Electronics 103924
Electronics 103924
Electronics 103924
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Nucleus is the central part of an atom and contains protons and neutrons.
Orbit is the region outside the nucleus of an atom through which electrons revolve
Proton is a positively charged particle found in the nucleus of an atom while the neutron has the
same mass as the proton, but has no charge.
Electrons are negatively charged particles that revolve around the nucleus in different orbits or
paths of an atom.
The number and arrangement of electrons in any orbit is determined by the following rules:
(i) The number of electrons in any orbit is given by 𝟐𝒏𝟐 where n is the number of the
orbit.
For example,
First orbit contains 2 × 12 = 2 electrons
Second orbit contains 2 × 22 = 8 electrons
Third orbit contains 2 × 32 = 18 electrons.
(ii) The last orbit cannot have more than 8 electrons.
(iii) The last but one orbit cannot have more than 18 electrons.
Draw a structure of the atoms of the following elements Hydrogen, Oxygen, Sodium, and
Copper and show the composition of the nucleus and orbits of each atom.
Classification of Materials
The electrons in the outermost orbit of an atom are known as valence electrons.
The outermost orbit can have a maximum of 8 electrons i.e. the maximum number of valence
electrons can be 8.
The valence electrons which are very loosely attached to the nucleus are known as free
electrons.
On the basis of electrical conductivity, materials are generally classified into conductors,
insulators and semi-conductors.
(i) A conductor is a substance which has a large number of free electrons and conducts
electric current. E.g. Most metals such as Copper, Aluminium, and Iron. Usually, they
have less than 4 valence electrons.
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(ii) An insulator is a substance which has practically no free electrons at ordinary
temperature and do not conduct any electric current. E.g. Rubber, plastic, sulphur,
neon, etc. Usually, they have more than 4 valence electrons.
(iii) A semiconductor is a substance which has very few free electrons at room
temperature and they practically don’t conduct current at room temperature. E.g.
Silicon, Germanium, and Carbon. Usually, they have 4 valence electrons.
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In terms of energy bands, it means that insulators have:
A full valence band
An empty conduction band
A large energy gap (of several eV) between them
At ordinary temperatures, the probability of electrons from full valence band gaining
sufficient energy so as to surmount energy gap and thus become available for
conduction in the conduction band, is low.
Figure (a) below shows the energy band diagram of an insulator, figure (b) shows an energy
band for conductors, and figure (c) shows the energy band for semi-conductors. As shown in
figure (b), conductors don’t have a forbidden gap and that the valence band and the conduction
band overlaps each other thus making electrons to flow freely from the valence band to the
conduction band.
In terms of energy bands, it means that electrical conductors are those which have overlapping
valence and conduction bands.
In terms of energy bands, semiconductors can be defined as those materials which have almost
an empty conduction band and almost filled valence band with a very narrow energy gap (of the
order of 1 eV) separating the two.
In terms of energy bands, insulators can be defined as those materials which have an empty
conduction band and a filled valence band with a very wide energy gap (of the order of several
eV) separating the two.
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Types of Semiconductors
There are two types of semiconductors:
Intrinsic or pure semiconductors
Extrinsic or impure semiconductors
Intrinsic Semiconductor
An intrinsic semiconductor is one which is made of the semiconductor material in its extremely
pure form. Examples of such semiconductors are: pure germanium and silicon.
The number of thermally-generated charge carriers per unit volume (i.e. intrinsic carrier density)
is given by: where N is constant for a given semiconductor, E g is the band
gap energy in joules, k is Boltzmann’s constant and T is the temperature in ºK.
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N-type semiconductors
P-type semiconductors
Doping agents
The usual doping agents are:
Pentavalent atoms: - these atoms have five valence electrons. E.g. arsenic, phosphorus,
antinomy, etc.
Trivalent atoms: - these atoms have three valence electrons. E.g. gallium, indium,
aluminium, boron, etc.
The pentavalent doping atoms are known as donor atoms because it donates or contributes one
electron to the conduction band of pure germanium atom.
The trivalent doping atoms are known as acceptor atoms because it accepts one electron from
the pure germanium atom.
Covalent Bond
This is a type of bond that is formed by sharing of electrons between atoms.
Each atom in this type of bond contributes equal number of electrons for sharing. For example,
germanium atom; which has 4 valence electrons, can bond covalently to 4 other germanium
atoms as shown in figure (i) and (ii) below. As seen in the figure below, the middle germanium
atom is surrounded by 4 other germanium atoms giving a total of 8 electrons in its outermost
energy level (orbit). A similar observation is made on the other germanium atoms as they bond
covalently to other 3 germanium atoms.
During doping, the middle germanium atom will be replaced by a doping agent (pentavalent or
trivalent) to form an extrinsic semiconductor.
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This type of semiconductor is obtained when a pentavalent impurity like arsenic (As) is added to
pure germanium crystal. Each arsenic atom forms covalent bonds with the surrounding four
germanium atoms with the help of four of its five electrons. The fifth electron is free. Since
several arsenic (As) atoms are added, several free electrons are produces and that helps in
electrical conductivity. Due to the presence of the free electrons (negatively charged), the
resulting semiconductor is of n-type.
NB: Silicon (Si) can be used in the place of Germanium (Ge) to produce the same type of
extrinsic semiconductor.
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Majority and Minority Charge carriers
In p-type semiconductors, holes (which are positively charged) are the majority charge carriers
while electrons (negatively charged) are the minority charge carriers.
In n-type semiconductor, electrons (negatively charged) are the majority charge carriers while
holes (positively charged) are the minority charge carriers.
The figure below shown the minority and majority charge carriers in both p-type and n-type
semiconductors.
P-N Junction
It is possible to manufacture a single piece of a semiconductor material half of which is doped by
P-type impurity and the other half by N-type impurity as shown in the figure below. The plane
dividing the two zones is called junction.
When a p-type semiconductor is suitably joined to n-type semiconductor, the contact surface is
called pn junction.
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Formation of pn junction
One common method of making pn junction is called alloying.
In this method, a small block of indium (trivalent impurity) is placed on an n-type germanium
slab as shown in figure (i). The system is then heated to a temperature of about 500ºC. The
indium and some of the germanium melt to form a small puddle of molten germanium-indium
mixture as shown in figure (ii). The temperature is then lowered and puddle begins to solidify.
The addition of indium overcomes the excess of electrons in the n-type germanium to such an
extent that it creates a p-type region. As the process goes on, the remaining molten mixture
becomes increasingly rich in indium. When all germanium has been re-deposited, the remaining
material appears as indium button which is frozen on to the outer surface of the crystallized
portion as shown in figure (iii). This button serves as a suitable base for soldering on leads.
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Forward Biased P-N Junction
In this connection, the p-region of the semiconductor is connected to a positive terminal of a
battery while the n-region of the semiconductor is connected to the negative terminal of the
battery.
In this type of connected, the electrons will be attracted to the side of holes while the holes
moves to the side of electrons. For this reason, the depletion layer decreases and electric current
starts to flow through the p-n junction.
Reverse Biased P-N Junction
In this connection, the n-region of the semiconductor is connected to a positive terminal of a
battery while the p-region of the semiconductor is connected to the negative terminal of the
battery.
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In this type of connected, the electrons will be attracted to the positive terminal of the battery
while the holes will be attracted to the negative terminal of the battery. For this reason, the
depletion layer increases and no current will flow through it.
P-N Junction Diode
It is a two-terminal device consisting of a P-N junction formed either in Ge or Si crystal. Its
circuit symbol is shown in figure (b) below. The P-type region is referred to as anode and while
the N-type region is referred to as the cathode.
In figure (b) below, arrowhead indicates the conventional direction of current flow when
forward-biased. It is the same direction in which hole current flow takes place.
One of the commercial pn-junction diodes is also shown in the figure below.
A PN-junction diode allows current to flow in one direction (during forward bias) and block
current when in reverse bias.
V/I Characteristics of a PN Junction Diode
It is a graph showing the current (usually on the y-axis) and voltage (usually on the x-axis)
through a diode in both forward and reverse bias. The figure below shows the VI characteristics
of a pn junction diode.
As shown in the figure above, when a diode is in the forward bias, the voltage barrier of 0.3V
(for Ge) or 0.7V (for Si) must be overcome before the diode can allow electric current to flow
through it. However, there is still some little forward leakage current that can flow when the
voltage is less than 0.3V or 0.7V.
Similarly, when the diode is in reverse bias, current is blocked from flowing until when the
applied voltage become equal or greater than the reverse breakdown voltage. However, there is
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still some reverse leakage current that flows through the diode in reverse bias even when the
applied voltage is below the reverse breakdown voltage.
Behold the reverse breakdown voltage, the diode allows very high amount of reverse current to
flow that damages the diode. Therefore, care should be taken when connecting a diode in the
reverse bias.
Or
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Resistor Construction
Resistors can be constructed out of a variety of materials. Most common, modern resistors are
made out of either a carbon, metal, or metal-oxide film. In these resistors, a thin film of
conductive (though still resistive) material is wrapped in a helix around and covered by an
insulating material. Metallic connection leads are then attached to the end caps of the resistor as
shown below.
Types of Resistors
There are two major types of resistors
Fixed resistors
Variable resistors
Fixed resistors are highly used in most electrical and electronic equipment than the variable
resistors.
Fixed resistors offer a fixed value of resistance to the flow of current. Usually, the resistance
value is set by the manufacturer.
Variable resistors on the other hand have a variable resistance value. These resistors consist of a
fixed resistor element and a slider which taps onto the main resistor element. This gives three
connections to the component: two connected to the fixed element, and the third is the slider. In
this way the component acts as a variable potential divider if all three connections are used.
Variable resistors and potentiometers are widely used for all forms of control; everything from
volume controls on radios and sliders in audio mixers to a host of areas where a variable
resistance is required.
The figure below shows the circuit symbol of a variable resistor
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There are various types of fixed resistors
Metal oxide film or Metal film resistors: - uses a metal film or metal oxide to surround the
ceramic rod. They are used for low power levels. used for medium power applications.
Carbon film resistors: - uses a carbon film to surround the ceramic rod. They are used for low
power levels.
Wire wound resistors: - they consists of a high resistance wire wound on a ceramic former. They
are used for high power applications.
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Light depended resistor (LDR): - these are resistors whose resistance value depends on light
intensity.
Thermistor: - is a type of variable resistor whose resistance value varies with temperature
Applications of resistors
Resistors are used in high frequency instrument.
Resistor is used in power control circuit.
It is used in DC power supplies.
Resistors are used in filter circuit networks.
It is used in wave generators.
Resistors are used in transmitters, modulators and demodulators.
It is used in medical instrument.
It is used in instrumentation applications.
Resistor is used in voltage regulators.
It is used in feedback amplifiers.
Capacitors
Capacitors are the most widely used electronic components after resistors. We find capacitors in
televisions, computers, and all electronic circuits.
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A capacitor is an electronic device that stores electric charge or electricity when voltage is
applied and releases stored electric charge whenever required. Capacitor acts as a small battery
that charges and discharges rapidly.
Capacitance is the capacitors ability (capacity) to store an electrical charge on its plates.
Standard Units of Capacitance
1
Microfarad (μF) 1μF = 1,000,000 = 0.000001 = 10−6 𝐹
1
Nanofarad (nF) 1nF = 1,000,000,000 = 0.000000001 = 10−9 𝐹
1
Picofarad (pF) 1pF = 1,000,000,000,000 = 0.000000000001 = 10−12 𝐹
Construction of a capacitor
A capacitor is made of two electrically conductive plates placed close to each other, but they do
not touch each other. These conductive plates are normally made of materials such as aluminum,
brass, or copper. The conductive plates of a capacitor is separated by a small distance. The empty
space between these plates is filled with a dielectric material such as air, vacuum, glass, liquid, or
solid (such as paper) as shown in the figure below.
Working of a capacitor
Charging a capacitor
When no voltage is applied to the capacitor, the total number of electrons and protons in the each
plate of the capacitor are equal thus making them electrically neutral. When voltage is applied to
the capacitor in such a way that, the positive terminal of the battery is connected to the left side
plate of the capacitor and the negative terminal of the battery is connected to the right side plate
of the capacitor, the charging of capacitor takes place. In this case, a large number of electrons
start moving from the negative terminal of the battery through the conductive wire. When these
electrons reach the right side plate of the capacitor, they experience a high resistance from
dielectric material. As a result, a large number of electrons build up on the right side plate of the
capacitor. On the other hand, the electrons on the left side plate experience a strong attractive
force from the positive terminal of the battery leaving behind concentration of holes on the left
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plate. This constitutes a potential difference between the two plates. Also, an electric field
develops between the two plates through the dielectric material as shown in the figure below.
This way, the capacitor is said to be charged.
Discharging a capacitor
If the external voltage source connected to the capacitor is removed, the capacitor remains
charged. However, when the capacitor is connected to an electric device such as an electric bulb
through a conductive wire, the electrons trapped on the right side plate starts flowing through the
circuit. We know that electric current is the flow of charge carriers (free electrons). Therefore,
when the free electrons or electric current reaches the light bulb, it glows with high intensity. The
electrons that started flowing from the right side plate through the conductive wire finally reach
the left side plate and fill the holes of the left side plate. As a result, the charge on the left side
plate and right side plate starts decreasing. This decreases the intensity of electric bulb, because
the electric current flowing through the electric bulb decreases. Finally, the charge stored on the
left plate and the right plate is completely released. As a result, the bulb will turn off, because no
electric current flows through the bulb. Thus, the charge stored on the left plate and right plate of
the capacitor is discharged.
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Types of capacitors
Capacitors are mainly grouped into
Fixed capacitors
Variable capacitors
Fixed capacitor is a type of capacitor which provides fixed amount of capacitance. They are
classified into different types based on the dielectric material used to construct them. The
different types of fixed capacitors are:
Paper capacitor: - uses paper as the dielectric material to store charge.
Plastic capacitor or plastic film capacitor: - uses plastic film as dielectric material to store
charge.
Ceramic capacitor: - uses ceramic material as dielectric to store charge. They are the most
widely used capacitors in the electronic circuits. These capacitors are used when large charge
storage and small physical size is required.
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Mica capacitor: - they are used in the applications where high accuracy and low capacitance
change over the time is desired. These capacitors can work efficiently at high frequencies.
Electrolytic capacitor: - is a type of capacitor which uses electrolyte as one of its electrodes to
achieve large capacitance. Electrolytic capacitors are mainly used when high charge storage in
small volume is required.
Variable capacitors are those whose capacitance can be varied. There are two most common
types of such capacitors named as a trimmer and rotor-stator capacitors.
\
Applications if capacitors
Energy storage
Pulsed power and weapons
Power conditioning
Power factor correction
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Suppression and coupling
Sensing
Oscillators
Inductors
An inductor is a passive component that is used in most power electronic circuits to store energy
in the form of magnetic energy when electricity is applied to it.
Inductance is the ability of an inductor to store energy.
The S.I. unit of inductance is henry (H) and when we measure magnetic circuits it is equivalent
to weber/ampere. It is denoted by the symbol L.
The general circuit symbol of an inductor is shown in the figure below
Air Core Inductor refers to coils wound on plastic, ceramic, or other nonmagnetic forms, as well
as those that have only air inside the windings as shown below.
Iron Powder Inductor are those that are made of iron oxide core
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Ferrite Core Inductor uses ferrite materials as core.
Applications of inductors
Diodes
A diode is defined as a two-terminal electronic component that only conducts current in one
direction (so long as it is operated within a specified voltage level).
The figure below shows the circuit symbol of a diode and some of the available diodes in the
market.
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The construction of a p-n junction diode is similar to the construction of a p-n junction explained
in the previous topic.
Working of a diode
A P-N junction diode is one-way device offering low resistance when forward-biased and behaving
almost as an insulator when reverse-biased. Hence, such diodes are mostly used as rectifiers i.e. for
converting alternating current (ac) into direct current (dc).
When the anode of the diode is connected to the positive terminal of a battery and the cathode to
the negative terminal of the battery, the diode is said to be forward bias. Due to forward bias,
majority charge carriers in both regions gets repelled. That is, the holes in the P-type region gets
repelled by the positive charge of the battery while electrons in the N-type region gets repelled
by the negative charges of the battery. This decreases the width of the depletion layer and
eventually destroys it if the applied voltage is high enough to overcome the barrier voltage. The
result is that the electrons and holes can now cross the opposite sides and constitute the electric
current from the battery to flow through the diode from the P-type to the N-type region and back
to the battery as shown in the figure below.
When the anode of the diode is connected to the negative terminal of a battery and the cathode to
the positive terminal of the battery, the diode is said to be reverse bias. Due to reverse bias,
majority charge carriers in both regions gets attracted to the side of the battery terminals they are
connected. That is, the holes in the P-type region gets attracted by the negative charge of the
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battery while electrons in the N-type region gets attracted by the positive charges of the battery.
This increases the width of the depletion layer thus increasing the barrier voltage. The result is
that the electrons and holes cannot cross the opposite sides and constitute the electric current
from the battery to flow through the diode. In this mode, the diode is said to be blocking the flow
of current.
As shown in the figure above, when a diode is in the forward bias, the voltage barrier of 0.3V
(for Ge) or 0.7V (for Si) must be overcome before the diode can allow electric current to flow
through it. However, there is still some little forward leakage current that can flow when the
voltage is less than 0.3V or 0.7V.
Similarly, when the diode is in reverse bias, current is blocked from flowing until when the
applied voltage become equal or greater than the reverse breakdown voltage. However, there is
still some reverse leakage current that flows through the diode in reverse bias even when the
applied voltage is below the reverse breakdown voltage.
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Behold the reverse breakdown voltage, the diode allows very high amount of reverse current to
flow that damages the diode. Therefore, care should be taken when connecting a diode in the
reverse bias.
Forward Characteristic
When the diode is forward-biased and the applied voltage is increased from zero, hardly any current
flows through the device in the beginning. It is so because the external voltage is being opposed by the
internal barrier voltage VB whose value is 0.7 V for Si and 0.3 V for Ge. As soon as VB is neutralized,
current through the diode increases rapidly with increasing applied battery voltage. It is found that as
little a voltage as 1.0 V produces a forward current of about 50 mA. A burnout is likely to occur if
forward voltage is increased beyond a certain safe limit.
Reverse Characteristic
When the diode is reverse-biased, majority carriers are blocked and only a small current (due to
minority carriers) flows through the diode. As the reverse voltage is increased from zero, the reverse
current very quickly reaches its maximum or saturation value Io which is also known as leakage current.
It is of the order of Nano-amperes (nA) for Si and microamperes (µA) for Ge. The value of Io (or Is ) is
independent of the applied reverse voltage but depends on (a) temperature, (b) degree of doping and
(c) physical size of the junction. As seen from the V/I characteristic curve above, when reverse voltage
exceeds a certain value called break-down voltage VBR (or Zener voltage Vz), the leakage current
suddenly and sharply increases, the curve indicating zero resistance at this point. Any further increase in
voltage is likely to produce burnout unless protected by a current-limiting resistor.
When P-N junction diodes are employed primarily because of this breakdown property as voltage
regulators, they are called Zener diodes.
Types of diode
P-N junction diodes or rectifier dioded
This is the basic diode formed with the interaction of p-type and n-type materials.
They are used mainly for rectification purposes.
The figure below shows the diode and its circuit symbol.
Zener diodes
It is the diode designed in such a way that it can operate in the reverse bias mode.
The figure below shows a zener diode and its circuit symbol.
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Light Emitting Diodes (LED)
This is a type of diode that emits light when a forward current passes through it.
Schottky diodes
These type of diodes are used for faster switching operations
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Photo diode
This is the type of diode that allows current to flow through it when light is shining on it and
blocks the current when in the dark.
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Applications of diodes
Rectifiers
Clipper Circuits
Clamping Circuits
Reverse Current Protection Circuits
In Logic Gates
Voltage Multipliers
As a Switch
Source isolation
As voltage reference
As a Light Source
As a Light Sensor
A solar cell or Photo-Voltaic cell
Protection against Surges
As the name indicates, it is a forward-biased P-N junction which emits visible light when energized.
Construction of LED
Broadly speaking, the LED structures can be divided into two categories:
1. Surface-emitting LEDs: These LEDs emit light in a direction perpendicular to the PN junction plane.
2. Edge-emitting LEDs: These LEDs emit light in a direction parallel to the PN junction plane
The figure below shows the construction of a surface-emitting LED. As seen from this figure, an N-type
layer is grown on a substrate and a P-type layer is deposited on it by diffusion. Since carrier
recombination takes place in the P-layer, it is kept upper most. The metal anode connections are made
at the outer edges of the P-layer so as to allow more central surface area for the light to escape. LEDs
are manufactured with domed lenses in order to lessen the reabsorption problem. A metal (gold) film is
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applied to the bottom of the substrate for reflecting as much light as possible to the surface of the
device and also to provide cathode connection. LEDs are always encased in order to protect their
delicate wires.
Working of LEDs
The forward voltage across an LED is considerably greater than for a silicon PN junction diode. Typically
the maximum forward voltage for LED is between 1.2 V and 3.2 V depending on the device. Reverse
breakdown voltage for an LED is of the order of 3 V to 10 V. For this reason, a current limiting resistor is
connected in series with the LED as shown in the figure (a) below. The LED emits light in response to a
sufficient forward current. The amount of power output translated into light is directly proportional to
the forward current as shown in figure (b). It is evident from this figure that greater the forward current,
the greater the light output.
Applications of LED
A liquid crystal is a material (usually, an organic compound) which flows like a liquid at room
temperature but whose molecular structure has some properties normally associated with solids
(examples of such compounds are: cholesteryl nonanoate and p-azoxyanisole).
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Construction of LCD
As shown in figure (a), a liquid crystal ‘cell’ consists of a thin layer (about 10 µm) of a liquid crystal
sandwiched between two glass sheets with transparent electrodes deposited on their inside faces. With
both glass sheets transparent, the cell is known as transmittive type cell. When one glass is transparent
and the other has a reflective coating, the cell is called reflective type. The LCD does not produce any
illumination of its own. It, in fact, depends entirely on illumination falling on it from an external source
for its visual effect.
Working of LCD
The two types of display available are known as (i) field-effect display and (ii) dynamic scattering
display. When field-effect display is energized, the energized areas of the LCD absorb the incident light
and, hence give localized black display. When dynamic scattering display is energized, the molecules of
energized area of the display become turbulent and scatter light in all directions. Consequently, the
activated areas take on a frosted glass appearance resulting in a silver display. Of course, the un-
energized areas remain translucent.
As shown in figure (b), a digit on an LCD has a segment appearance. For example, if number 5 is
required, the terminals 8, 2, 3, 6 and 5 would be energized so that only these regions would be activated
while the other areas would remain clear
Applications of LCDs
Field-effect LCDs are normally used in watches and portable instruments where source of
energy is a prime consideration.
Thousands of tiny LCDs are used to form the picture elements (pixels) of the screen in one type
of B & W pocket TV receiver.
Used in recent desk top LCD monitors.
Used in note book computer display
Used in cellular phone display
A bipolar junction transistor consists of two pn junctions formed by sandwiching either p-type or
n-type semiconductor between a pair of opposite types.
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Types of BJTs
There are two types of transistors, namely; n-p-n transistor and p-n-p transistor
The figure below shows a commercial transistor and its circuit symbol
A transistor (pnp or npn) has three sections of doped semiconductors. The section on one side is
the emitter (E) and the section on the opposite side is the collector (C). The middle section is
called the base (B) and forms two junctions between the emitter and collector.
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(i) Emitter is the section on one side that supplies charge carriers (electrons or holes). It
is always forward biased w.r.t. base so that it can supply a large number of majority
charge carriers. In figure (i) below, the emitter (p-type) of pnp transistor is forward
biased and supplies hole charges to its junction with the base. Similarly, in figure (ii),
the emitter (n-type) of npn transistor has a forward bias and supplies free electrons to
its junction with the base.
(ii) Collector is thehe section on the other side that collects the charges. It is always
reverse biased. Its function is to remove charges from its junction with the base. In
figure (i), the collector (p-type) of pnp transistor has a reverse bias and receives hole
charges that flow in the output circuit. Similarly, in figure (ii), the collector (n-type)
of npn transistor has reverse bias and receives electrons.
(iii) Base is the middle section which forms two pn-junctions between the emitter and
collector. The base-emitter junction is forward biased, allowing low resistance for the
emitter circuit. The base-collector junction is reverse biased and provides high
resistance in the collector circuit.
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Working of NPN transistor
The figure below shows an npn transistor with forward bias to emitter-base junction and reverse
bias to collector-base junction. The forward bias causes the electrons in the n-type emitter to
flow towards the base. This constitutes the emitter current IE. As these electrons flow through the
p-type base, they tend to combine with holes. As the base is lightly doped and very thin,
therefore, only a few electrons (less than 5%) combine with holes to constitute base current I B.
The remainder (more than 95%) cross over into the collector region to constitute collector
current IC. In this way, almost the entire emitter current flows in the collector circuit.
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Figure (i) below shows the circuit symbol of an NPN transistor while figure (ii) shows the circuit
symbol of a PNP transistor.
From the circuits shown above, it is evident that the emitter current is equal to the sum of the
base current and collector current.
𝐼𝐸 = 𝐼𝐶 + 𝐼𝐵
Transistor Connection Methods
There are three methods of connecting a transistor;
Common base connection method
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Common emitter connection method
Common collector connection method
Common base connection
In this circuit arrangement, input is applied between emitter and base and output is taken from collector
and base. Here, base of the transistor is common to both input and output circuits and hence the name
common base connection.
Emitter Current amplification factor (α) is the ratio of output current to input current. In a common
base
𝐼𝐶
∝=
𝐼𝐸
At constant VCB
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𝑉𝐶𝐶 − 𝑉𝐶𝐵
𝐼𝐶 =
𝑅𝐶
Example
1. In a common base connection, IE = 1mA, IC = 0.95mA. Calculate the value of IB and the current
amplification factor ∝.
Solution
𝐼𝐸 = 𝐼𝐶 + 𝐼𝐵
𝐼𝐵 = 0.05 𝑚𝐴
𝐼𝐶 0.95𝑚𝐴
∝= = = 0.95
𝐼𝐸 1𝑚𝐴
2. In a common base connection, current amplification factor is 0.9. If the emitter current is 1mA,
determine the value of base current.
Solution
𝐼𝐸 = 𝐼𝐶 + 𝐼𝐵
𝐼𝐶
∝=
𝐼𝐸
𝐼𝐶 =∝ 𝐼𝐸 = 0.9 × 1𝑚𝐴 = 0.9𝑚𝐴
𝐼𝐸 = 𝐼𝐶 + 𝐼𝐵
1 × 10−3 = 0.9 × 10−3 + 𝐼𝐵
𝐼𝐵 = 1 × 10−3 − 0.9 × 10−3
𝐼𝐵 = 0.1 × 10−3 𝐴
𝐼𝐵 = 0.1𝑚𝐴
3. For the common base circuit shown in the figure below, determine I C and VCB. Assume the
transistor to be of silicon (𝑉𝐵𝐸 = 0.7𝑉).
Solution
𝐼𝐸 = 𝐼𝐶 + 𝐼𝐵
Page 35 of 65
𝑉𝐸𝐸 = 𝐼𝐸 𝑅𝐸 + 𝑉𝐵𝐸 thus,
𝑉𝐸𝐸 − 𝑉𝐵𝐸 8 − 0.7
𝐼𝐸 = = = 4.87𝑚𝐴
𝑅𝐸 1.5 × 103
𝐼𝐶 ≅ 𝐼𝐸 = 4.87𝑚𝐴
𝑉𝐶𝐶 = 𝐼𝐶 𝑅𝐶 + 𝑉𝐶𝐵
18 = (4.87 × 10−3 × 1.2 × 103 ) + 𝑉𝐶𝐵
𝑉𝐶𝐵 = 18 − (4.87 × 10−3 × 1.2 × 103 )
𝑉𝐶𝐵 = 12.15𝑉
Common emitter connection
In this circuit arrangement, input is applied between base and emitter and output is taken from collector
and emitter. Here, emitter of the transistor is common to both input and output circuits and hence the
name common emitter connection as shown in the figures below.
This is the highly used transistor biasing method because of its large current gain 𝛽 and high
voltage and power gains. It has a high input impedance too which is good for amplification
purposes.
Base current amplification factor (β) is the ratio of output current (IC) to the input current (IB).
𝐼𝐶
𝛽=
𝐼𝐵
Relationship between 𝜷 𝒂𝒏𝒅 ∝
𝐼𝐶
∝= − − − (𝑖 )
𝐼𝐸
𝐼
𝛽 = 𝐼 𝐶 − − − (𝑖𝑖)
𝐵
Page 36 of 65
𝐼𝐵 = 𝐼𝐸 − 𝐼𝐶
This is a rarely used transistor biasing method. Although it has high current gain, its voltage gain
is low (less than 1).
Current amplification factor ( 𝛾 ) is the ratio of output current (IE) to the input current (IB).
𝐼𝐸
𝛾=
𝐼𝐵
Relationship between 𝜸 and ∝
𝐼𝐸
𝛾= − − − (𝑖 )
𝐼𝐵
𝐼𝐶
∝= − − − (𝑖𝑖 )
𝐼𝐸
From equation
𝐼𝐸 = 𝐼𝐶 + 𝐼𝐵
→ 𝐼𝐵 = 𝐼𝐸 − 𝐼𝐶
Substitute 𝐼𝐵 in equation (i) to get
𝐼𝐸
𝛾= − − − (𝑖𝑖𝑖)
𝐼𝐸 − 𝐼𝐶
From equation (ii), make 𝐼𝐶 the subject and substitute it in equation (iii)
𝐼𝐶 =∝ 𝐼𝐸
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𝐼𝐸
𝛾=
𝐼𝐸 −∝ 𝐼𝐸
𝐼𝐸
𝛾=
𝐼𝐸 (1−∝)
1
𝛾=
1−∝
Transistor Load line analysis
Load line is a line joining the saturation point to the cut-off point of a transistor.
Saturation point is the point when the transistor is fully on. At this point, the transistor acts as a
closed switch.
Cut-off point is the point when the transistor is fully off. At this point, the transistor acts as an
open switch.
Operating point (Q) is the point that gives the values of IC and VCE when no signal is applied to
the input circuit of the transistor.
To get the cut-off and saturation points, we use the output circuit loop and apply the
Kirchhoff’s voltage law.
𝑉𝐶𝐶 = 𝐼𝐶 𝑅𝐶 + 𝑉𝐶𝐸 − − − (𝑖)
Saturation point
At saturation, the transistor is fully closed (acts like a conductor/wire). Thus there is no voltage
drop across the transistor at this point, that is 𝑉𝐶𝐸 = 0𝑉.
Thus equation (i) becomes
𝑉𝐶𝐶 = 𝐼𝐶 𝑅𝐶
𝑉𝐶𝐶
𝐼𝐶,𝑠𝑎𝑡 =
𝑅𝐶
Page 39 of 65
Cut-off point
At cut-off, the transistor is fully open (acts like an open circuit). Thus there is no current through
the transistor at this point, that is 𝐼𝐶 = 0𝐴.
Thus equation (i) becomes
𝑉𝐶𝐶 = 𝑉𝐶𝐸
The load line is thus drawn as shown in figure (ii) above. From the figure, point A is the
saturation-point while point B is the cut-off point.
To get the operating point, the value of 𝐼𝐶 𝑎𝑛𝑑 𝑉𝐶𝐸 is found using equation (i) also. As seen in
figure (ii) above, the operating points varies depending on the base current 𝐼𝐵 . This is true
because 𝐼𝐶 depends on 𝐼𝐵 𝑎𝑛𝑑 𝐼𝐸 .
Example
In the circuit diagram shown in the figure below, if VCC = 12V and RC = 6 kΩ, draw the d.c. load line. What
will be the Q point if zero signal base current is 20µA and β = 50?
Solution
𝑉𝐶𝐶 = 𝐼𝐶 𝑅𝐶 + 𝑉𝐶𝐸
At saturation
𝑉𝐶𝐶
𝐼𝐶,𝑠𝑎𝑡 =
𝑅𝐶
12
𝐼𝐶,𝑠𝑎𝑡 = = 2𝑚𝐴
6 × 103
At cut-off point
𝑉𝐶𝐸 = 𝑉𝐶𝐶 = 12𝑉
Operating point Q is, we find 𝐼𝐶 and 𝑉𝐶𝐸
𝐼𝐶
𝛽=
𝐼𝐵
Page 40 of 65
𝐼𝐶
50 =
20 × 10−6
𝐼𝐶 = 50 × 20 × 10−6 = 1𝑚𝐴
To get 𝑉𝐶𝐸 , we substitute 𝐼𝐶 in the equation 𝑉𝐶𝐶 = 𝐼𝐶 𝑅𝐶 + 𝑉𝐶𝐸
The dc load line and the operating point Q are shown in the figure below
In this method, a high resistance RB (several hundred kΩ) is connected between the base and +ve end of
supply for npn transistor and between base and negative end of supply for pnp transistor as shown in
the figure below.
Page 41 of 65
Circuit analysis
𝐼𝐶
𝛽=
𝐼𝐵
𝐼𝐶
𝐼𝐵 =
𝛽
Applying Kirchhoff’s voltage law at the input circuit (loop),
𝑉𝐶𝐶 = 𝐼𝐵 𝑅𝐵 + 𝑉𝐵𝐸
𝑉𝐶𝐶 − 𝑉𝐵𝐸
𝑅𝐵 =
𝐼𝐵
Applying Kirchhoff’s voltage law at the output circuit (loop),
𝑉𝐶𝐶 = 𝐼𝐶 𝑅𝐶 + 𝑉𝐶𝐸
𝑉𝐶𝐶 − 𝑉𝐶𝐸
𝑅𝐶 =
𝐼𝐶
Example
The figure below shows biasing with base resistor method.
(i) Determine the collector current IC and collector-emitter voltage VCE. Neglect small base-
emitter voltage. Given that β = 50.
(ii) If RB in this circuit is changed to 50 kΩ, find the new operating point
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Solution
𝑉𝐵𝐵 = 𝐼𝐵 𝑅𝐵 + 𝑉𝐵𝐸
2 = 100 × 103 𝐼𝐵 + 0
2
𝐼𝐵 = = 0.02𝑚𝐴
100 × 103
𝐼𝐶
50 =
0.02 × 10−3
𝐼𝐶 = 50 × 0.02 × 10−3 = 1𝑚𝐴
To get 𝑉𝐶𝐸 we use the output circuit
𝑉𝐶𝐶 = 𝐼𝐶 𝑅𝐶 + 𝑉𝐶𝐸
9 = 1 × 10−3 × 2 × 103 + 𝑉𝐶𝐸
𝑉𝐶𝐸 = 9 − (1 × 10−3 × 2 × 103 ) = 7𝑉
The operating point is 7V, 1mA
ii. To get the new operating point we repeat the above steps
𝑉𝐵𝐵 = 𝐼𝐵 𝑅𝐵 + 𝑉𝐵𝐸
2 = 50 × 103 𝐼𝐵 + 0
2
𝐼𝐵 = = 0.04𝑚𝐴
50 × 103
𝐼𝐶
50 =
0.04 × 10−3
Page 43 of 65
𝐼𝐶 = 50 × 0.04 × 10−3 = 2𝑚𝐴
To get 𝑉𝐶𝐸 we use the output circuit
𝑉𝐶𝐶 = 𝐼𝐶 𝑅𝐶 + 𝑉𝐶𝐸
9 = 2 × 10−3 × 2 × 103 + 𝑉𝐶𝐸
𝑉𝐶𝐸 = 9 − (2 × 10−3 × 2 × 103 ) = 5𝑉
The new operating point is 5V, 2mA
Assignment
Draw the dc load line and show the two operating points for the above circuit
Emitter bias method
This circuit differs from base-bias circuit in two important respects. First, it uses two separate dc voltage
sources; one positive (+ VCC) and the other negative (– VEE). Normally, the two supply voltages will be
equal. For example, if VCC = + 20V (dc), then VEE = – 20V (dc). Secondly, there is a resistor RE in the
emitter circuit.
Circuit analysis
Applying Kirchhoff’s voltage law at the input circuit loop
𝑉𝐸𝐸 = 𝐼𝐵 𝑅𝐵 + 𝑉𝐵𝐵 + 𝐼𝐸 𝑅𝐸 − − − (𝑖)
𝐼𝐶
𝛽=
𝐼𝐵
𝐼𝐶 = 𝛽𝐼𝐵
𝐼𝐸 ≅ 𝐼𝐶 = 𝛽𝐼𝐵
𝐼
Thus 𝐼𝐵 = 𝐸 . Substitute it in equation (i) to get
𝛽
Page 44 of 65
𝐼𝐸
𝑉𝐸𝐸 = 𝑅 + 𝑉𝐵𝐵 + 𝐼𝐸 𝑅𝐸
𝛽 𝐵
𝐼𝐸
𝑉𝐸𝐸 − 𝑉𝐵𝐵 = 𝑅 + 𝐼𝐸 𝑅𝐸
𝛽 𝐵
𝑅𝐵
(𝑉𝐸𝐸 − 𝑉𝐵𝐵 ) = 𝐼𝐸 ( + 𝑅𝐸 )
𝛽
𝑉𝐸𝐸 − 𝑉𝐵𝐵
𝐼𝐸 =
𝑅𝐵
𝛽 + 𝑅𝐸
But 𝐼𝐸 ≅ 𝐼𝐶
Thus
𝑉𝐸𝐸 − 𝑉𝐵𝐵
𝐼𝐶 =
𝑅𝐵
𝛽 + 𝑅𝐸
Applying Kirchhoff’s law at the output circuit loop
𝑉𝐶𝐶 = 𝐼𝐶 𝑅𝐶 + 𝑉𝐶𝐸 + 𝐼𝐸 𝑅𝐸 − 𝑉𝐸𝐸 − − − (𝑖𝑖)
But 𝐼𝐸 ≅ 𝐼𝐶
𝑉𝐶𝐸 = 𝑉𝐶𝐶 + 𝑉𝐸𝐸 − 𝐼𝐶 𝑅𝐶 − 𝐼𝐶 𝑅𝐸
𝑉𝐶𝐸 = 𝑉𝐶𝐶 + 𝑉𝐸𝐸 − 𝐼𝐶 (𝑅𝐶 + 𝑅𝐸 )
Biasing with Collector Feedback Resistor
In this method, one end of RB is connected to the base and the other end to the collector as shown in
the figure below.
Circuit Analysis
Page 45 of 65
𝑉𝐶𝐶 = 𝐼𝐶 𝑅𝐶 + 𝐼𝐵 𝑅𝐵 + 𝑉𝐵𝐸
Applying Kirchhoff’s voltage law at the output circuit loop
𝑉𝐶𝐶 = 𝐼𝐶 𝑅𝐶 + 𝑉𝐶𝐸
Voltage Divider Bias Method
This is the most widely used method of providing biasing and stabilization to a transistor.
In this method, two resistances R1 and R2 are connected across the supply voltage VCC and provide
biasing. The emitter resistance RE provides stabilization.
The name ‘‘voltage divider’’ comes from the voltage divider formed by R1 and R2. The voltage drop
across R2 forward biases the base emitter junction. This causes the base current and hence collector
current flow in the zero signal conditions.
Circuit Analysis
𝑅2
The biasing voltage across R2 is given by 𝑉2 = 𝑅 𝑉𝐶𝐶
1 +𝑅2
Page 46 of 65
Used as a switch
Used in impedance marching circuits
Silicon versus Germanium
Although both silicon and germanium are used in semiconductor devices.
Page 47 of 65
drain (D) taken out from the bar as shown.
A JFET has three terminals namely Gate (G), Source (S), and Drain (D).
JFET polarities
Regardless of the type of JFET, the voltage between the gate and source is such that the gate is
reverse biased as shown in figure (i) and (ii) below. This is the normal way of JFET connection.
Working of a JFET
When a voltage VDS is applied between drain and source terminals and voltage on the gate (VGS)
is zero as shown in figure (i), the two p-n junctions at the sides of the bar establish depletion
layers. The electrons will flow from source to drain through a channel between the depletion
layers. The size of these layers determines the width of the channel and hence the current
conduction through the bar.
When a reverse voltage VGS is applied between the gate and source as shown in figure (ii), the
width of the depletion layers is increased. This reduces the width of conducting channel, thereby
increasing the resistance of n-type bar. Consequently, the current from source to drain is
decreased. On the other hand, if the reverse voltage on the gate is decreased, the width of the
depletion layers also decreases. This increases the width of the conducting channel and hence
source to drain current.
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Schematic (circuit) symbol of a JFET
The figure below shows the circuit symbols of the two types of JFETs.
Applications of JFET
Used as a switch
Used as an amplifier
Metal Oxide Semiconductor FET (MOSFET)
Unlike a JFET that can only work in the depletion mode only (reducing the width of the
conduction channel), a MOSFET can be operated in two modes, that is, the depletion mode
(decreasing the width of the conduction channel) and the enhancement mode (increasing the
width of the conduction channel).
Page 49 of 65
Types of MOSFETs
i. Depletion-type MOSFET or D-MOSFET.
The D-MOSFET can be operated in both the depletion-mode and the enhancement-mode. For
this reason, a D-MOSFET is sometimes called depletion/enhancement MOSFET.
Types of D-MOSFET
N-channel D-MOSFET
P-channel D-MOSFET
ii. Enhancement-type MOSFET or E-MOSFET.
The E-MOSFET can be operated only in enhancement-mode.
Types of E-MOSFET
N-channel E-MOSFET
P-channel E-MOSFET
Construction of D-MOSFET
It consists of a p-type or n-type silicon bar containing a n-type or p-type substrate at right hand
side as shown in below. The bar forms the conducting channel for the charge carriers. If the bar
is of n-type, it is called n-channel D-MOSFET as shown in figure below and if the bar is of p-
type, it is called a p-channel D-MOSFET. The gate terminal is deposited on a thin layer of
silicon (IV) oxide which is deposited at one side (left hand side) of the channel. Other terminals
are source (S) and drain (D) taken out from the bar as shown. The substrate is internally
connected to the source terminal.
Construction of E-MOSFET
It consists of a p-type or n-type silicon bar (channel) containing a n-type or p-type substrate at
right hand side that extends across the channel thus dividing the channel into two parts as shown
in below. The bar forms the conducting channel for the charge carriers. If the bar is of n-type, it
is called n-channel E-MOSFET as shown in figure below and if the bar is of p-type, it is called a
p-channel E-MOSFET. The gate terminal is deposited on a thin layer of silicon (IV) oxide which
is deposited at one side (left hand side) of the channel. Other terminals are source (S) and drain
(D) taken out from the bar as shown. The substrate is internally connected to the source terminal.
Page 50 of 65
Circuit symbol of the types of MOSFETs
Figure (i) below shows a n-channel E-MOSFET while figure (ii) shows a p-channel E-MOSFET.
Operation of D-MOSFET
Depletion Mode
Page 51 of 65
Considering the circuit shown below, the current is initially flowing through the n-channel from
the drain to the source. When the gate source voltage (VGG) is increased, the gate terminal
become negatively charged thus repelling the electrons in the n-channel. The repelled electrons
leaves behind holes near the gate terminal and the p-layer grows towards the gate terminal. As
more gate source voltage is applied, the p-layer (substrate) eventually cuts the channel in two
parts thus depleting the flow of electrons (current) through the channel.
Enhancement Mode
Considering the circuit shown below, the current is initially flowing through the n-channel from
the drain to the source. When the gate source voltage (VGG) is increased, the gate terminal
become positively charged thus attracting the electrons in the n-channel. The attracted electrons
increases the concentration of electrons near the gate terminal thus diffuse into the p-layer
(substrate) to fill the holes. As a result, the substrate (p-layer) decreases and thus increases
(enhances) the n-channel and consequently increases the current through the channel.
Operation of E-MOSFET
This type of MOSFET only operates in the enhancement mode.
Initially when there is no gate source voltage applied at the gate, no current that can flow through
the channel of the MOSFET. When a positive gate source voltage (VGS) is applied, electrons gets
attracted from the n-channel to fill the holes in the p-substrate near the gate terminal and current
starts to flow through the n-channel. As more gate voltage is applied, more electrons gets
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attracted to the gate terminal leading to decreasing in the length of the p-substrate. Consequently,
the current through the channel increases (conduction of the n-channel is enhanced).
Applications of MOSFTETs
Used in switching circuits
Used in power control circuits
Used in amplifier circuits
Used in DC motor drives
Values and ratings of electronic components
Resistor colour codes
We can determine the value of a resistor using its colour code or using an ohmmeter (or a
multimeter).
Here, we will determine the value or size of a resistor using its color code. A resistor (fixed) has
different colour bands printed on its body like the one shown in the figure below. To measure the
resistance of a resistor using a multimeter, the appropriate ohmmeter scale is selected and the
two leads (red and black leads) of the multimeter connected to the two ends of the resistor. The
value of the resistance will be displayed on the screen of the multimeter.
The colour bands and their corresponding values are shown in the table below.
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Depending on the size of the transistor, it can have three, four, five or six colour bands. The last
one two colour bands are for tolerance and temperature coefficients respectively.
For a 3-band or 4-band resistor
The first two bands always denote the first two digits of the resistance value in ohms .
On a three or four-band resistor, the third band represents the multiplier (power of 10).
The fourth color band signifies tolerance. Keep in mind that if this band is absent and you
are looking at a three-band resistor, the default tolerance is ±20%.
Page 54 of 65
For 5 or 6 Band Resistors
Resistors with high precision have an extra color band to indicate a third significant digit. If your
resistor has five or six color bands, the third band becomes this additional digit of the
resistance value along with bands one and two. Everything else shifts to the right, making the
fourth color band the multiplier and the fifth band the tolerance. A six-band indicates the
reliability, or the temperature coefficient (ppm/K) specification. Using brown, the most
common sixth band color, as an example, every temperature change of 10°C changes the
resistance value by 0.1%.
Page 55 of 65
For ceramic capacitors like the one shown in the figure below, to get the capacitance of the
capacitor, we take the first two digits on the capacitor body to be the first two figures of the
capacitance. The third digit on the capacitor body is called the multiplier and is usually written as
the power of 10 and it is multiplied to the first two figures obtained previously. The letter written
together with the number on the capacitor body is used to indicate the tolerance of the capacitor.
If the number is two digits, the multiplier is assumed to be zero. The tolerances corresponding to
the respective letters are shown in the table in the figure below. The value is expressed in Pico
Farads (pF).
Note: 1 pF = 10−12 𝐹
For example, the value of the capacitance of the capacitor shown below is 10 × 104 𝑝𝐹
Page 56 of 65
Assignment
Determine the capacitance of the following capacitors. Leave your answer in farads (F).
This process of converting ac voltage into dc voltage is called rectification and is accomplished with the
help of a rectifier, filter, and voltage regulator circuit. These elements put together constitute dc power
supply.
There are two types of dc power supplies: unregulated dc power supply, and regulated dc power
supply.
An unregulated power supply is one whose dc terminal voltage is affected significantly by the amount of
load. As the load draws more current, the dc terminal voltage becomes less.
It is that dc power supply whose terminal voltage remains almost constant regardless of the amount of
current drawn from it. An unregulated supply can be converted into a regulated power supply by adding
a voltage regulating circuit to it.
The figure below shows a basic block diagram of a regulated dc power supply
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A typical dc power supply consists of five stages as shown in Fig. 55.1.
1. Transformer
Its job is either to step up or (mostly) step down the ac supply voltage to suit the requirement of the
solid-state electronic devices and circuits fed by the dc power supply. It also provides isolation from the
supply line–an important safety consideration.
2. Rectifier
It is a circuit which employs one or more diodes to convert ac voltage into pulsating dc voltage.
3. Filter
The function of this circuit element is to remove the fluctuations or pulsations (called ripples) present in
the output voltage supplied by the rectifier. Of course, no filter can, in practice, gives an output voltage
as ripple-free as that of a dc battery but it approaches it so closely that the power supply performs as
well.
4. Voltage Regulator
Its main function is to keep the terminal voltage of the dc supply constant even when
(i) Ac input voltage to the transformer varies (deviations from 220 V are common); or
Usually, Zener diodes and transistors are used for voltage regulation purposes. Again, it is impossible to
get 100% constant voltage but minor variations are acceptable for most of the jobs.
5. Voltage Divider
Its function is to provide different dc-voltages needed by different electronic circuits. It consists of a
number of resistors connected in series across the output terminals of the voltage regulator. Obviously,
it eliminates the necessity of providing separate dc power supplies to different electronic circuits
working on different dc levels.
Rectifier
Working
During the positive half-cycle of the input ac voltage, the diode D is forward-biased (ON) and conducts.
While conducting, the diode acts as a short-circuit so that circuit current flows and hence, positive half-
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cycle of the input ac voltage is dropped across RL. During the negative input half-cycle, the diode is
reverse-biased (OFF) and so, does not conduct i.e. there is no current flow. Hence, there is no voltage
drop across RL. For this reason, the negative half cycle of the supply voltage is not part of the output
voltage. It constitutes the output voltage VL as shown in figure (b). Waveform of the load voltage is also
shown in figure (b). It consists of halfwave rectified sinusoids of peak value VLM.
The full-wave rectifier circuit can be made from a center tap transformer and two diodes or a normal
transformer and bridge circuit of four diodes.
Working
The figure above shows a full-wave center-tapped rectifier circuit. During the positive half cycle of the
supply voltage, M is positive while N is negative w.r.t G. For this reason, the diodes D1 is forward bias
while D2 is reverse bias. Load current will flow through the D1. During the negative half cycle of the
supply voltage, N is positive while M is negative w.r.t G, thus D2 is forward bias while D1 is reverse bias.
The load current now flows through D2.
The waveforms for the input and output voltages are similar to those of a full-wave bridge rectifier
shown below.
(i) It requires only two diodes unlike the bridge rectifier which requires four diodes.
(ii) As during each half-cycle of ac input only one diode conducts, the voltage drop in the
internal resistance is low compared to that of bridge rectifier.
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(i) It is difficult to locate the center tap on the secondary winding.
(ii) The dc output is small as each diode utilizes only one-half of the transformer secondary
voltage.
(iii) The diodes used must have high peak inverse voltage
Working
This circuit consist of a transformer without center tap and four diodes as shown in the figure below.
During the positive half cycle of supply voltage, M is positive w.r.t N. Thus, diodes D1, D3 are forward
bias while diodes D2, D4 are reverse bias. Load current flows through D1 and D3. During the negative
half cycle of the supply voltage, N is positive w.r.t M and thus diodes D2, D4 are forward bias while
diodes D1, D3 are reverse bias. The load current now flows through D2 and D4. The waveforms of the
input and output voltage are shown in figure (b) below. A shown in the output voltage waveforms,
voltage appear across the load (current flows through the load) in both half cycles and that is why it is
known as full-wave rectifier.
Filter Circuits
As shown in the above waveforms, for full-wave and half-wave rectifiers, there is oscillating factor which
is not needed by dc components operated by the dc supply. Therefore, smoothing or filtering is needed
to remove (reduce) the ripple effect and regulation to stabilize the output voltage.
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A filter circuit is a device which removes the ac component of rectifier output but allows the dc
component to reach the load.
NB: A capacitor passes ac readily but does not pass dc at all while on the other hand, an inductor
opposes ac but allows dc to pass through it.
Capacitor filter,
Choke input filter
Capacitor input filter or π-filter.
Capacitor Filter
It consists of a capacitor C placed across the rectifier output in parallel with load R L.
Operation
The pulsating direct voltage of the rectifier is applied across the capacitor. As the rectifier voltage
increases, it charges the capacitor and also supplies current to the load. At the end of quarter cycle
[Point A in figure (iii)], the capacitor is charged to the peak value Vm of the rectifier voltage. Now, the
rectifier voltage starts to decrease. As this occurs, the capacitor discharges through the load and voltage
across it (i.e. across parallel combination of R-C) decreases as shown by the line AB in figure (iii). The
voltage across load will decrease only slightly because immediately the next voltage peak comes and
recharges the capacitor. This process is repeated again and again and the output voltage waveform
becomes ABCDEFG.
NB: The capacitor filter circuit is extremely popular because of its low cost, small size, little weight and
good characteristics. For small load currents (say upto 50 mA), this type of filter is preferred. It is
commonly used in transistor radio battery eliminators.
The figure below shows a typical choke input filter circuit. It consists of a choke L connected in series
with the rectifier output and a filter capacitor C across the load. Only a single filter section is shown, but
several identical sections are often used to reduce the pulsations as effectively as possible.
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Operation
The pulsating output of the rectifier is applied across terminals 1 and 2 of the filter circuit. The choke
offers high opposition to the passage of ac component but negligible opposition to the dc component.
The result is that most of the ac component appears across the choke while whole of dc component
passes through the choke on its way to load. This results in the reduced pulsations at terminal 3. At
terminal 3, the rectifier output contains dc component and the remaining part of ac component which
has managed to pass through the choke. Now, the low reactance of filter capacitor bypasses the ac
component but prevents the dc component to flow through it. Therefore, only dc component reaches
the load.
Operation
The pulsating output from the rectifier is applied across the input terminals (i.e. terminals 1 and 2) of
the filter. The filtering action of the three components C1, L and C2 of this filter is described below
(a) The filter capacitor C1 offers low reactance to ac component of rectifier output while it offers infinite
reactance to the dc component. Therefore, capacitor C1 bypasses an appreciable amount of ac
component while the dc component continues its journey to the choke L.
(b) The choke L offers high reactance to the ac component but it offers almost zero reactance to the dc
component. Therefore, it allows the dc component to flow through it, while the un-bypassed ac
component is blocked.
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(c) The filter capacitor C2 bypasses the ac component which the choke has failed to block. Therefore,
only dc component appears across the load and that is what we desire.
The figures below shows simple circuits of how the filter circuits are incorporated to the rectifier circuit.
Voltage Multipliers
A voltage multiplier is a circuit which produces a greater dc output voltage than ac input voltage to the
rectifiers.
Multipliers are required in many circuit applications where it is necessary to have high voltages with low
currents as for electron accelerating purposes in a cathode-ray tube (CRT).
For example, a voltage doubler will provide a dc output that is twice the peak input ac voltage, a voltage
tripler will provide a dc output that is three times the peak input ac voltage and so on.
While voltage multipliers provide dc output that is much greater than the peak input ac voltage, there is
no power amplification and law of conservation of energy holds good. When a voltage multiplier
increases the peak input voltage by a factor n, the peak input current is decreased by approximately the
same factor.
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Operation
During the positive half-cycle of the input voltage, D1 conducts (not D2) and charges C1 to peak value of
secondary voltage (V m) with the polarity as shown in figure (a). During the negative half-cycle, D2
conducts (not D1) and charges C2. The voltage across C2 is the sum of peak supply voltage and the
voltage across C1 (C1 will be discharging to C2). At this point the voltage across the capacitor C2 and
hence across the load will be twice that at the secondary coil of the transformer. During the next
positive half-cycle, D2 is open and C2 will discharge through the load while C1 charges to peak voltage
and the above process is repeated for the subsequent half cycles.
Note: This circuit has very poor regulation and its ripple content is also high. This circuit has a common
connection between the line and load (which a full-wave doubler does not have).
Operation
During the positive half-cycle of the input voltage, D1 conducts (but not D2) and charges capacitor C1 to
the peak voltage Vm with the polarity as shown. During the negative half-cycle, D2 conducts (but not
D1) charging C2 to Vm. As far as the load is concerned, voltages across C1 and C2 are in series-aiding. If
there is a load connected across the output (across the two capacitors), then load voltage VL = 2Vm as
shown in figure (a).
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Operation
During the first positive half-cycle, C1 charges to Vm as diode D1 conducts. During negative half-cycle,
C2 is charged through D2 to 2Vm (i.e. to the sum of voltage across C1 and peak input voltage Vm).
During the second positive half-cycle, D3 conduct and voltage across C2 charges C3 to same voltage 2Vm
(C1 cannot charge C3 because it is shorted by D1). During the negative half-cycle, diodes D2 and D4
conduct allowing C3 to charge C4 to the same peak voltage 2Vm. If is seen from Fig. 55.34 that voltage
across C2 is 2V m, across C1 and C3 is 3V m and across C2 and C4 is 4V m. If additional diodes and
capacitors are used, each capacitor would be charged to a peak voltage of 2Vm.
When voltage is taken across diode D3, its value would be triple that of the supply (tripler) and if voltage
is taken across diode d4, its value would be four times that of the supply (quadrupler).
Note: The basic idea in a voltage multiplier is to charge each capacitor to the peak input a.c. voltage and
to arrange the capacitors so that their stored voltages will add.
Voltage Stabilization
In many electronic applications, it is desired that the output voltage should remain constant regardless
of the variations in the input voltage or load. In order to ensure this, a voltage stabilizing device, called
voltage stabilizer is used.
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