in intrinsic silicon, trivalent impurity atoms are
added. These are atoms with three valence electrons such as aluminum (Al), boron (B),
indium (In) and gallium (Ga). As illustrated in figure 1-16, each trivalent atom (boron, in
this case) forms covalent bonds with four adjacent silicon atoms. All three of the boron
atom’s valence electrons are used in the covalent bonds; and, since four electrons are
required, a hole results when each trivalent atom is added.

Because the trivalent atom can take an electron, it is often referred to as acceptor
atom. The number of holes can be carefully controlled by the number of trivalent impurity
atoms added to the silicon. A hole created by this doping process is not accompanied by
a conduction (free) electron.

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1.2.5 Majority and Minority Carriers

Holes can be thought of as positive charges because the absence of an electron
leaves a net positive charge on the atom. The holes are majority carriers in p-type material.
Although the majority of current carriers in p-type material are holes, there are also a few
free electrons that are created when electron-hole pairs are thermally generated. These
free electrons are not produced by the addition of the trivalent impurity atoms. Electrons
in p-type material are the minority carriers.
EXAMPLE 1.1

i. What is the basic difference between conductors and insulators?
ii. How do semiconductors differ from conductors and insulators?
iii. How many valence electrons does a conductor such as copper have?
iv. How many valence electrons does a semiconductor have?
v. Name three of the best conductive materials.
vi. What is the most widely used semiconductive material?
vii. Why does a semiconductor have fewer free electrons than a

conductor?

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SUMMARY

In this module we have studied that…
1. The process of adding impurities to a semiconductor crystal is called doping.
2. Doping semiconductor crystal changes its electrical characteristics.
3. Donor impurities have five valence electrons and produce free electrons in this
crystal. This forms N type semiconductor material.
4. Free electrons serve as current carriers.
5. Acceptor impurities have three valence electrons and produce holes in the crystal.

REFERENCE

1. ELECTRONIC PRINCIPLES

- BY ALBERT PAUL MALVINO
- SIXTH EDITION.

2. ELECTRONIC FUNDAMENTALS : CIRCUIT DEVICES AND APPLICATIONS.

- BY THOMAS L. FLOYD.
- SIXTH EDITION.

3. ELECTRONICS PRINCIPLES AND APPLICATIONS.

- BY CHARLES A. SCHULER.
- SIXTH EDITION.

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MODULE NO 2 : DIODES Application
(GENERAL CHARACTERISTICS)

2.1 THE DIODES

Example: Circuit symbol:

Function

Diodes allow electricity to flow in only one direction. The arrow of the circuit symbol
shows the direction in which the current can flow. Diodes are the electrical version of a
valve and early diodes were actually called valves.

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2.1.1 Forward Voltage Drop

Electricity uses up a little energy pushing its way through the diode, rather like a
person pushing through a door with a spring. This means that there is a small voltage
across a conducting diode, it is called the forward voltage drop and is about 0.7V for all
normal diodes which are made from silicon. The forward voltage drop of a diode is almost
constant whatever the current passing through the diode so they have a very steep
characteristic (current-voltage graph).

2.1.2 Reverse Voltage

When a reverse voltage is applied a perfect diode does not conduct, but all real
diodes leak a very tiny current of a few µA or less. This can be ignored in most circuits
because it will be very much smaller than the current flowing in the forward direction.
However, all diodes have a maximum reverse voltage (usually 50V or more) and if this
is exceeded the diode will fail and pass a large current in the reverse direction, this is
called breakdown.

Ordinary diodes can be split into two types: Signal diodes which pass small
currents of 100mA or less and Rectifier diodes which can pass large currents. In addition
there are LEDs (which have their own page) and Zener diodes (at the bottom of this page).

2.1.3 Connecting and soldering

Diodes must be connected the correct way round, the diagram may be labelled a
or + for anode and k or - for cathode (yes, it really is k, not c, for cathode!). The cathode
is marked by a line painted on the body. Diodes are labelled with their code in small print,
you may need a magnifying glass to read this on small signal diodes!

Small signal diodes can be damaged by heat when soldering, but the risk is small
unless you are using a germanium diode (codes beginning OA...) in which case you
should use a heat sink clipped to the lead between the joint and the diode body. A standard
crocodile clip can be used as a heat sink.

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Rectifier diodes are quite robust and no special precautions are needed for

soldering them.

2.1.4 Testing diodes

You can use a multimeter or a simple tester (battery, resistor and LED) to check
that a diode conducts in one direction but not the other. A lamp may be used to test a
rectifier diode, but do NOT use a lamp to test a signal diode because the large current
passed by the lamp will destroy the diode!

2.1.5 Signal diodes (small current)

Signal diodes are used to process information (electrical signals) in circuits, so
they are only required to pass small currents of up to 100mA.

General purpose signal diodes such as the 1N4148 are made from silicon and
have a forward voltage drop of 0.7V.

Germanium diodes such as the OA90 have a lower forward voltage drop of 0.2V
and this makes them suitable to use in radio circuits as detectors which extract the audio
signal from the weak radio signal.

For general use, where the size of the forward voltage drop is less important,
silicon diodes are better because they are less easily damaged by heat when soldering,
they have a lower resistance when conducting, and they have very low leakage currents
when a reverse voltage is applied.

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2.1.6 Protection diodes for relays

Signal diodes are also used with relays to protect transistors and integrated circuits

from the brief high voltage produced when the relay coil is switched off. The diagram

shows how a protection diode is connected across the relay coil, note that the diode is

connected 'backwards' so that it will normally NOT conduct. Conduction only occurs when

the relay coil is switched off, at this moment current tries to continue flowing through the

coil and it is harmlessly diverted through the diode. Without the diode no current could

flow and the coil would produce a damaging high voltage 'spike' in its attempt to keep the

current flowing.

2.1.6 Rectifier diodes (large current)

Rectifier diodes are used in power supplies to convert alternating current (AC) to
direct current (DC), a process called rectification. They are also used elsewhere in circuits
where a large current must pass through the diode.

All rectifier diodes are made from silicon and therefore have a forward voltage drop
of 0.7V. The table shows maximum current and maximum reverse voltage for some
popular rectifier diodes. The 1N4001 is suitable for most low voltage circuits with a current
of less than 1A.

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2.1.7 Bridge rectifiers

There are several ways of connecting diodes to make a rectifier to convert AC to
DC. The bridge rectifier is one of them and it is available in special packages containing
the four diodes required. Bridge rectifiers are rated by their maximum current and
maximum reverse voltage. They have four leads or terminals: the two DC outputs are
labelled + and -, the two AC inputs are labelled .

The diagram shows the operation of a bridge rectifier as it converts AC to DC.
Notice how alternate pairs of diodes conduct.

Various types of Bridge Rectifiers

Note that some have a hole through their centre for attaching to a heat sink

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2.1.8 Zener diodes

Example: Circuit symbol:

a = anode, k = cathode

Zener diodes are used to maintain a fixed voltage. They are designed to
'breakdown' in a reliable and non-destructive way so that they can be used in reverse to
maintain a fixed voltage across their terminals. The diagram shows how they are
connected, with a resistor in series to limit the current.

Zener diodes can be distinguished from ordinary diodes by their code and
breakdown voltage which are printed on them. Zener diode codes begin BZX... or BZY...
Their breakdown voltage is printed with V in place of a decimal point, so 4V7 means 4.7V
for example.

Zener diodes are rated by their breakdown voltage and maximum power:

 The minimum voltage available is 2.7V.
 Power ratings of 400mW and 1.3W are common.

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2.2 Series diode configuration with DC inputs
• When connected to voltage sources in series, the diode is on if the applied voltage
is in the direction of forward-bias and it is greater than the VT of the diode
• When a diode is on, we can use the approximate model for the on state
Series diode configuration

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Parallel diode configuration

Determine I1 , VD1 , VD2 and V0 for the parallel diode circuit in below figure

Solution

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Examples
1. Find diode current and output voltage

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Solution:
2. Solve for I , v1,v2 and vo

3.Determine unknown parameters

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4.Determine unknown parameters

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Here’s the next article of the tutorial on Semiconductor Power switching devices.
You can also write an article and send it to us by mail.
Diodes are connected inside the circuit in two configurations. These configurations
are:
Series configuration
Parallel configuration
Both of the connection patterns are widely used and will be discussed in this article
in detail along with diagrams.

2.3 Series Configuration
Series connection means a side by side connection. When two components are
connected in series, they have one common junction. The variation of voltage and
current in a series connection is as follows:
Potential difference across every component is different.
The current across every component connected in series remains the same.
The same properties also hold true for diodes when they are connected in a series
configuration.
Diode Characteristics in Series Configuration
When connected in series, we observe the following properties to hold true
among the diodes:
Resultant diode’s forward voltage increases.
Reverse blocking capabilities of diodes are increased in series connection.
Consider two diodes connected in series. The thing to be kept in mind over here
is that all the diodes connected in series won’t have the same characteristics as
shown in the graph below.

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V-I characteristics show that the diodes have different blocking voltages. In forward
biased state, the voltage drop and the forward current would be same on the
diodes. While in the reverse biased the blocking voltage is different as the diodes
have to carry the same leakage current.
This problem can be solved by connected resistances across every diode. Voltage
would be shared equally; hence the leakage current would differ.

Total leakage current would now be:
Our requirement is:

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We know,

So we get,

Area of Application
A single diode cannot meet higher voltage requirements, unless it is connected in
series. So the major areas of application are:
HVDC (High voltage direct current) transmission lines.
Commercial areas where regulated voltage supply is needed.

2.3 Parallel configuration
Parallel connection means the components are connected across each other,
having two common points. Current differs across each component while voltage
drop is same. When diodes are connected in parallel, this same trend is observed.
Diode Characteristics in Parallel Configuration
Current carrying capacity increases.
No conduction in resultant diode in both sides.
Consider two diodes connected in parallel configuration. Current would be shared
among the two diodes. To make this sharing equal, inductors (with same
inductances) are connected. When current at D1 increases, the voltage drop
across L1 increases, generating an opposite polarity value at L2.
Inductors are used for dynamic conditions. Inductors are usually bulky and
expensive and generate spikes which can cause problems.

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Diodes of same type having same voltage drops can be used for steady state
conditions. In this case, the parallel diodes would have the same reverse blocking
voltages. Some precautions are to be kept in mind while using the diodes with
same forward voltage drops, which are:
The diodes should have same heat sinks.
They should be cooled equally when necessary.
Negligence would change the temperature of the diodes unequally. This will in turn
cause the forward characteristics to differ which can create problems.
Areas of Application
High power applications.
Several diodes connected in parallel can match the desired current rating.
Finally, which configuration is to be used? Well, this depends on the voltage and
current ratings of the application, as I have discussed earlier. Both the
configurations can fulfill our requirements which a single diode cannot.

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EXERCISE 2.1
1. Define doping.
2. What is the difference between a pentavalent atom and a trivalent
atom? What are other names for these atoms?
3. How is an n-type semiconductor formed?
4. How is an p-type semiconductor formed?
5. What is the majority carrier in an n-type semiconductor?
6. What is the majority carrier in an p-type semiconductor?
7. By what process are the majority carriers produced?
8. By what process are the minority carriers produced?
9. What is the difference between intrinsic and extrinsic semiconductors?

SUMMARY

In this module we have studied that…………….
 One of most basic and useful electronic component is the PN-junction diode.
 2.When the diode is formed,a depletion region appears that acts as an
insulator.
 3.Forward bias forces the majority carries to the junction and collapses the
depletion region. The diode conducts.(Technically speaking, it semiconducts.)
 Reverse bias widens the depletion region. The diode does not conduct.

 5.Reverse bias forces the minority carriers to the junction. This causes a small
leakage current to flow. It can usully be ignored.

 6, Volt-ampere characteristic curves are used very often to describe the behavior
of electronic devices.

 7.The volt-ampere characteristic curve of a resistor is linear(a straight line)
 The volt-ampere characteristic curve of a diode is nonlinear.

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 9.It takes about 0.2V of forward bias to turn on a germanium diode, about 0.6V for
a silicon rectifier, and about 2V for an LED.

 10.A silicon diode will avalanche at some high value of reverse voltage.
 11.Diode leads are identified as the cathode lead and the anode lead.
 The anode must be made positive with respect to the cathode to make a diode

conduct .
 Manufacturers mark the cathode lead with a band , bevel, flange, or plus(+)sign.
 14.If there is doubt,the ohmmeter test can identify the cathode lead. It will be

connected to the negative terminal. A low resistance reading indicates that the
negative terminal of the ohmmeter is connected to the cathode.
 Caution should be used when applying the ohmmeter test . Some ohmmeter have
reversed polarity . The voltage of some ohmmeter is to low to turn on a PN-
junctions diode. Some ohmmeter”s voltages are to high and may damage delicate
PN junctions.
 A diode used to change alternatining current to direct current is called a rectifier
diode.
 Schottky diode do not have a depletion region and turn off much faster than silicon
diodes.
 Semiconductors with free holes are classified as P – type materials.
 Impurities with five valence electrons produce N – type semiconductors.
 Impurities with three valence electrons produce P – type semiconductors.
 Holes drift toward the negative end of a voltage source.
 Majority carriers are electrons for N – type material. Hole are majority carriers for
P – type material.
 Minority carriers are holes for N – type material. Electrons are minority carriers for
P – type material.

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REFERENCE

1. ELECTRONIC PRINCIPLES

- BY ALBERT PAUL MALVINO
- SIXTH EDITION.

2. ELECTRONIC FUNDAMENTALS : CIRCUIT DEVICES AND APPLICATIONS.

- BY THOMAS L. FLOYD.
- SIXTH EDITION.

3. ELECTRONICS PRINCIPLES AND APPLICATIONS.

- BY CHARLES A. SCHULER.
- SIXTH EDITION.

4. ELECTRONIC DEVICES AND CIRCUIT THEORY

- BY ROBERT L. BOYLESTAD AND LOUIS NASHELSKY
- EIGHTH EDITION.
- www.prenhall.com/boylestad

5. ELECTRONIC DEVICES AND CIRCUIT THEORY.

- BY ROBERT L. BOYLESTAD AND LOUIS NASHELSKY.
- NINTH EDITION.

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MODULE NO 3: ZENERS AND OTHER TWO TERMINAL
DEVICES

INTRODUCTION

The analysis of networks employing Zener diodes is quite similar to the
analysis of semiconductor diodes in previous sections .First the state of the diode must be
determined, followed by a substitution of the appropriate model and a determination of the
other unknown quantities of the network.

LEARNING OBJECTIVES

The objectives of this module are to:
1. Identifies the Zener diodes.
2. Identifies the characteristics and notations.

LEARNING OUTCOMES

After completing the module, students should be able to:
1. Identifies the Zener diodes.
2. identifies the characteristics and notations.

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3.1 ZENER AND OTHER TWO TERMINAL DEVICES.

3.1.1 ZENER DIODES
The analysis of networks employing Zener diodes is quite similar to the analysis of

semiconductor diodes in previous sections .First the state of the diode must be
determined, followed by a substitution of the appropriate model and a determination of the
other unknown quantities of the network. Figure 2.102 reviews the approximate equivalent
circuits for each region of a Zener diode assuming the straight-line approximations at each
break point. Note that the forward-bias region is included because occasionally an
application will skip into this region also.

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3.2.1 ZENER DIODE CHARACTERISTIC AND NOTATION

The first examples will demonstrate how a Zener diode can be used to establish
reference voltage levels and act as a protection device. The use of a Zener diode as a
regulator will then be described in detail because it is one of its major areas of application.
A regulator is combination of elements designed to ensure that the output voltage of a
supply remains fairly constant

EXAMPLE 3.1
Determine the reference voltages provided by the network of fig.2.103, which uses

a white LED to indicate that the power is on. What is the level of current through the LED
and the power delivered by the supply? How does the power absorbed by the LED
compare to that of the 6-V Zener diode?

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SOLUTION:

First we have to check that there is sufficient applied voltage to turn on all the
series diode elements. The while LED will have a drop of about 4 V across it, the 6-V and
3.3-V Zener diodes have a total of 9.3 V, and the forward-biased silicon diode has 0.7 V,
for a total of 14 V. the applied 40 V is then sufficient to turn on all the elements and, one
hopes,
Establish a proper operating current.

Note that the silicon diode was used to create a reference voltage of 4 V because

V01 = Vz2 + Vk = 3.3 V + 0.7 V = 4.0 V

Combination the voltage of the 6-V Zener diode with the 4 V results in.

V02 = V01 + Vz1 = 4 V + 6 V = 10 V
Finally, the 4 V across the white LED will leave a voltage of 40 V – 14 V =26 V
across the resistor, and

IR = I LED =VR = 40 V - V02 - V LED = 40 V – 10 V – 4 V = 26 V = 20 Ma

R 1.3 kΩ 1.3 kΩ 1.3 kΩ

That will establish the proper brightness for the LED.

The power delivered by the supply is simply the product of the supply voltage and
current drain as follows:

Ps = EIs = EIR = (40 V) (20 Ma) = 800 Mw
The power absorbed by the LED is

PLED = VLEDILED = (4 V) (20 mA) = 80 mW

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And the power absorbed by the 6-v Zener diode is
PZ = VZIZ = (6 V) (20 mA) = 120 mW

The power absorbed by the Zener diode exceeds that of the LED 40 mW.
3.2.2 ZENER DIODE AS A REGULATOR

The use of the Zener diode as a regulator is so common that three conditions
surrounding the analysis of the basic Zener regulator are considered. The analysis
provides an excellent opportunity to become better acquainted with the response of the
Zener diode to different operating condition. The basic configuration appears in fig.2.106.
the analysis is first for fixed quantities, followed by a fixed supply voltage and a variable
load, and finally a fixed load and a variable supply.

3.2.2.1 VI AND R FIXED
The simplest of Zener diode regulator networks appears in fig.2.106. the applied

dc volt-age is fixes, as is the load resistor. The analysis can fundamentally be broken down
into
two steps.

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1. Determine the state of the Zener diode by removing it from the network and
calculating the voltage across the resulting open circuit.

Applying step 1 to the network of fig.2.106 results in the network of fig.2.107, where
an application of the voltage divider results in
V = VL = RLVi

R + RL
If V ≥ Vz the Zener diode is on, and the appropriate equivalent model can be substituted.
If V < Vz the diode is off, and the open-circuits equivalent is substituted.

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2. Substituted the appropriate equivalent circuit and solve for the desired
unknowns.

For the network of fig.2.106, the ‘on’ state will result in the equivalent network of
fig.2.108.since voltages across parallel elements must be the same, we find that
VL = Vz

The zener diode current must be determined by an application of kirchof`s law.
That is,

IR = Iz + IL
Iz = IR –IL
Iz = VL and IR = VR= Vi-VL

RL R R
The power dissipated by the zener diode is determined by
PZ = Vz.Iz
That must be less than the Pzm specified for the device.

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Before continuing, it is particularly important to realize that the first step was
employed only to determine the state of the zener diode. If the zener diode is in the ‘on’
state, the voltage across the diode is not V volts. When the system is turned on, the zener
diode will turn on as soon as the voltage across the zener diode is Vz volts. It will then
‘lock’ at this level and never reach he higher level of V volts.

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SUMMARY

In this module we have studied that…..
1. A diode used to stabilize or regulate voltage is the zener diode.
2. Zener diodes conduct from anode to cathode when they are working as
regulators. This is just the opposite from the way rectifier diodes conduct.
3. A diode clipper or limiter can be used to stabilize the peak to peak amplitude
of a signal. It may also be used to change the shape of a signal or reduce its
noise content.
4. Clamps or dc restorers add a dc component to an ac signal.
5. Light – emitting diodes are used as indicators, transmitters and in optoisolators.
6. Varicap diodes are solid – state variable capacitors. They are operated under
conditions of reverse bias.
7. Varicap diodes show minimum capacitance at maximum bias. They show
maximum capacitance at minimum bias.

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REFERENCE

1. ELECTRONIC PRINCIPLES

- BY ALBERT PAUL MALVINO
- SIXTH EDITION.

2. ELECTRONIC FUNDAMENTALS : CIRCUIT DEVICES AND APPLICATIONS.

- BY THOMAS L. FLOYD.
- SIXTH EDITION.

3. ELECTRONICS PRINCIPLES AND APPLICATIONS.

- BY CHARLES A. SCHULER.
- SIXTH EDITION.

4. ELECTRONIC DEVICES AND CIRCUIT THEORY

- BY ROBERT L. BOYLESTAD AND LOUIS NASHELSKY
- EIGHTH EDITION.
- www.prenhall.com/boylestad

5. ELECTRONIC DEVICES AND CIRCUIT THEORY.

- BY ROBERT L. BOYLESTAD AND LOUIS NASHELSKY.
- NINTH EDITION.

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MODULE NO 4: BIPOLAR JUNCTION TRANSISTORS

INTRODUCTION

Materials can be divided into three main types according to the way they react to current
when a voltage is applied across them Insulators (nonconductor), for example, are
materials that have a very high resistance and therefore oppose current, whereas
conductors are materials that have a very low resistance and therefore pass current
easily.
Semiconductor is a material that is neither a good conductor nor a good insulator but,
rather, lies halfway between the two. Under certain circumstances, the resistive properties
of a semiconductor can be varied between those of a conductor and those of an insulator.
This characteristic of semiconductor element that makes them useful as a amplifiers and
rectifier. Diodes, transistors and ICs are all made from semiconductor materials such a
germanium (Ge) and silicon (Si).
This topic describes introduction to bipolar junction transistor(emitter, base and collector),
current and voltage characteristics of BJT, type of transistor (NPN & PNP), Three
operation region of BJT, depletion region and barrier potentials at each jjunction.

LEARNING OBJECTIVES

The objectives of these topics are to :
1. impart to students the knowledge, skill and understanding of bipolar junction

transistor
2. equip students with the basic to apply knowledge of bipolar junction transistor
3. describe bipolar junction transistor operating characteristics.

LEARNING OUTCOMES

After completing the topics, student should be able to :

1. describe the basic structure of the bipolar junction transistors
2. discuss the difference between the structure of a NPN and PNP transistor
3. identify current and voltages characteristics
4. identify various type of transistor package configuration
5. troubleshoot various faults in transistor circuit

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4.1 INTRODUCTION TO BIPOLAR JUNCTION TRANSISTORS

Bipolar Transistor essentially consists of a pair of PN Junction Diodes that are joined back-
to-back. This forms a sort of a sandwich where one kind of semiconductor is placed in
between two others. There are therefore two kinds of Bipolar sandwich, the NPN and PNP
varieties. The three layers of the sandwich are conventionally called the Collector, Base,
and Emitter. The reasons for these names will become clear later once we see how the
transistor works.

Figure 2.1 (a) : Bipolar Junction Transistor is formed by two diodes

Figure 2.1 (b) : Bipolar Junction Transistor

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4.1.1 THE TRANSISTOR CONSTRUCTION

The BJT is constructed with three doped semiconductor regions separed by two
pn junctions. The three region are called emitter, base and collector.

(a) : npn transistor (b) : pnp transistor

Figure 2.1 (c) : BJT construction

One type consists of two n regions separated by a p region (npn), and the other
consists of two p regions separated by n region (pnp). The pn junction joining the
base region and the emitter regions is called the base -emitter junction. The
junction joining the base region and the collector region is called the base-
collector junction.

The base region is lightly doped and very thin compared to the heavily doped
emitter and the moderately doped collector regions. The term bipolar refers to
the use of the both holes and electrons as carriers in the transistor
structure.

In summary, the following points about the construction of a transistor;

1) The emitter region is heavily doped. Its job is to emit or inject current
carriers into the base region. For npn transistors the n-type emitter injects
free electrons into the base. For pnp transistors the p-type emitter injects
holes into the base.

2) The base is very thin and lightly doped. Most of the current carriers
injected into the base region cross over into the collector side and do not
flow out the base lead.

3) The collector region is moderately doped. It is also the largest region
within the transistor. Its job is to collect or attract current carriers injected
into the base region.

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4.2 CURRENT AND VOLTAGES CHARACTERISTICS OF BJT

Figure 2.2
The Figure 2.2 above and circuit symbols for both the NPN and PNP transistors are shown
above with the arrow in the circuit symbol always showing the direction of conventional
current flow between the base terminal and its emitter terminal, with the direction of the
arrow pointing from the positive P-type region to the negative N-type region, exactly the
same as for the standard diode symbol.
IB : dc base current
IE : dc emitter current
IC : dc collector current
VBE : dc voltage at base with respect to emitter
VCB : dc voltage at collector with respect to base
VCE : dc voltage at collector with respect to emitter

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4.3 TYPE OF TRANSISTOR

There is 2 types of BJT:

1) npn
2) pnp

(a) : npn (b) : pnp

Figure 2.3: Schematic symbols for npn and pnp transistors.

The arrow on the schematic symbol is important for three reason;

1) It identifies the compenant terminals. The arrow is always drawn on
the emitter terminal.

2) The arrow always points toward the n-type material.
3) The arrow indicates the direction of the emitter current.

In order for the transistor to operate properly as an amplifier, the two PN junctions
must be correctly biased with external dc voltages.

The proper bias arrangement for both NPN and PNP transistor is the base-
emitter (BE) junction is forward biased and the base-collector (BC) junction
is reverse biased. This is called forward-reverse bias.

(c) : NPN circuit (d) : PNP circuit

Figure 2.3 : Forward-reverse bias of a bipolar transistor

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Example :-

A transistor has the following currents; IB = 20 mA and IC = 4.98 A. Calculate IE.

Answer :- = IB + IC
IE = 20 mA + 4.98 A
= 0.02 A + 4.98 A
= 5A

Transistor Parameters

DC Beta ( βDC ) and DC Alpha (αDC )

The ratio of the collector current IC to the base current IB is the DC Beta ( βDC
), which is the current gain of a transistor.

βDC = IC
IB

Typical values of βDC range from less than 20 to 200 or higher. βDC is usually
designated as hFE on transistor data sheet.

hFE = βDC

Example 1:

A transistor has the following current ; IC = 10 mA, IB = 50 μA. Calculate βDC

Answer : IC / IB
10 mA
βDC = 50 μA
= 200

=

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Example 2:

A transistor has βDC = 150 and IB = 75 μA. Calculate IC .

Answer :

βDC = IC
IB
IC = βDC x IB
= 150 x 75 μA
= 11.25 mA.

The ratio of the collector current, IC , to the emitter current, IE ,is the DC Alpha
(αDC).

αDC = IC
IE

Typically, values of αDC range from 0.95 to 0.99 or greater, but αDC is always
less than 1. The reason is that IC is always slightly less than IE by the amount of IB.

Example 1 :

A transistor has the following currents ; IE = 15 mA , IB = 60 μA. Calculate αDC .

Answer :

First calculate IC ;

IC = IE - IB
= 15 mA - 60 μA
= 15 mA - 0.06 mA
= 14.94 mA.

αDC = IC
IE

= 14.94 mA
15 mA

= 0.996

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Relationship of βDC and αDC
Let's start with the current formula IE = IB + IC and divide each current by IC ;

IE = IC + IB = 1 + IB

IC IC IC IC

Since βDC = IC / IB and αDC = IC / IE ; we can substitute the reciprocals into the
equation :

1=1+1

αDC βDC

By rearranging and solving for βDC , we get ;

1= βDC + 1
αDC βDC

βDC = αDC ( βDC + 1)
βDC = αDC βDC + αDC
βDC - αDC βDC = αDC
βDC ( 1 - αDC ) = αDC

βDC = αDC
1 - αDC

If βDC is known, αDC can be found using ;

αDC = βDC
1 + βDC

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Example 1 :

A transistor has βDC = 100. Calculate αDC

Answer :

αDC = βDC

1 + βDC

= 100

1 + 100

= 0.99

Example 2 :

A transistor has αDC = 0.995. Calculate βDC.

Answer :

βDC = αDC

1 - αDC

= 0.995

1 - 0.995

= 199

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