GXEST104: Introduction to Electrical & Electronics Engineering

Module III
INTRODUCTION TO ELECTRONIC DEVICES
1. Passive and active components in electronics
1.1 Active Components
• Definition: Components that can inject power into a circuit and require an external power
source to operate.
• Function: Capable of amplifying signals, controlling current flow, and generating signals.
• Examples:
o Transistors: Used for amplification and switching.
o Integrated Circuits (ICs): Contain multiple transistors and other components to
perform complex functions.
o Diodes: Allow current to flow in one direction, used for rectification.
o Light Emitting Diodes (LEDs): Emit light when current flows through them.
1.2 Passive Components
• Definition: Components that do not require an external power source and cannot amplify
signals. They can only store or dissipate energy.
• Function: Primarily used to store, filter, or control the flow of electrical signals.
• Examples:
o Resistors: Control the flow of electrical current by offering resistance.
o Capacitors: Store electrical energy temporarily in an electric field.
o Inductors: Store energy temporarily in a magnetic field and resist changes in current.
o Transformers: Transfer electrical energy between circuits through electromagnetic
induction.

Parameter Active Component Passive Component

Nature of Source It delivers energy/power to the circuit. It utilizes energy/power of the circuit.

Function These are the devices which produces These are the devices which store energy
energy in the form of voltage/ current. in the form of voltage/ current.

Power gain Capable of providing power gain. Incapable of providing power gain
Requirement of Requires external source for operation. Does not require any external source for
external source operation.

Nature of Energy It acts as energy donor. It acts as energy acceptor.

Examples Diode, Transistors, ICs Resistors, Capacitors, Inductors

1
 GXEST104: Introduction to Electrical & Electronics Engineering

❖ Resistors Classification
Unit: Ohm (Ω).

1.Fixed Resistors: Fixed resistors are electronic components with a resistance value that does not
change. They are used to limit the current flow, divide voltages, and in various other applications where
a stable and predictable resistance is required.
Examples:
1. Carbon Composition Resistors: Made from a mixture of carbon powder and a binding
material. They are inexpensive but have higher noise and lower precision.
2. Metal Film Resistors: Made by depositing a thin layer of metal onto a ceramic substrate. They
offer better precision and stability compared to carbon composition resistors.
3. Wire-Wound Resistors: Made by winding a metal wire (usually nichrome) around a ceramic,
plastic, or fiberglass core. They can handle higher power levels and are very precise.
4. Thick and Thin Film Resistors: Made by applying a thick or thin layer of resistive material
onto an insulating substrate. They are used in applications requiring high precision and stability.
2.Variable Resistors: Variable resistors are components whose resistance can be adjusted. They are
commonly used to control current or voltage in a circuit. By changing the resistance, they can regulate
the flow of electrical energy.
Examples:
1. Potentiometers: These have three terminals and are used to adjust voltage. They function as
adjustable voltage dividers.
2. Rheostats: These have two terminals and are used to control current. They function by varying
the resistance in a circuit.
3. Preset: A preset is a type of variable resistor that is designed to be set or adjusted during the
manufacturing or calibration process and then left in that position. Unlike regular variable
resistors, presets are not meant to be adjusted frequently by the end user.

❖ Capacitor Classification
Unit: Farad (F).

Capacitors are electronic components that store and release electrical energy. They are classified based
on their construction, dielectric material, and application. Here are the main classifications and some
examples:
1. Fixed Capacitors: These capacitors have a fixed capacitance value that cannot be changed.
• Ceramic Capacitors: Made from ceramic materials. They are non-polarized and used in high-
frequency applications.
o Example: Multilayer Ceramic Capacitor (MLCC)
• Film Capacitors: Use a thin plastic film as the dielectric. They are known for their stability
and low inductance.
o Example: Polyester Film Capacitor

2
 GXEST104: Introduction to Electrical & Electronics Engineering

• Electrolytic Capacitors: Use an electrolyte as the dielectric. They are polarized and have high
capacitance values.
o Example: Aluminum Electrolytic Capacitor
• Tantalum Capacitors: A type of electrolytic capacitor with tantalum as the anode. They are
known for their reliability and stability.
o Example: Solid Tantalum Capacitor
• Mica Capacitors: Use mica as the dielectric. They are stable and used in high-frequency
applications.
o Example: Silver Mica Capacitor
2. Variable Capacitors: These capacitors have an adjustable capacitance value.
• Tuning Capacitors: Used in radio frequency circuits to tune the frequency.
o Example: Air Variable Capacitor
• Trimmer Capacitors: Small capacitors used for fine adjustments in circuits.
o Example: Ceramic Trimmer Capacitor
3. Supercapacitors: Also known as ultracapacitors, they have very high capacitance values and are
used for energy storage.
• Double-Layer Capacitors: Store energy in an electrostatic double layer.
o Example: Electric Double-Layer Capacitor (EDLC)
• Pseudo-Capacitors: Store energy through electrochemical reactions.
o Example: Hybrid Supercapacitor
Applications:
• Ceramic Capacitors: Used in decoupling and filtering applications.
• Film Capacitors: Used in power supplies and audio circuits.
• Electrolytic Capacitors: Used in power supply filtering and energy storage.
• Tantalum Capacitors: Used in space-constrained applications like mobile phones.
• Supercapacitors: Used in backup power supplies and energy harvesting systems.

❖ Inductors Classifications
Unit: Henry (H).
Inductors are passive electronic components that store energy in a magnetic field when electric current
flows through them. They are classified based on their core material, construction, and application. Here
are the main classifications:
1. Air Core Inductors:
• Description: These inductors use air as the core material.

3
 GXEST104: Introduction to Electrical & Electronics Engineering

• Features: They have low inductance and are used in high-frequency applications.
• Applications: RF tuning coils, filter circuits, and high-frequency applications like TV and radio
receivers.
2. Iron Core Inductors:
• Description: These inductors use iron as the core material.
• Features: They have higher inductance and are used in low-frequency applications.
• Applications: Power supplies, audio equipment, and transformers.
3. Ferrite Core Inductors:
• Description: These inductors use ferrite, a ceramic compound consisting of iron oxide mixed
with other metals.
• Features: They offer high inductance and are used in both high-frequency and low-frequency
applications.
• Applications: Power inductors, EMI filters, and transformers.
4. Laminated Core Inductors:
• Description: These inductors have a core made of laminated steel sheets.
• Features: They reduce eddy current losses and are used in power applications.
• Applications: Power transformers, inductors in power supplies, and line filters.
5. Toroidal Inductors:
• Description: These inductors have a doughnut-shaped core, usually made of ferrite or
powdered iron.
• Features: They offer high inductance with minimal electromagnetic interference (EMI).
• Applications: Power supplies, audio equipment, and EMI suppression.
6. Powdered Iron Core Inductors:
• Description: These inductors use powdered iron as the core material.
• Features: They provide stable inductance over a wide range of frequencies.
• Applications: RF circuits, power supplies, and signal processing.
7. Multi-Layer Inductors:
• Description: These inductors are made by stacking multiple layers of conductive material
separated by insulating layers.
• Features: They are compact and suitable for high-frequency applications.
• Applications: Mobile devices, RF circuits, and high-density electronic circuits.

4
 GXEST104: Introduction to Electrical & Electronics Engineering

Colour Coding of Resistors

Homework: Prepare notes on Semiconductors, dopping, P type and N type Semiconductors

2. Working of PN junction diode

Fig: PN junction diode symbol

2.1 Structure:
• P-type Region: Contains holes (positive charge carriers).
• N-type Region: Contains electrons (negative charge carriers).
• Depletion Region: Area around the junction where no free charge carriers exist.

5
 GXEST104: Introduction to Electrical & Electronics Engineering

2.2 Formation of Depletion Region

• When the P-type and N-type materials are joined, electrons from the N-region diffuse into the
P-region and recombine with holes.
• This creates a region devoid of free charge carriers, known as the depletion region, which acts
as a barrier.
2.3 Biasing of the Diode

• Forward Bias: Positive voltage applied to the P-region and negative to the N-region.
o Reduces the width of the depletion region.
o Allows current to flow through the diode.
• Reverse Bias: Positive voltage applied to the N-region and negative to the P-region.
o Increases the width of the depletion region.
o Prevents current from flowing through the diode.
2.4 Current Flow
• Forward Bias: Electrons move from N to P region, and holes move from P to N region,
allowing current to flow.
• Reverse Bias: The depletion region widens, blocking the flow of current.

3. V-I characteristics of PN Junction diode

3.1 Forward Biased Condition

• Behaviour:
o Initially, the current is very small due to the barrier potential.
o Once the applied voltage exceeds the barrier potential (cut-in voltage), the current
increases rapidly.

6
 GXEST104: Introduction to Electrical & Electronics Engineering

o Cut-in Voltage: For silicon diodes, it’s approximately 0.7V; for germanium diodes, it’s
about 0.3V.

Knee Voltage
• Definition: The minimum forward voltage at which a diode starts to
conduct significantly.
• Characteristics:
o Also known as cut-in voltage.
o For silicon diodes, it’s approximately 0.7V.
o For germanium diodes, it’s around 0.3V.
• Behaviour: Below this voltage, the current through the diode is very
small. Once the knee voltage is reached, the current increases rapidly.

3.2 Reverse Bias

• Behaviour:
o The current is very small (almost zero) because the depletion region widens, increasing
resistance.
o Breakdown Voltage: If the reverse voltage exceeds a certain threshold, the diode will
conduct a large current, potentially damaging it.

Avalanche Breakdown
• Definition: A phenomenon where a high reverse voltage causes a
large increase in current through the diode.
• Mechanism:
o When the reverse voltage is high, the electric field across the
junction becomes strong enough to accelerate free electrons.
o These high-energy electrons collide with atoms, creating
more free electrons.
o This chain reaction results in a sudden increase in current.
• Characteristics:
o Occurs at high reverse voltages, typically above 5V.
o Can cause permanent damage to the diode if not controlled.

7
 GXEST104: Introduction to Electrical & Electronics Engineering

3.3 VI Characteristics Curve

• Forward Bias Region: Shows a rapid increase in current after the cut-in voltage.
• Reverse Bias Region: Shows minimal current until the breakdown voltage is reached.

Reverse Saturation Current: A

small current that flows through a
diode even when it is reverse-biased.

4. Zener diode and avalanche breakdown

Fig: Zener diode symbol

❖ Zener diode
Definition:
• A Zener diode is a special type of diode designed to allow current to flow in the reverse direction
when a specific reverse voltage, known as the Zener breakdown voltage, is reached.
Working Principle:
• Forward Bias: Operates like a regular diode, allowing current to flow from the anode to the
cathode.
• Reverse Bias: When the reverse voltage exceeds the Zener breakdown voltage, the diode
conducts in the reverse direction without damage.

8
 GXEST104: Introduction to Electrical & Electronics Engineering

Applications:
• Voltage Regulation: Maintains a constant output voltage despite variations in the input voltage
or load conditions.
• Protection Circuits: Protects sensitive electronic components from voltage spikes.

❖ Avalanche Breakdown
Definition
• A phenomenon where a high reverse voltage causes a large increase in current through the
diode due to the ionization of atoms.
Mechanism: When a significant reverse voltage (Typically above 5V) is applied, the electric field
across the junction becomes strong and the electrons gain enough energy to accelerate and collide with
atoms, creating more free electrons. This process continues, leading to a rapid increase in current. This
will permanently damage the diode if not controlled.

Comparison with Zener Breakdown

• Zener Breakdown: Occurs at lower voltages (typically below 5V) and is due to a strong
electric field in a heavily doped junction.
• Avalanche Breakdown: Occurs at higher voltages and is due to the collision of electrons with
atoms.

5. Basics of Zener voltage regulator

Zener diode is a silicon semiconductor with a p-n junction that is specifically designed to work in the
reverse biased condition. When forward biased, it behaves like a normal signal diode, but when the
reverse voltage is applied to it, the voltage remains constant for a wide range of currents. Due to this
feature, it is used as a voltage regulator in d.c. circuit. The primary objective of the Zener diode as
a voltage regulator is to maintain a constant voltage. Let us say if Zener voltage of 5 V is used then, the
voltage becomes constant at 5 V, and it does not change.

9
 GXEST104: Introduction to Electrical & Electronics Engineering

5.1 What is a Voltage Regulator?

A voltage regulator is a device that regulates the voltage level. It essentially steps down the input voltage
to the desired level and keeps it at that same level during the supply. This ensures that even when a load
is applied the voltage doesn’t drop. The voltage regulator is used for two main reasons, and they are:
• To vary or regulate the output voltage
• To keep the output voltage constant at the desired value despite variations in the supply voltage.
Voltage regulators are used in computers, power generators, alternators to control the output of the plant.

5.2 Zener diode as a voltage regulator

There is a series resistor connected to the circuit in order to limit the current into the diode. It is
connected to the positive terminal of the d.c. It works in such a way the reverse-biased can also work
in breakdown conditions. We do not use ordinary junction diode because the low power rating diode
can get damaged when we apply reverse bias above its breakdown voltage. When the minimum input
voltage and the maximum load current is applied, the Zener diode current should always be minimum.
Since the input voltage and the required output voltage is known, it is easier to choose a Zener diode
with a voltage approximately equal to the load voltage, i.e. VZ = VL.
The circuit diagram of a voltage regulator using a Zener diode is shown:
RS=(VL-VZ) IL

Current through the diode increases when the voltage across the diode tends to increase which results
in the voltage drop across the resistor. Similarly, the current through the diode decreases when the
voltage across the diode tends to decrease. Here, the voltage drop across the resistor is very less, and
the output voltage results normally.

6. DC Power Supply
A regulated power supply can convert unregulated an AC (alternating current or voltage) to a constant
DC (direct current or voltage). A regulated power supply is used to ensure that the output remains
constant even if the input changes. A regulated DC power supply is also called as a linear power supply,
it is an embedded circuit and consists of various blocks.
The regulated power supply will accept an AC input and give a constant DC output. Figure below shows
the block diagram of a typical regulated DC power supply

10
 GXEST104: Introduction to Electrical & Electronics Engineering

The basic building blocks of a regulated DC power supply are as follows:

1. A step-down transformer
2. A rectifier
3. A DC filter
4. A regulator

Step Down Transformer

A step-down transformer will step down the voltage from the ac mains to the required voltage level.
The turn’s ratio of the transformer is so adjusted such as to obtain the required voltage value. The output
of the transformer is given as an input to the rectifier circuit.

Rectification

Rectifier is an electronic circuit consisting of diodes which carries out the rectification process.
Rectification is the process of converting an alternating voltage or current into corresponding direct
(DC) quantity. The input to a rectifier is ac whereas its output is unidirectional pulsating DC. Usually,
a full wave rectifier or a bridge rectifier is used to rectify both the half cycles of the ac.

DC Filter

The rectified voltage from the rectifier is a pulsating DC voltage having very high ripple content. But
this is not we want, we want a pure ripple free DC

Regulation

This is the last block in a regulated DC power supply. The output voltage or current will change or
fluctuate when there is change in the input from ac mains or due to change in load current at the output
of the regulated power supply or due to other factors like temperature changes. This problem can be
eliminated by using a regulator. A regulator will maintain the output constant even when changes at the

11
 GXEST104: Introduction to Electrical & Electronics Engineering

input or any other changes occur. Transistor series regulator, Fixed and variable IC regulators or a Zener
diode operated in the Zener region can be used depending on their applications.

7. Diode as Rectifiers
The main application of p-n junction diode is in rectification circuits. These circuits are used to describe
the conversion of a.c signals to d.c in power supplies. Diode rectifier gives an alternating voltage which
pulsates in accordance with time. The filter smoothes the pulsation in the voltage and to produce d.c
voltage, a regulator is used which removes the ripples.
There are two primary methods of diode rectification:
• Half Wave Rectifier
• Full Wave Rectifier
7.1 Half Wave Rectifier
A half-wave rectifier is the simplest form of the rectifier and requires only one diode for the construction
of a halfwave rectifier circuit.
A halfwave rectifier circuit consists of three main components as follows:
• A diode
• A transformer
• A resistive load

Working of Half Wave Rectifier

1. A high AC voltage is applied to the primary side of the step-down transformer. The obtained
secondary low voltage is applied to the diode.
2. The diode is forward biased during the positive half cycle of the AC voltage and reverse biased
during the negative half cycle.
3. The final output voltage waveform is as shown in the figure below:

12
 GXEST104: Introduction to Electrical & Electronics Engineering

Half wave Rectifier with capacitor filter

The half wave rectifier converts the Alternating Current (AC) into Direct Current (DC). But the
obtained Direct Current (DC) at the output is not a pure Direct Current (DC). It is a pulsating Direct
Current (DC). The pulsating Direct Current (DC) is not constant. It fluctuates with respect to time.
When this fluctuating Direct Current (DC) is applied to any electronic device, the device may not work
properly. Sometimes the device may also be damaged. So, the fluctuating Direct Current (DC) is not
useful in most of the applications. Therefore, we need a Direct Current (DC) that does not fluctuate with
respect to time. The only solution for this is smoothing the fluctuating Direct Current (DC). This can be
achieved by using a device called filter.
The pulsating Direct Current (DC) contains both AC and DC components. DC components are useful,
but AC components are not useful. So, we need to reduce or completely remove the AC components. By
using the filter, we can reduce the AC components at the output. The filter is an electronic device that
allows dc components and blocks the ac components of the rectifier output. The filter is made up of a
combination of components such as capacitors, resistors, and inductors. The capacitor allows the ac
component and blocks the dc component. In the below circuit diagram, the capacitor C is connected in
shunt with load resistor (RL).

Working:
When AC voltage is applied, during the positive half cycle, the diode D is forward biased and
allows electric current through it. As we already know that the capacitor provides high resistive path to
dc components (low-frequency signal) and low resistive path to ac components (high-frequency signal).
Electric current always prefers to flow through a low resistance path. So, when the electric current

13
 GXEST104: Introduction to Electrical & Electronics Engineering

reaches the filter, the dc components experience a high resistance from the capacitor and ac components
experience a low resistance from the capacitor. The dc components do not like to flow through the
capacitor (high resistance path). So, they find an alternative path (low resistance path) and flows to the
load resistor (RL) through that path.

On the other hand, the ac components experience a low resistance from the capacitor. So the ac
components easily passes through the capacitor. Only a small part of the ac components passes through
the load resistor (RL) producing a small ripple voltage at the output. The passage of ac components
through the capacitor is nothing but charging of the capacitor. During the conduction period, the
capacitor charges to the maximum value of the supply voltage. When the voltage between the plates of
the capacitor is equal to the supply voltage, the capacitor is said to be fully charged. When the capacitor
is fully charged, it holds the charge until the input AC supply to the rectifier reaches the negative half
cycle.
When the negative half cycle is reached, the diode D gets reverse biased and stops allowing electric
current through it. During this non-conduction period, the input voltage is less than that of the capacitor
voltage. So, the capacitor discharges all the stored charges through the load resistor RL. This prevents
the output load voltage from falling to zero. The capacitor discharges until the input supply voltage is
less than the capacitor voltage. When the input supply voltage is greater than the capacitor voltage, the
capacitor again starts charging. When the positive half cycle is reached again, the diode D is forward
biased and allows electric current. This makes capacitor to charge again.

Ripple Factor of Half Wave Rectifier

Ripple factor determines how well a halfwave rectifier can convert AC voltage to DC voltage.

14
 GXEST104: Introduction to Electrical & Electronics Engineering

Ripple factor can be quantified using the following formula:

𝑉𝑟𝑚𝑠 2
Γ = √( ) −1
𝑉𝑑𝑐

The ripple factor of a halfwave rectifier is 1.21.

Efficiency of Halfwave Rectifier
The efficiency of a halfwave rectifier is the ratio of output DC power to the input AC power.
The efficiency formula for halfwave rectifier is given as follows.
𝑃𝑑𝑐
𝜂=
𝑃𝑎𝑐
RMS value of Half Wave Rectifier
The RMS value of the load current for a half-wave rectifier is given by the formula:
𝐼𝑚
𝐼𝑟𝑚𝑠 =
2
Form factor of a Halfwave Rectifier
The form factor is the ratio between RMS value and average value and is given by the formula:
𝑅𝑀𝑆 𝑣𝑎𝑙𝑢𝑒
𝐹𝑜𝑟𝑚 𝑓𝑎𝑐𝑡𝑜𝑟 =
𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝑣𝑎𝑙𝑢𝑒
Applications of Half Wave Rectifier
Here are a few common applications of half wave rectifiers:
• They are used for signal demodulation purpose
• They are used for rectification applications
• They are used for signal peak applications
Disadvantages of Half Wave Rectifier
• Power loss
• Low output voltage
• The output contains a lot of ripples

7.2 Full Wave Rectifier

A full wave rectifier is defined as a rectifier that converts the complete cycle of alternating current into
pulsating DC. Unlike halfwave rectifiers that utilize only the halfwave of the input AC cycle, full wave
rectifiers utilize the full cycle. The lower efficiency of the half wave rectifier can be overcome by the
full wave rectifier.

15
 GXEST104: Introduction to Electrical & Electronics Engineering

The circuit of the full wave rectifier consists of a step-down transformer and two diodes that are
connected, and centre tapped. The output voltage is obtained across the connected load resistor.
Working of Full Wave Rectifier
The input AC supplied to the full wave rectifier is very high. The step-down transformer in the rectifier
circuit converts the high voltage AC into low voltage AC. The anode of the centre tapped diodes is
connected to the transformer’s secondary winding and connected to the load resistor. During the positive
half cycle of the alternating current, the top half of the secondary winding becomes positive while the
second half of the secondary winding becomes negative.
During the positive half cycle, diode D1 is forward biased as it is connected to the top of the secondary
winding while diode D2 is reverse biased as it is connected to the bottom of the secondary winding. Due
to this, diode D1 will conduct acting as a short circuit and D2 will not conduct acting as an open circuit
During the negative half cycle, the diode D1 is reverse biased and the diode D2 is forward biased because
the top half of the secondary circuit becomes negative and the bottom half of the circuit becomes
positive. Thus in a full wave rectifiers, DC voltage is obtained for both positive and negative half cycle.

Full wave Rectifier with capacitor filter

The working of the full wave rectifier with filter is almost similar to that of the half wave rectifier with
filter. The only difference is that in the half wave rectifier only one half cycle (either positive or
negative) of the input AC current will charge the capacitor but the remaining half cycle will not charge
the capacitor. But in full wave rectifier, both positive and negative half cycles of the input AC current
will charge the capacitor.
The main duty of the capacitor filter is to short the ripples to the ground and blocks the pure DC (DC
components), so that it flows through the alternate path and reaches output load resistor RL.
When input AC voltage is applied, during the positive half cycle, the diode D1 is forward biased and
allows electric current whereas the diode D2 is reverse biased and blocks electric current. On the other
hand, during the negative half cycle the diode D2 is forward biased (allows electric current) and the
diode D1 is reverse biased (blocks electric current).

16
 GXEST104: Introduction to Electrical & Electronics Engineering

The pulsating Direct Current (DC) produced by the full wave rectifier contains both AC and DC
components. We know that the capacitor allows the AC components and blocks the DC components of
the current. When the DC current that contains both DC components and AC components reaches the
filter, the DC components experience a high resistance from the capacitor whereas the AC components
experience a low resistance from the capacitor. Electric current always prefers to flow through a low
resistance path. So, the AC components will flow through the capacitor whereas the DC components
are blocked by the capacitor. Therefore, they find an alternate path and reach the output load resistor
RL. The flow of AC components through the capacitor is nothing but the charging of a capacitor. Thus,
the filter converts the pulsating DC into pure DC.
Rectification Efficiency
The rectification efficiency of the full-wave rectifier can be obtained using the following formula:
𝑃𝑑𝑐
𝜂=
𝑃𝑎𝑐
The efficiency of the full wave rectifiers is 81.2%.
Advantages of Full Wave Rectifier
• The rectification efficiency of full wave rectifiers is double that of half wave rectifiers. The
efficiency of half wave rectifiers is 40.6% while the rectification efficiency of full wave
rectifiers is 81.2%.

17
 GXEST104: Introduction to Electrical & Electronics Engineering

• The ripple factor in full wave rectifiers is low hence a simple filter is required. The value of
ripple factor in full wave rectifier is 0.482 while in half wave rectifier it is about 1.21.
• The output voltage and the output power obtained in full wave rectifiers are higher than that
obtained using half wave rectifiers.
The only disadvantage of the full wave rectifier is that they need more circuit elements than the half
wave rectifier which makes, making it costlier.

7.3 Bridge Rectifier

The construction of a bridge rectifier is shown in the figure below. The bridge rectifier circuit is made
of four diodes D1, D2, D3, D4, and a load resistor RL. The four diodes are connected in a closed-loop
configuration to efficiently convert the alternating current (AC) into Direct Current (DC). The main
advantage of this configuration is the absence of the expensive centre-tapped transformer. Therefore,
the size and cost are reduced.

The input signal is applied across terminals A and B, and the output DC signal is obtained across the
load resistor RL connected between terminals C and D. The four diodes are arranged in such a way that
only two diodes conduct electricity during each half cycle. D1 and D3 are pairs that conduct electric
current during the positive half cycle/. Likewise, diodes D2 and D4 conduct electric current during a
negative half cycle.
Working
When an AC signal is applied across the bridge rectifier, terminal A becomes positive during the positive
half cycle while terminal B becomes negative. This results in diodes D1 and D3 becoming forward
biased while D2 and D4 becoming reverse biased. The current flow during the positive half-cycle is
shown in the figure below:

18
 GXEST104: Introduction to Electrical & Electronics Engineering

During the negative half-cycle, terminal B becomes positive while terminal A becomes negative. This
causes diodes D2 and D4 to become forward biased and diode D1 and D3 to be reverse biased. The
current flow during the negative half cycle is shown in the figure below:

From the figures given above, we notice that the current flow across load resistor RL is the same during
the positive and negative half-cycles. The output DC signal polarity may be either completely positive
or negative. In our case, it is completely positive. If the diodes’ direction is reversed, we get a complete
negative DC voltage.
Thus, a bridge rectifier allows electric current during both positive and negative half cycles of the input
AC signal.
The output waveforms of the bridge rectifier are shown in the below figure.

19
 GXEST104: Introduction to Electrical & Electronics Engineering

Advantages
• The efficiency of the bridge rectifier is higher than the efficiency of a half-wave rectifier.
However, the rectifier efficiency of the bridge rectifier and the centre-tapped full-wave rectifier
is the same.
• The DC output signal of the bridge rectifier is smoother than the output DC signal of a half-
wave rectifier.
• In a half-wave rectifier, only half of the input AC signal is used, and the other half is blocked.
Half of the input signal is wasted in a half-wave rectifier. However, in a bridge rectifier, the
electric current is allowed during both positive and negative half cycles of the input AC signal.
Hence, the output DC signal is almost equal to the input AC signal.
Disadvantages
• The circuit of a bridge rectifier is complex when compared to a half-wave rectifier and centre-
tapped full-wave rectifier. Bridge rectifiers use 4 diodes while half-wave rectifiers and centre-
tapped full wave rectifiers use only two diodes.
• When more diodes are used more power loss occurs. In a centre-tapped full-wave rectifier, only
one diode conducts during each half cycle. But in a bridge rectifier, two diodes connected in
series conduct during each half cycle. Hence, the voltage drop is higher in a bridge rectifier.

Homework: Prepare notes on Bridge rectifier with capacitor filter

8. Ripple Factor
A ripple is defined as the fluctuating AC component in the rectified DC output.
The rectified DC output could be either DC current or DC voltage. When the fluctuating AC
component is present in DC current it is known as the current ripple while the fluctuating AC
component in DC voltage is known as the voltage ripple.
The ripple factor is defined as
The ratio of the RMS value of an alternating current component in the rectified output to the average
value of rectified output.
The ripple factor is denoted as Γ (𝑔𝑎𝑚𝑚𝑎). It is a dimensionless quantity.

20
 GXEST104: Introduction to Electrical & Electronics Engineering

𝑉𝑟𝑚𝑠 2
Γ = √( ) −1
𝑉𝑑𝑐

8.1 Half-Wave Rectifier

• Without Filter:
o The half-wave rectifier allows only one half of the AC cycle to pass through, resulting
in a pulsating DC output.
o The ripple factor for a half-wave rectifier without a filter is approximately 1.211.
• With Filter:
o Adding a capacitor filter smooths the output by charging during the peak voltage and
discharging when the voltage drops.
o The ripple factor decreases significantly with a filter. The exact value depends on the
capacitance and load resistance but is generally much lower than 1.21
8.2 Full-Wave Rectifier
• Without Filter:
o A full-wave rectifier converts both halves of the AC cycle into DC, resulting in a higher
average output voltage and lower ripple compared to a half-wave rectifier.
o The ripple factor for a full-wave rectifier without a filter is approximately 0.48
• With Filter:
o A capacitor filter further smooths the output, reducing the ripple.
o The ripple factor with a filter is significantly lower and depends on the filter’s
capacitance and the load resistance
8.3 Bridge Rectifier

• Without Filter:
o A bridge rectifier uses four diodes to convert both halves of the AC cycle into DC,
similar to a full-wave rectifier but without the need for a center-tapped transformer.
o The ripple factor for a bridge rectifier without a filter is also approximately 0.48
• With Filter
o Adding a Capacitor filter reduces the ripple in the output
o Ripple factor with a filter is much lower and depends on the capacitance and load
resistor

21
 GXEST104: Introduction to Electrical & Electronics Engineering

9. Bipolar Junction Transistor (BJT)

A bipolar junction transistor is a three-terminal semiconductor device that consists of two p-n junctions
which can amplify or magnify a signal. It is a current controlled device. The three terminals of the BJT
are the base, the collector, and the emitter. Bipolar junction transistor (BJT) is a bidirectional device
that uses both electrons and holes as charge carriers. While Unipolar transistor i.e. field effect
transistor uses only one type of charge carrier. BJT is a current controlled device. The current flows
from emitter to collector or from collector to emitter depending on the type of connection. This main
current is controlled by a very small current at the base terminal.

Fig.: Types of BJT

9.1 Construction of BJT

Bipolar junction transistor is formed by the combination of two back-to-back doped semiconductor
materials. In other words, BJT is formed by the “sandwich” of back-to-back extrinsic semiconductor
materials. These extrinsic semiconductors are PN junction diodes. Two PN junctions’ diodes are
sandwiched together to form a three-terminal device known as BJT transistor. BJT is a three-terminal
device having two junctions. After doping an intrinsic semiconductor with Trivalent or Pentavalent
impurities a P-type semiconductor or N-type semiconductor respectively is made. If the number of
electrons are greater than the number of holes (positive carriers) then that is known as N-type
semiconductor material. While in P-type semiconductor, the number of holes is greater than the number
of electrons. When P-type and N-type material are connected then it becomes a PN-junction diode. BJT
transistors are formed after connecting two PN junctions back-to-back. These transistors are known
as PNP or NPN bipolar junction transistors depending on whether P or N-type is sandwiched.

22
 GXEST104: Introduction to Electrical & Electronics Engineering

Basically, transistors have three portions and two junctions. These three portions are
called Emitter, Collector, and Base. The emitter and collector sandwich the base in between them. The
middle portion (base) forms two junctions with the emitter & collector. The junction of the base with
emitter is known as the Emitter-Base junction while the junction of the base with the collector is
known as the Collector-Base junction.

➢ Terminals of BJT

There are three terminals of BJT. These terminals are knowns as collector, emitter and base. These
terminals are briefly discussed here.

• Emitter

The emitter is the portion on one side of the transistor which emits electrons or holes to the other two
portions. The base is always reverse bias with respect to emitter so that it can emit a large number
of majority carriers. It is the most heavily doped region of the BJT. The emitter-base junction should
be always forward bias in both PNP and NPN transistors. Emitter supplies electrons to the emitter-base
junction in NPN while it supplies holes into the same junction in PNP transistor.

• Collector

The portion on the opposite side of the Emitter that collects the emitted charge carriers (i.e. electrons or
holes) is known as collector. The collector is heavily doped but the doping level of the collector is in
between the lightly doping level of base and heavily doped level of emitter. Collector-base junction
should be always reversed biased in both PNP and NPN transistors. The reason for reverse biasing is to
remove charge carriers (electrons or holes) from the collector-base junction. The collector of NPN
transistor collects electrons emitted by emitter. While in PNP transistor, it collects holes emitted by
emitter.

• Base

The base is the middle portion between collector and emitter & it forms two PN junctions between
them. The base is the most lightly doped portion of the BJT. Being the middle portion of the BJT allows
it to control the flow of charge carriers between emitter and collector. The base-collector junction shows
high resistance because this junction is reversed bias.

PNP Construction

In PNP bipolar transistor, the N-type semiconductor is sandwiched between two P-type semiconductors.
PNP transistors can be formed by connecting cathodes of two diodes. The cathodes of the diodes are
connected together at a common point known as base. While the anodes of the diodes that are on the
opposite sides are known as the collector and the emitter.

23
 GXEST104: Introduction to Electrical & Electronics Engineering

The emitter-base junction is forward bias while collector-base junction is reverse bias. So, in PNP type
current flows from emitter to collector. The emitter, in this case, is at high potential to both collector
and base.

NPN Construction

NPN type is exactly opposite to PNP type. In NPN bipolar transistor, the P-type semiconductor is
sandwiched between two N-type semiconductors. When the anodes of two diodes are connected
together it forms an NPN transistor. The current will flow from the collector to emitter because the
collector terminal is more positive than emitter in NPN connection.

24
 GXEST104: Introduction to Electrical & Electronics Engineering

The difference between PNP and NPN symbol is the arrow mark at the emitter which shows the
direction of flow of current. The current will either flow from emitter to collector or from collector to
emitter. The arrow mark in PNP transistor is inward, which shows the flow of current from emitter to
collector. In case of NPN collector, the arrow mark is outward, which shows the flow of current from
collector to emitter.

9.2 Working Principle of BJT

The emitter-base junction of BJT is forward-biased, whereas the collector-base junction is reverse
biased. The forward bias of the emitter-base junction causes the emitter current to flow and this emitter
current entirely flows in the collector circuit. Therefore, the collector current depends upon the emitter
current and nearly equal to the emitter current.

➢ Working of NPN Transistor

With the forward-biased emitter-base junction and reverse-biased collector-base junction, it can be seen
that the forward bias causes the flow of electrons from the n-type emitter into the p-type base. This
constitutes the emitter current (IE). As these electrons flow through the p-type base, they tend to combine
with the holes.

Since the base is lightly doped and very thin, hence, only a small number of electrons (less than 5%)
combine with the holes to constitute the base current (IB). The remaining (more than 95%) electrons
cross over the base region and reach to the collector region to constitute the collector current (IC). In
this manner, the entire emitter current flows in the collector circuit.

The emitter current is the sum of base and collector currents, that is, 𝐼𝐸 = 𝐼𝐵 + 𝐼𝐶

25
 GXEST104: Introduction to Electrical & Electronics Engineering

➢ Working of PNP Transistor

For the PNP-transistor, the forward bias of emitter-base junction causes the flow of holes in the p-type
emitter region towards the n-type base and constitutes the emitter current (IE). As these holes cross into
the n-type base region, they tend to combine with the electrons. Since the base is lightly doped and very
thin, hence only a small number of holes (less than 5%) combine with the electrons. The remaining
(more than 95%) cross the base and reach into the collector region to constitute the collector current
(IC).

In this manner, the entire emitter current flows into the collector circuit. It may be noted that the current
conduction inside the pnp-transistor is due to the movement of holes. However, in the external
connecting wires, the current is still due to the flow of electrons.

The emitter current is the sum of base and collector currents, that is, 𝐼𝐸 = 𝐼𝐵 + 𝐼𝐶

9.3 BJT Biasing

A BJT has two pn-junctions viz. emitter-base junction and collector-base junction. Application of
proper DC voltage at the two junctions of the BJT is known as BJT or Transistor Biasing.

When a transistor used as an amplifier, the emitter-base junction is forward biased and collector-base
junction is reverse biased. If the transistor is operated under this bias condition, then it is said to be
operating in the active region.

When both the junctions are forward biased then the transistor is said to be operating in the saturation
region. The transistor operated in saturation region acts like a closed switch and the collector current
becomes maximum.

When both the junctions are reverse biased, the transistor is said to be operating in the cut off region.
The BJT operated in cut off region acts as an open switch and a very small collector current (in µA)
flows from emitter to collector. This current is called reverse leakage current and is due to minority
charge carriers (electrons in p-region and holes in n-region).

Emitter-Base Junction Collector-Base Junction Operating Region

Forward Biased Reverse Biased Active Region
Forward Biased Forward Biased Saturation Region
Reverse Biased Reverse Biased Reverse Biased

26
 GXEST104: Introduction to Electrical & Electronics Engineering

10.BJT Configurations
Transistor configurations refer to different ways of connecting transistors in electronic circuits to
perform specific functions. The three main configurations are common emitter, common base, and
common collector, each with its unique characteristics and advantages. Selecting the appropriate
configuration is crucial to achieve the desired circuit performance.

Accordingly, a transistor can be connected in a circuit in the following ways.

• Common base (CB) configuration (or) Grounded base configuration,

• Common emitter (CE) configuration (or) Grounded emitter configuration.
• Common collector (CC) configuration (or) Grounded collector configuration.

✓ Common emitter configuration: Offers high voltage gain and moderate current gain, commonly
used for amplification applications.
✓ Common base configuration: Provides high current gain and moderate voltage gain, suitable
for impedance matching and RF amplifier circuits.
✓ Common collector configuration: Offers high voltage gain and unity current gain, commonly
used for impedance buffering and voltage amplification.

27
 GXEST104: Introduction to Electrical & Electronics Engineering

10.1 Common Emitter configuration of BJT

In this circuit arrangement, input is applied between the base and emitter, and output is taken from the
collector and emitter. Here, the emitter of the transistor is common to both input and output circuits and
hence the name common emitter connection.

𝑽𝑪𝑪

𝑽𝑩𝑩

Fig.: CE configuration of an NPN transistor, practical circuit

➢ Input Characteristics

Input characteristics are the relationship between the input current and the input voltage keeping output
voltage constant. Here the input current is the base current IB, input voltage is base emitter voltage
VBE, and the output voltage is collector emitter voltage VCE.

28
 GXEST104: Introduction to Electrical & Electronics Engineering

First the output voltage VCE is kept at zero and the input voltage VBE is gradually increased and the input
current IB is noted. Then again, the output voltage VCE is increased like 10V, 20V and kept constant and
by increasing the input voltage VBE, the input current IB is noted. The cut in voltage of a silicon transistor
is 0.7 volts and germanium transistor is 0.3 volts. In our case, it is a silicon transistor. So from the above
graph, we can see that after 0.7 volts, a small increase in input voltage (VBE) will rapidly increases the
input current (IB).

From the results it is observed that when the input voltage VBE is increased initially there is no current
produced, further when it is increased the input current IB increases steeply. When the output voltage
VCE is further increased the curve shifts right side.

➢ Output Characteristics

Output characteristics is the relationship between the output current and the output voltage keeping
input current constant. Here the values of output current IC and the output voltage VCE is noted keeping
input current IB constant.
In active region when the output voltage is increased there is very slight change in the output voltage.
The curve looks almost flat in the active region. Cut off region is the region where the input current is
below zero. When both the junctions are forward biased, it is in saturation region.

➢ Current gain (α)

The current gain of a transistor in CE configuration is defined as the ratio of output current or collector
current (IC) to the input current or base current (IB).
𝐼𝐶
𝛼=
𝐼𝐵
The current gain of a transistor in CE configuration is high. Therefore, the transistor in CE configuration
is used for amplifying the current.

29
 GXEST104: Introduction to Electrical & Electronics Engineering

11.Comparison between Common Base, Common Emitter and Common

Collector configuration

Parameter CB-Common Base CE-Common Emitter CC-Common Collector

Circuit

Input Resistance Low (20 to 100Ω) Moderate (1KΩ to 3KΩ) High (In terms of MΩ)

Output
High (1MΩ to 3MΩ) Moderate (40KΩ to 80KΩ) Low (In terms of Ω)
Resistance

Phase difference 0𝑜 180𝑜 0𝑜

𝐼𝐶 𝐼𝐶 𝐼𝐸
Current Gain 𝛼= 𝛽= 𝛾=
𝐼𝐸 𝐼𝐵 𝐼𝐵

Voltage Gain Greater than 1 Greater than 1 Less than 1

Power Gain Low High Maximum(1)

Amplifier, Filter, Impedance matching,

Application Amplifier, Filter, Oscillator
Oscillator Attenuator

12.Concept of Biasing and Load line

Biasing is the process of providing DC voltage which helps in the functioning of the circuit. A transistor
is biased in order to make the emitter base junction forward biased and collector base junction reverse
biased, so that it maintains in active region, to work as an amplifier.

12.1 Need for DC biasing:

If a signal of very small voltage is given to the input of BJT, it cannot be amplified. Because, for a BJT,
to amplify a signal, two conditions must be met.

1. The input voltage should exceed cut-in voltage for the transistor to be ON.
2. The BJT should be in the active region, to be operated as an amplifier.

If appropriate DC voltages and currents are given through BJT by external sources, so that BJT operates
in active region and superimpose the AC signals to be amplified, then this problem can be avoided. The
given DC voltage and currents are so chosen that the transistor remains in active region for entire input
AC cycle. Hence DC biasing is needed.

30
 GXEST104: Introduction to Electrical & Electronics Engineering

12.2 Types of DC Biasing Circuits

The commonly used methods of transistor biasing are

• Base Resistor method

• Collector to Base bias
• Biasing with Collector feedback resistor
• Voltage-divider bias

Fig.: Base Resistor Method Fig.: Collector to Base Bias

Fig.: Biasing with collector feedback resistor Fig.: Voltage divider biasing

12.3 DC load line & Operating Point

When the output characteristics of a transistor in CE configuration are considered, the curve looks as
below for different input values. In the figure, the output characteristics are drawn between collector
current IC and collector voltage VCE for different values of base current IB. These are considered here
for different input values to obtain different output curves.

31
 GXEST104: Introduction to Electrical & Electronics Engineering

Operating point

When a value for the maximum possible collector current is considered, that point will be present on
the Y-axis (IC), which is nothing but the saturation point. As well, when a value for the maximum
possible collector emitter voltage (VCE) is considered, that point will be present on the X-axis, which is
the cutoff point.

When a line is drawn joining these two points, such a line can be called as Load line. This is called so
as it symbolizes the output at the load. This line, when drawn over the output characteristic curve, makes
contact at a point called as Operating point.

This operating point is also called as quiescent point or simply Q-point. There can be many such
intersecting points, but the Q-point is selected in such a way that irrespective of AC signal swing, the
transistor remains in active region. This can be better understood through the figure below.

The load line must be drawn to obtain the Q-point. A transistor acts as a good amplifier when it is in
active region and when it is made to operate at Q-point, faithful amplification is achieved. Faithful
amplification is the process of obtaining complete portions of input signal by increasing the signal
strength. This is done when AC signal is applied at its input.

DC Load line

When the transistor is given the bias and no signal is applied at its input, the load line drawn at such
condition, can be understood as DC condition. Here there will be no amplification as the signal is
absent. The circuit will be as shown below.

32
 GXEST104: Introduction to Electrical & Electronics Engineering

The value of collector emitter voltage at any given time will be

VCE=VCC−ICRC

As VCC and RC are fixed values, the above one is a first-degree equation and hence will be a straight
line on the output characteristics. This line is called as D.C. Load line. The figure below shows the
DC load line.

To obtain the load line, the two end points of the straight line are to be determined. Let those two
points be A and B.

• To obtain A

When collector emitter voltage VCE = 0, the collector current is maximum and is equal to VCC/RC. This
gives the maximum value of VCE. This is shown as

VCE=VCC−ICRC

0=VCC−ICRC

𝑉𝐶𝐶
𝐼𝐶 =
𝑅𝐶

This gives the point A (OA = VCC/RC) on collector current axis, shown in the above figure.

• To obtain B

33
 GXEST104: Introduction to Electrical & Electronics Engineering

When the collector current IC = 0, then collector emitter voltage is maximum and will be equal to the
VCC. This gives the maximum value of IC. This is shown as

VCE=VCC−ICRC

=VCC

(As IC = 0)

This gives the point B, which means (OB = VCC) on the collector emitter voltage axis shown in the
above figure.

Hence, we got both the saturation and cutoff point determined and learnt that the load line is a straight
line. So, a DC load line can be drawn.

Factors affecting the operating point

The main factor that affects the operating point is the temperature. The operating point shifts due to
change in temperature.

As temperature increases, the values of ICE, β, VBE gets affected.

• ICBO gets doubled (for every 10o rise)

• VBE decreases by 2.5mv (for every 1o rise)

So, the main problem which affects the operating point is temperature. Hence operating point should
be made independent of the temperature so as to achieve stability. To achieve this, biasing circuits are
introduced.

Need for Stabilization

Stabilization of the operating point has to be achieved due to the following reasons.

1. Temperature dependence of IC

As the expression for collector current IC is

IC=βIB+ICEO
=βIB+(β+1)ICBO
The collector leakage current ICBO is greatly influenced by temperature variations. To come out of this,
the biasing conditions are set so that zero signal collector current IC = 1 mA. Therefore, the operating
point needs to be stabilized i.e. it is necessary to keep IC constant.

2. Individual variations

As the value of β and the value of VBE are not same for every transistor, whenever a transistor
is replaced, the operating point tends to change. Hence it is necessary to stabilize the
operating point.

3. Thermal runaway

34
 GXEST104: Introduction to Electrical & Electronics Engineering

As the expression for collector current IC is

IC=βIB+ICEO
=βIB+(β+1)ICBO

The flow of collector current and also the collector leakage current causes heat dissipation. If the
operating point is not stabilized, there occurs a cumulative effect which increases this heat dissipation.

The self-destruction of such an unstabilized transistor is known as Thermal run away.

In order to avoid thermal runaway and the destruction of transistor, it is necessary to stabilize the
operating point, i.e., to keep IC constant.

Stability Factor: the rate of change of collector current IC with respect to the collector leakage current
ICO at constant β and IB is called Stability factor.
𝐼𝐶
𝑆= 𝑎𝑡 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 𝐼𝐵 𝑎𝑛𝑑 𝛽
𝐼𝐶𝑂

13.Transistor as a Switch
A transistor can be used as a solid state switch. If the transistor is operated in the saturation region, then
it acts as closed switch and when it is operated in the cut off region then it behaves as an open switch.

The transistor operates as a Single Pole Single Throw (SPST) solid state switch. When a zero-input
signal applied to the base of the transistor, it acts as an open switch. If a positive signal applied at the
input terminal, then it acts like a closed switch.

When the transistor operating as switch, in the cut off region the current through the transistor is zero
and voltage across it is maximum, and in the saturation region the transistor current is maximum and
voltage across is zero. Therefore, both the on – state and off – state power loss is zero in the transistor
switch.

➢ Cut Off State (Open Switch)

When transistor operates in the cut off region shows the following characteristics −

• The input is grounded i.e. at zero potential.

• The VBE is less that cut – in voltage 0.7 V.
• Both emitter – base junction and collector – base junction are reverse biased.
• The transistor is fully – off acting as open switch.
• The collector current IC = 0 A and output voltage Vout = VCC.

35
 GXEST104: Introduction to Electrical & Electronics Engineering

➢ Saturation State (Closed Switch)

The transistor operating in the saturation region exhibits following characteristics −

• The input is connected to VCC.

• Base – Emitter voltage is greater than cut – in voltage (0.7 V).
• Both the base – emitter junction and base – collector junction are forward biased.
• The transistor is fully – ON and operates as closed switch.
• The collector current is maximum

𝑉𝐶𝐶
𝐼𝐶 = 𝑎𝑛𝑑 𝑉𝑜 = 0𝑉
𝑅𝐿

14.Transistor as an Amplifier
For a transistor to act as an amplifier, it should be properly biased. For a transistor to work as an
amplifier, we usually use the common-emitter configuration. The figure below shows how the transistor
is set up when it is connected to a circuit as an amplifier.

In the figure given above, the input is connected in forward-biased, and the output is connected in
reverse-biased. The input signal is applied on the base-emitter junction, and the output is taken through
the load in the emitter-collector junction. There is also an application of DC voltage in the input circuit
for amplification. Besides, a small change in signal voltage results in the change of emitter current,
which is mainly due to the low resistance in the input circuit.

36
 GXEST104: Introduction to Electrical & Electronics Engineering

15.RC Coupled Amplifier

An RC-coupled amplifier is a type of electronic amplifier that uses a resistor-capacitor (RC) network to
couple the output of one stage of amplification to the input of the next stage. The capacitor blocks any
DC voltage, while allowing AC signals to pass through. The resistor and capacitor values are carefully
selected to optimize the frequency response of the amplifier. This configuration is commonly used in
audio amplifiers and radio receivers due to its simplicity and effectiveness.
It is a single stage amplifier in common emitter (emitter terminal is common to both input and output)
configuration and voltage divider biasing. This circuit is simple and provides good stabilization

Construction:

• The resistors R1, R2, RE provides voltage divider biasing to the transistor, which helps the
transistor to work in the active region. It sets proper operating point for CE (Common Emitter)
amplifier.
• The Coupling capacitors CC1 & CC2, are known as input and output coupling capacitors
respectively. These capacitors pass AC signal from one side to the other side. At the same time,
it does not allow the DC voltages to pass through it, hence it is also called blocking capacitors.
• The bypass capacitor, CE & Emitter resistor, RE: CE works as a bypass capacitor, it bypasses all
the ac currents from the emitter to the ground. If this capacitor is not there in the circuit, the AC
voltage developed across RE will affect the input AC voltage. The resistor RE provides thermal
stabilization to the circuit.
Working:
The input signal is applied to the base of the transistor through the input coupling capacitor CC1, which
blocks the DC components in the AC signal. The transistor circuit amplifies the signal, and the output
is taken across the load resistor RL, through the coupling capacitor CC2, which blocks the DC
components present in the amplified output signal. The output signal will be a phase shifted version of
the input signal by 180o.

Voltage Gain (Av):

The voltage gain Av of an RC coupled amplifier is the ratio between output voltage (Vo) and input
voltage (Vi)

37
 GXEST104: Introduction to Electrical & Electronics Engineering

𝑉𝑜
𝐴𝑉 =
𝑉𝑖

Frequency Response:

• Low Frequencies
At low frequencies (< 50 Hz), the reactance of coupling capacitor is quite high and hence very small
part of the signal will pass from one stage to the next stage. The reactance of CE is also high at low
frequencies. This high reactance cannot shunt the emitter resistance RE effectively. Due to these two
factors, the voltage gains fall at low frequencies.

• High Frequencies
At high frequencies (> 20 kHz), the reactance CC is very small, and it behaves as a short circuit. This
increases the loading effect of the next stage and serves to reduce the voltage gain. Also, the capacitive
reactance of the base-emitter junction is low, and at high frequencies increases the base current.
Therefore, the ββ value reduces. Due to these two reasons, the voltage gains fall at high frequencies.

• Mid Frequencies
At mid frequencies (50 Hz to 20 kHz), the voltage gain of the amplifier is constant. The effect of
coupling capacitors in this frequency range is such as maintaining a uniform voltage gain. Thus, as the
frequency increases in this range, the reactance of CC decreases which tends to increase the gain.
However, at the same time, lower reactance means higher loading of the first stage and hence lower
gain. These two factors almost cancel each other, resulting in a uniform gain at mid-frequency.

16. Introduction to FET

A Field Effect Transistor (FET) is a type of transistor that controls the flow of electric current using
an electric field. FETs are essential in electronics because they act as switches or amplifiers in circuits
with high efficiency and low power consumption. Unlike Bipolar Junction Transistors (BJTs), which
rely on current control, FETs control current using voltage, making them voltage-controlled devices.

Structure of FET

The basic structure of a FET includes three terminals:

38
 GXEST104: Introduction to Electrical & Electronics Engineering

• Source (S): Where current enters the FET.

• Drain (D): Where current exits the FET.
• Gate (G): Controls the flow of current between the Source and Drain by applying a voltage.

How a FET Works

In a FET, a thin channel exists between the Source and Drain for current flow. The Gate terminal is
placed near this channel, separated by a thin insulating layer. By applying a voltage to the Gate, we
create an electric field that influences the conductivity of the channel, either allowing or preventing
current flow.

• When there is no voltage at the Gate: The channel might be open or closed depending on
the FET type (e.g., n-channel or p-channel), meaning it can naturally conduct or block
current.
• When voltage is applied to the Gate: The electric field created affects the charge carriers in
the channel, controlling the current between the Source and Drain.

Types of FETs

• Junction FET (JFET): Uses a pn junction to control current.

• Metal-Oxide-Semiconductor FET (MOSFET): Uses a metal oxide layer for insulation,
which gives better control and efficiency, making it common in digital circuits.

16.1 Metal Oxide Field Effect Transistor

MOSFETs are a type of Field Effect Transistor (FET) widely used in electronic devices for switching
and amplification. They are known for their high input impedance and fast switching speeds.

MOSFETs are classified into two main types:

1. Enhancement MOSFET (E-MOSFET): Normally off; requires a positive gate voltage to

turn on (for n-channel) or a negative gate voltage (for p-channel).
2. Depletion MOSFET (D-MOSFET): Normally on; can conduct without any gate voltage.
Applying voltage to the gate can either increase or decrease the conductivity.

MOSFETs have three main regions:

1. Source (S): Terminal through which current enters.

2. Drain (D): Terminal through which current exits.
3. Gate (G): Controls the current flow by applying voltage.

39
 GXEST104: Introduction to Electrical & Electronics Engineering

A thin insulating layer of silicon dioxide (SiO₂) separates the Gate from the channel (between the
Source and Drain), preventing direct current flow from the Gate to the channel and ensuring that only
an electric field affects the channel conductivity.

• n-channel MOSFET: Has an n-type channel; conducts when a positive voltage is applied to
the Gate.
• p-channel MOSFET: Has a p-type channel; conducts when a negative voltage is applied to
the Gate.

➢ N-Channel Enhancement Mode MOSFET

1. No Gate Voltage (VGS = 0):

• With zero voltage applied to the Gate, no conductive channel exists between the Source and
Drain.
• The MOSFET remains off, preventing current flow from the Drain to the Source.
2.Positive Gate Voltage Applied (VGS > 0):
• Applying a positive voltage at the Gate creates an electric field that attracts electrons
(negatively charged carriers) towards the region just under the Gate.
• As electrons accumulate, they form an inversion layer, creating an n-type conductive channel
between the Source and Drain.
3.Threshold Voltage (VTH):
• The Gate voltage must reach a certain level, called the threshold voltage (VTH), for the channel
to form fully and allow significant current to flow between the Source and Drain.
4.Channel Formation and Current Flow (VGS> VTH):
• When the Gate voltage exceeds the threshold voltage, a conductive n-type channel is fully
established.
• Electrons flow from the Source to the Drain when a voltage is applied between the Drain and
Source (VDS), allowing current (ID) to flow through the MOSFET.
5.Increasing Gate Voltage (VGS >> VTH):
• Increasing the Gate voltage widens the channel, reducing resistance and allowing more current
(ID) to flow between the Source and Drain.

40
 GXEST104: Introduction to Electrical & Electronics Engineering

• The MOSFET operates in the linear or ohmic region when VDS is low and in the saturation
region when VDS is high.
6.Controlling Current Flow:
• The MOSFET acts as a voltage-controlled switch, where the amount of current from Drain to
Source depends on the Gate voltage.
• This property is widely used for switching and amplifying signals in electronic circuits.

➢ N-Channel Depletion Mode MOSFET

1.No Gate Voltage (VGS = 0):

• In the depletion mode MOSFET, a conductive n-type channel already exists between the
Source and Drain, allowing current to flow even without any Gate voltage.
• This makes the MOSFET normally on.
2.Negative Gate Voltage Applied (VGS < 0):
• Applying a negative voltage to the Gate creates an electric field that repels electrons from the
channel.
• This reduces the channel width, increasing its resistance, which depletes the channel of
charge carriers and reduces current flow (ID).
3.Threshold Voltage (Negative VTH):
• At a specific negative Gate voltage, called the threshold voltage (VTH), the channel is
pinched off completely, and current flow from Drain to Source stops.
4.Positive Gate Voltage Applied (VGS > 0):
• When a positive voltage is applied to the Gate, it attracts more electrons into the channel,
enhancing its conductivity.
• This increases current flow from Drain to Source, effectively enhancing the channel beyond
its natural conductive state.
5.Operating Modes:

41
 GXEST104: Introduction to Electrical & Electronics Engineering

• The MOSFET can operate in a depletion mode (by reducing current flow with a negative
Gate voltage) or in an enhancement mode (by increasing current flow with a positive Gate
voltage).
6.Current Control:
• This ability to control current with both positive and negative Gate voltages makes the N-
Channel Depletion Mode MOSFET versatile for applications requiring both normally-on and
controlled-off states.

Homework: Construction and working of P channel Enhancement & Depletion mode MOSFET

42