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GNDU Question Paper-2024
Bachelor of Computer Application (BCA) (Hons.)
5
th
Semester (Batch 2024-28) (CBGS)
PHYSICS: Paper-B
(Electronics)
Time Allowed: Three Hours Max. Marks:35
Note: Attempt Five questions in all, selecting at least One question from each section. The
Fifth question may be attempted from any section. All questions carry equal marks.
SECTION-A
1. (a) What is a rectifier? Explain construction and working of full wave bridge rectifier.
Write its advantages.
(b) What are logic gates? Write truth tables for AND, OR, NOT gates.
2. (a) What are photonics devices? Write short notes on:
(i) Photodiode
(ii) LED.
(b) What are filter circuits ? Explain with diagram working of L-C filter circuit.
SECTION-B
3. What are different configurations of a bipolar transistor ? Draw circuit diagram and
explain working of common base NPN transistor.
4. Explain construction and working of JFET. What are advantages of JFET over Bipolar
transistor ?
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SECTION-C
5. What are different types of feedbacks in amplifiers ? Explain how negative feedback can
decrease non-linear distortions of an amplifier and improve its gain stability.
6. Draw and explain working of common emitter amplifier circuit. How it amplifies input
signal and produces 180° phase difference in input and output signals ?
SECTION-D
7. Draw circuit diagram of Phase shift oscillator, explain its working and find an expression
for its frequency of oscillations.
8. Explain, with circuit diagram working of Tuned oscillator. How it can produce oscillation
in output signal and find its frequency of oscillations.
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GNDU Answer Paper-2024
Bachelor of Computer Application (BCA) (Hons.)
5
th
Semester (Batch 2024-28) (CBGS)
PHYSICS: Paper-B
(Electronics)
Time Allowed: Three Hours Max. Marks:35
Note: Attempt Five questions in all, selecting at least One question from each section. The
Fifth question may be attempted from any section. All questions carry equal marks.
SECTION-A
1. (a) What is a rectifier? Explain construction and working of full wave bridge rectifier.
Write its advantages.
(b) What are logic gates? Write truth tables for AND, OR, NOT gates.
Ans: 1. (a) What is a Rectifier? Explain the Construction and Working of a Full Wave Bridge
Rectifier. Write its Advantages.
Introduction
Electricity is available in two main forms:
AC (Alternating Current): The current changes its direction continuously. Electricity
supplied to our homes is AC.
DC (Direct Current): The current flows only in one direction. Batteries, mobile
phones, laptops, and many electronic devices work on DC.
Since most electronic devices require DC power, we need a device that converts AC into DC.
This device is called a Rectifier.
What is a Rectifier?
A Rectifier is an electronic circuit or device that converts Alternating Current (AC) into
Direct Current (DC).
Simple Definition
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A rectifier is a device that changes AC supply into DC supply so that electronic devices can
work properly.
Everyday Example
Imagine you have water flowing in a pipe.
In AC, the water flows forward and backward repeatedly.
In DC, the water flows only in one direction.
A rectifier acts like a special valve that allows the current to move in only one direction.
Full Wave Bridge Rectifier
A Full Wave Bridge Rectifier converts both positive and negative halves of the AC input into
DC output.
Unlike a half-wave rectifier, it uses both halves of the AC signal, making it much more
efficient.
Construction of Full Wave Bridge Rectifier
A Full Wave Bridge Rectifier mainly consists of:
1. Four PN Junction Diodes (D1, D2, D3, D4)
2. AC Input Supply
3. Load Resistance (RL)
4. Output Terminals
Diagram
D1 D2
+---->|-------+-------|<----+
AC ~ | | | ~ AC
| RL |
| | |
+----|<-------+------->|----+
D3 D4
D1, D2, D3, D4 = Four diodes
RL = Load resistor where DC output is obtained
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Working of Full Wave Bridge Rectifier
The working can be understood in two cycles.
1. Positive Half Cycle
During the positive half of AC:
Upper terminal becomes positive.
Lower terminal becomes negative.
Now,
D1 and D4 become Forward Biased (they conduct electricity).
D2 and D3 become Reverse Biased (they do not conduct).
Current Path:
AC → D1 → Load (RL) → D4 → AC
Current flows through the load in one direction.
2. Negative Half Cycle
Now the AC polarity reverses.
Upper terminal becomes negative.
Lower terminal becomes positive.
Now,
D2 and D3 become Forward Biased.
D1 and D4 become Reverse Biased.
Current Path:
AC → D2 → Load (RL) → D3 → AC
Again, current flows through the load in the same direction.
Important Observation
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Although the AC input changes its direction every half cycle,
the current through the load resistor always flows in one direction.
Hence, the output becomes pulsating DC.
Input and Output Waveform
Input AC
/\ /\ /\
/ \ / \ / \
----/----\--/----\--/----\----
/ \/ \/
Output DC
/\ /\ /\ /\
/ \ / \ / \ / \
---/----\/----\/----\/----\----
Notice that both positive and negative halves are converted into positive pulses.
Advantages of Full Wave Bridge Rectifier
1. Converts both halves of AC into DC.
2. Gives higher efficiency than a half-wave rectifier.
3. Produces more output voltage.
4. Ripple is less.
5. Better performance and smoother DC output.
6. No center-tapped transformer is required.
7. Widely used in power supplies, chargers, TVs, computers, and electronic circuits.
Applications
Mobile chargers
Laptop adapters
Television power supplies
UPS systems
Computer SMPS
Battery chargers
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Conclusion
A Full Wave Bridge Rectifier is one of the most commonly used rectifier circuits because it
uses four diodes to convert both halves of AC into DC. It provides higher efficiency, better
output, and smoother DC, making it suitable for almost all electronic devices.
1. (b) What are Logic Gates? Write Truth Tables for AND, OR, and NOT Gates.
Introduction
Every digital devicesuch as a computer, mobile phone, calculator, ATM, washing
machine, or traffic light controllermakes decisions using Logic Gates.
They are called the building blocks of digital electronics because all digital circuits are made
using logic gates.
What are Logic Gates?
A Logic Gate is an electronic circuit that performs a logical operation on one or more binary
inputs to produce a single binary output.
Logic gates understand only two numbers:
0 = OFF = False = Low Voltage
1 = ON = True = High Voltage
Think of a logic gate like a decision-maker. It checks the input conditions and then decides
what the output should be.
1. AND Gate
The AND gate gives an output of 1 only when all inputs are 1.
Simple Example
Imagine two switches connected in series to a bulb.
The bulb will glow only if both switches are ON.
Symbol
A ----\
) AND ---- Output
B ----/
Truth Table
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A
B
Output (A AND B)
0
0
0
0
1
0
1
0
0
1
1
1
2. OR Gate
The OR gate gives an output of 1 if any one input is 1.
Simple Example
Imagine two switches connected in parallel.
If either switch is ON, the bulb glows.
Symbol
A ----\
) OR ---- Output
B ----/
Truth Table
A
B
Output (A OR B)
0
0
0
0
1
1
1
0
1
1
1
1
3. NOT Gate
The NOT gate has only one input.
It simply reverses the input.
Input 0 → Output 1
Input 1 → Output 0
Everyday Example
Think of a room light controlled by an automatic inverter circuit:
If there is power (1), the emergency light stays OFF (0).
If power goes OFF (0), the emergency light turns ON (1).
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The output is always the opposite of the input.
Symbol
A ----|>o---- Output
Truth Table
A
Output (NOT A)
0
1
1
0
Summary of Logic Gates
Gate
Condition for Output = 1
AND
All inputs must be 1
OR
At least one input must be 1
NOT
Gives the opposite of the input
Conclusion
Logic gates are the foundation of digital electronics. They process binary data (0 and 1) and
help computers and electronic devices make decisions. The AND gate outputs 1 only when
all inputs are 1, the OR gate outputs 1 when at least one input is 1, and the NOT gate simply
reverses the input. Together, these basic gates are used to build more complex circuits such
as adders, memory units, processors, and control systems. Understanding these three gates
is the first step toward learning digital electronics and computer hardware.
2. (a) What are photonics devices? Write short notes on:
(i) Photodiode
(ii) LED.
(b) What are filter circuits ? Explain with diagram working of L-C filter circuit.
Ans: Introduction to Photonics Devices
The word Photonics comes from the word Photon, which means a tiny particle of light. Just
as electronics deals with the movement of electrons, photonics deals with the generation,
detection, transmission, and control of light.
So, Photonics devices are electronic devices that work using light. These devices either
produce light, detect light, or control light signals. Today, photonics plays a very important
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role in communication systems, medical equipment, security systems, mobile phones,
television remote controls, optical fiber communication, and automatic street lights.
Think of it this way:
Electronics = Electricity
Photonics = Light
Some common examples of photonics devices are:
LED
Photodiode
Laser diode
Optical fiber devices
(i) Photodiode
Definition
A Photodiode is a semiconductor device that converts light energy into electrical current.
Whenever light falls on the photodiode, it produces a small electric current. The stronger
the light, the larger the current produced.
In simple words,
Photodiode works as a light sensor.
Simple Working
Imagine you are standing in a dark room.
When someone switches on the light, you immediately know that light has appeared.
Similarly, a photodiode "feels" the light and converts it into an electrical signal.
Working Steps
1. Light falls on the photodiode.
2. Light photons hit the semiconductor material.
3. Electrons are released inside the device.
4. Electric current starts flowing.
5. The circuit detects this current.
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The brighter the light,
➡ More electrons are released.
➡ More current is produced.
Simple Diagram of Photodiode
Light Rays
☀ ☀ ☀ ☀ ☀
↓↓↓↓
___________
| |
|Photodiode |
|___________|
│ │
│ │
Electrical Output
Applications of Photodiode
TV Remote Receiver
Optical Fiber Communication
Smoke Detectors
Automatic Street Lights
Barcode Scanner
Camera Light Sensors
Solar Radiation Measurement
Advantages
Very fast response
Small size
High sensitivity
Low power consumption
Reliable operation
(ii) LED (Light Emitting Diode)
Definition
LED stands for Light Emitting Diode.
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It is a semiconductor device that converts electrical energy into light energy.
Unlike a normal bulb, an LED produces light with very little heat and consumes very less
electricity.
Simple Working
Imagine water flowing through a pipe.
Similarly, electricity flows through an LED.
When electric current passes through the semiconductor material, electrons combine with
holes and release energy in the form of light.
That is why LEDs glow.
Working Steps
1. Battery supplies electricity.
2. Current flows through LED.
3. Electrons move across the junction.
4. Energy is released.
5. Light is produced.
Diagram of LED
Battery
(+) ---->|---- (-)
LED
↑
Light Rays
Applications of LED
Home Lighting
Mobile Flashlight
Traffic Signals
TV Screens
Computer Monitor
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Decorative Lights
Digital Display Boards
Car Headlights
Advantages
Saves electricity
Long life (up to 50,000 hours)
Produces less heat
Eco-friendly
Small size
Bright illumination
Difference Between Photodiode and LED
Photodiode
LED
Converts light into electricity
Converts electricity into light
Works as a light detector
Works as a light source
Used in sensors
Used for lighting
Receives light
Emits light
2. (b) What are Filter Circuits?
Definition
A Filter Circuit is an electronic circuit that removes unwanted AC (alternating current)
fluctuations or ripple from the output of a rectifier and provides a smooth DC (direct
current) voltage.
A rectifier converts AC into DC, but the output is not perfectly smooth. It contains small
unwanted waves called ripples.
The job of a filter is to remove these ripples.
Real-Life Example
Imagine you are filtering muddy water.
Mud = Ripples
Water Filter = Filter Circuit
Clean Water = Smooth DC Output
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Similarly,
Rectifier Output = Rippled DC
Filter Circuit = Removes Ripple
Output = Pure DC
L-C Filter Circuit
The L-C Filter uses two components:
L = Inductor (Coil)
C = Capacitor
These components work together to remove ripples more effectively than using only one
component.
Diagram of L-C Filter
Rectifier
│
│
───LLLL───┬──────── Output
Inductor│
│
─||─
Capacitor
│
GND
Working of L-C Filter
Step 1: Rectifier Output
The rectifier produces DC with ripples.
Rippled DC
/\ /\ /\
__/ \__/ \__/ \__
Step 2: Inductor Action
The inductor opposes sudden changes in current.
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It blocks most of the AC ripple while allowing DC current to pass.
So, ripple becomes smaller.
Step 3: Capacitor Action
The capacitor is connected parallel to the output.
It stores electrical charge when the voltage rises and releases it when the voltage falls.
Thus, it fills the gaps between the ripples.
Step 4: Smooth Output
Finally, the output becomes nearly constant DC.
Smooth DC
__________________________
Why Both Components Are Used Together?
Inductor
Blocks AC ripple
Allows DC current to pass
Reduces sudden current changes
Capacitor
Stores electric charge
Removes remaining ripple
Maintains steady output voltage
Together they provide much smoother DC than either component alone.
Advantages of L-C Filter
Produces smooth DC output
Removes ripple efficiently
Improves power supply performance
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Increases efficiency of electronic circuits
Suitable for high-current applications
Applications of L-C Filter
Power supply units
Radio receivers
Television circuits
Audio amplifiers
Communication systems
Industrial electronic equipment
Conclusion
Photonics devices are modern electronic components that work with light instead of only
electricity. A Photodiode detects light and converts it into electrical current, making it useful
in sensors, optical communication, and automatic control systems. An LED performs the
opposite function by converting electrical energy into visible light, providing efficient and
long-lasting illumination in homes, displays, and electronic devices.
A filter circuit is an essential part of a power supply because it removes unwanted ripple
from the rectifier output. In an L-C filter, the inductor reduces AC ripple by opposing
changes in current, while the capacitor smooths the remaining fluctuations by storing and
releasing charge. Together, they produce a steady DC voltage that is required for the
reliable operation of electronic equipment. Understanding these devices and circuits helps
students grasp the basics of modern electronics and communication systems.
SECTION-B
3. What are different configurations of a bipolar transistor ? Draw circuit diagram and
explain working of common base NPN transistor.
Ans: A Bipolar Junction Transistor (BJT) is one of the most important semiconductor devices
used in electronic circuits. It is mainly used to amplify signals and act as an electronic
switch. The word "bipolar" means that the transistor works with the movement of both
electrons and holes. A transistor has three terminals:
Emitter (E) Supplies charge carriers (electrons in an NPN transistor).
Base (B) Controls the flow of current.
Collector (C) Collects the charge carriers coming from the emitter.
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A transistor can be connected in different ways depending on which terminal is common to
both the input and output circuits. These connections are called transistor configurations.
Different Configurations of a Bipolar Transistor
There are three basic configurations of a BJT:
1. Common Base (CB) Configuration
The Base terminal is common to both the input and output circuits.
Input is applied between Emitter and Base.
Output is taken between Collector and Base.
It has low input resistance and high output resistance.
Current gain is less than 1.
Voltage gain is high.
Mainly used in high-frequency applications.
2. Common Emitter (CE) Configuration
The Emitter is common to both input and output.
Input is applied between Base and Emitter.
Output is taken between Collector and Emitter.
It provides high current gain and high voltage gain.
This is the most commonly used transistor configuration because it gives excellent
amplification.
3. Common Collector (CC) Configuration
The Collector is common to both input and output.
Input is applied between Base and Collector.
Output is taken between Emitter and Collector.
It has high input resistance and low output resistance.
Voltage gain is nearly 1.
It is mainly used as a buffer circuit for impedance matching.
Common Base (CB) Configuration of an NPN Transistor
In the Common Base (CB) configuration, the Base terminal is connected to ground
(common) and remains common for both input and output circuits.
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Circuit Diagram
+VCC
|
RC
|
|
Collector (C)
|
|\
| \
Base (Common)---| > NPN Transistor
(Ground) | /
|/
Emitter (E)
|
RE
|
Input Signal
|
-VEE
Simple Representation
Output
|
C
|
|\
Base ------| >------ NPN
(Common) |/
|
E
|
Input
Working of Common Base (CB) NPN Transistor
To understand the working, imagine a water pipe.
The Emitter is like a water pump that pushes water.
The Base acts like a control valve.
The Collector is like a storage tank collecting water.
In a transistor, instead of water, electrons flow.
Step 1: Forward Bias at the Emitter-Base Junction
The Emitter-Base junction is forward biased.
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This means:
Emitter is connected to the negative terminal.
Base is slightly positive.
Because of forward bias, a large number of electrons leave the emitter and enter the base.
Step 2: Reverse Bias at the Collector-Base Junction
The Collector-Base junction is reverse biased.
This means:
Collector is connected to the positive supply.
Base remains at a lower potential.
The reverse bias creates a strong electric field that pulls the electrons from the base into
the collector.
Step 3: Current Flow
Most of the electrons entering the base do not remain there because the base is very thin
and lightly doped.
Approximately:
9599% of the electrons move directly to the collector.
Only 15% combine with holes in the base.
Therefore,
Emitter Current (IE) is the largest.
Collector Current (IC) is almost equal to emitter current.
Base Current (IB) is very small.
The relationship between these currents is:
Since IB is very small, IC is nearly equal to IE.
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Why is it called "Common Base"?
It is called Common Base because the Base terminal is shared by both the input and output
circuits.
Input: Emitter to Base
Output: Collector to Base
The Base remains fixed (common) throughout the circuit.
Characteristics of Common Base Configuration
Base is common to input and output.
Input is applied between emitter and base.
Output is taken between collector and base.
Low input resistance because the emitter-base junction is forward biased.
High output resistance because the collector-base junction is reverse biased.
Current gain (α) is less than 1, usually between 0.95 and 0.99.
High voltage gain.
Suitable for high-frequency communication circuits because it has very little
feedback and good frequency response.
Advantages
Excellent performance at high frequencies.
High voltage amplification.
Better stability than some other configurations.
Low signal distortion.
Disadvantages
Low current gain (less than 1).
Low input resistance makes interfacing difficult.
Not commonly used for general-purpose amplification.
Applications
The Common Base configuration is used in:
High-frequency amplifiers
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Radio Frequency (RF) circuits
Television receivers
Communication systems
Impedance matching circuits
Conclusion
A Bipolar Junction Transistor (BJT) can be connected in three configurations: Common
Base (CB), Common Emitter (CE), and Common Collector (CC). Among these, the Common
Base configuration has the Base terminal common to both input and output circuits. In an
NPN transistor, the emitter-base junction is forward biased, while the collector-base
junction is reverse biased. Electrons flow from the emitter, pass through the thin base, and
are collected by the collector, allowing the transistor to amplify signals. Although the
Common Base configuration provides low current gain, it offers high voltage gain and
excellent high-frequency performance, making it especially useful in RF and communication
applications.
4. Explain construction and working of JFET. What are advantages of JFET over Bipolar
transistor ?
Ans: Explain the Construction and Working of JFET. What are the Advantages of JFET over
Bipolar Transistor?
Introduction
A JFET (Junction Field Effect Transistor) is a type of transistor that is widely used in
electronic circuits. It is a voltage-controlled semiconductor device, which means it controls
the flow of current using voltage rather than current. JFET is mainly used in amplifiers,
switches, oscillators, communication equipment, and electronic measuring instruments
because it consumes very little power and produces less noise.
To understand JFET easily, imagine a water pipe. Water flows through the pipe from one
end to the other. Now imagine you can press the pipe from the sides to reduce the water
flow without completely blocking it. In a JFET, electrons are like water, the channel is the
pipe, and the gate controls the width of the channel, thereby controlling the flow of
current.
Construction of JFET
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A JFET has three terminals:
1. Source (S)
2. Drain (D)
3. Gate (G)
Main Parts
1. Source (S)
The source is the terminal through which electrons enter the channel.
It supplies charge carriers to the transistor.
2. Drain (D)
The drain is the terminal through which electrons leave the channel.
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Current flows from Drain to Source (conventional current), while electrons move
from Source to Drain.
3. Gate (G)
The gate controls the amount of current flowing through the channel.
It is formed by a P-type semiconductor surrounding the N-type channel (in an N-
channel JFET).
4. Channel
The channel is the path through which current flows.
It can be either:
o N-channel JFET (more common because electrons move faster)
o P-channel JFET
Simple Construction Diagram
Drain (D)
│
┌─────────┐
Gate (P) │ N-Channel│ Gate (P)
─────│ │─────
│ │
└─────────┘
│
Source (S)
The two P-type regions connected together form the Gate, while the middle N-type region
forms the Channel.
Working of JFET
The working of a JFET is very simple if you remember one rule:
The gate voltage controls the width of the channel, and the channel width controls the
current flow.
Step 1: Apply Drain Voltage
A positive voltage is applied to the Drain, while the Source is connected to ground.
Electrons start moving from the Source to the Drain through the N-channel.
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Current begins to flow.
Step 2: Gate is Reverse Biased
The Gate is connected with a negative voltage (for an N-channel JFET).
Because of reverse bias:
Almost no current enters the Gate.
A depletion region forms around the gate.
The depletion region behaves like a barrier.
Step 3: Channel Becomes Narrow
As the gate voltage becomes more negative:
The depletion region becomes wider.
The channel becomes narrower.
Less current can pass.
This is similar to squeezing a water pipe.
Step 4: Pinch-Off Condition
If the gate voltage becomes sufficiently negative:
The depletion regions almost touch each other.
The channel becomes extremely narrow.
Current stops increasing.
This condition is called the Pinch-Off Condition.
The corresponding voltage is called the Pinch-Off Voltage.
Step 5: Cut-Off Region
If the gate voltage becomes even more negative:
The channel is completely closed.
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No drain current flows.
The transistor behaves like an OFF switch.
Simple Everyday Example
Imagine a garden hose.
Water = Electric Current
Pipe = Channel
Hand pressing pipe = Gate Voltage
When you don't press the pipe:
➡ Water flows freely.
When you press gently:
➡ Less water flows.
When you press fully:
➡ Water almost stops.
Exactly the same thing happens in a JFET.
Characteristics of JFET
Some important characteristics are:
It is a voltage-controlled device.
Gate current is almost zero.
It has high input impedance.
It consumes very little power.
It generates less electrical noise.
It is suitable for signal amplification.
Advantages of JFET over Bipolar Transistor (BJT)
JFET offers several advantages compared to a Bipolar Junction Transistor (BJT).
1. High Input Impedance
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JFET draws almost no input current.
BJT requires base current.
Therefore, JFET does not load the previous circuit.
2. Low Power Consumption
Since gate current is almost zero, power consumption is very low.
BJT consumes more power because base current is needed.
3. Voltage-Controlled Device
JFET operates using voltage.
BJT operates using current.
Voltage control makes JFET easier to interface with many electronic circuits.
4. Less Noise
JFET produces much less electrical noise.
It is ideal for audio amplifiers and communication systems.
5. Better Thermal Stability
JFET is less affected by temperature changes.
It has a lower chance of thermal runaway.
BJT is more sensitive to heat.
6. Faster Switching
JFET switches quickly in many electronic applications.
It is useful in high-speed circuits.
7. Longer Life
Because very little current flows through the gate, the device experiences less stress
and generally offers better reliability.
8. Simpler Biasing
JFET biasing circuits are comparatively simpler because almost no gate current flows.
Difference Between JFET and Bipolar Transistor (BJT)
JFET
Bipolar Transistor (BJT)
Voltage-controlled device
Current-controlled device
High input impedance
Low input impedance
Gate current is almost zero
Base current is required
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Low power consumption
Higher power consumption
Low noise
More noise
Better thermal stability
More affected by temperature
Uses only one type of charge carrier (majority carriers)
Uses both electrons and holes
Applications of JFET
JFET is commonly used in:
Audio amplifiers
Electronic switches
Oscillators
Communication systems
Measuring instruments
Buffer amplifiers
Radio frequency (RF) circuits
Conclusion
A JFET (Junction Field Effect Transistor) is a three-terminal, voltage-controlled
semiconductor device consisting of Source, Drain, and Gate. Current flows through a
semiconductor channel, while the Gate voltage controls the width of this channel by
creating a depletion region. As the gate voltage becomes more negative (in an N-channel
JFET), the channel narrows, reducing the current. At the pinch-off condition, the current
reaches its controlled limit, and with further increase in reverse gate voltage, the channel
closes completely, switching the device OFF.
Compared to a Bipolar Junction Transistor (BJT), a JFET has higher input impedance, lower
power consumption, less electrical noise, better thermal stability, and voltage-controlled
operation, making it an excellent choice for amplifiers, switching circuits, communication
systems, and other electronic applications where efficiency and signal quality are important.
SECTION-C
5. What are different types of feedbacks in amplifiers ? Explain how negative feedback can
decrease non-linear distortions of an amplifier and improve its gain stability.
Ans: Introduction
Imagine you are speaking through a microphone connected to a loudspeaker. If the sound
coming out of the speaker is sent back to the microphone, it forms a feedback loop.
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Sometimes this feedback improves the sound quality, while other times it creates an
annoying loud whistle.
The same concept is used in electronic amplifiers.
An amplifier is an electronic circuit that increases the strength (amplitude) of a weak
electrical signal. However, while amplifying the signal, the amplifier may also introduce
noise, distortion, and unstable gain.
To solve these problems, engineers use a technique called feedback.
Feedback means taking a part of the output signal and sending it back to the input of the
amplifier.
Depending on how the output is returned to the input, feedback is divided into different
types.
What is Feedback?
Definition:
Feedback is the process of taking a portion of the output signal of an amplifier and feeding it
back to its input.
There are two main types:
1. Positive Feedback
2. Negative Feedback
Simple Diagram of Feedback
Input Signal
│
▼
+----------------+
| Amplifier |
+----------------+
│
▼
Output Signal
│
┌─────┴─────┐
│ Feedback │
└─────┬─────┘
│
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Back to Input
1. Positive Feedback
In positive feedback, the feedback signal is added to the original input signal.
Input + Feedback
│
▼
Amplifier
Characteristics
Increases overall gain.
Can make the amplifier unstable.
May produce oscillations.
Mainly used in oscillators and signal generators.
Example
Suppose the input signal is 10 V.
The amplifier produces 100 V.
If 20 V of the output is added back to the input:
New Input = 10 + 20 = 30 V
The amplifier output becomes much larger.
Although gain increases, stability decreases.
2. Negative Feedback
In negative feedback, the feedback signal is subtracted from the input signal.
Input
│
▼
(+)──────► Amplifier ─────► Output
▲
│
(-) Feedback
Instead of increasing the input, the returned signal opposes it.
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This slightly reduces gain but greatly improves the overall performance.
Negative feedback is the most widely used type in practical amplifiers.
Different Types of Feedback in Amplifiers
Feedback is also classified according to how the signal is connected at the input and output.
There are four types.
1. Voltage-Series Feedback
Voltage Output
│
▼
Series Connection
│
▼
Amplifier
Features
Output voltage is sampled.
Feedback is connected in series with input.
Increases input resistance.
Decreases output resistance.
Commonly used in voltage amplifiers.
2. Voltage-Shunt Feedback
Voltage Output
│
▼
Parallel Connection
│
▼
Amplifier
Features
Voltage is sampled.
Feedback is connected in parallel.
Reduces input resistance.
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Used in current-controlled circuits.
3. Current-Series Feedback
Current Output
│
▼
Series Feedback
│
▼
Amplifier
Features
Output current is sampled.
Feedback is connected in series.
Increases input resistance.
Used in transconductance amplifiers.
4. Current-Shunt Feedback
Current Output
│
▼
Parallel Feedback
│
▼
Amplifier
Features
Output current is sampled.
Feedback is connected in parallel.
Reduces input resistance.
Used in current amplifiers.
Why is Negative Feedback Preferred?
Although negative feedback reduces the gain slightly, it provides many important
advantages:
Better stability
Less distortion
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Lower noise
Wider bandwidth
Better frequency response
Improved linearity
That is why almost all modern amplifiers use negative feedback.
What is Non-linear Distortion?
Before understanding how negative feedback reduces distortion, we must first know what
distortion means.
An amplifier should produce an output that is exactly like the input, only larger.
Ideal Case
Input Signal
/\
/ \
____/ \____
Output Signal
/\
/ \
________/ \________
The shape remains the same.
Distorted Output
Input
/\
/ \
____/ \____
Output
__
_/ \__
__/ \___
The output shape has changed.
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This change is called distortion.
What is Non-linear Distortion?
Non-linear distortion occurs when the amplifier does not amplify every part of the signal
equally.
Some portions are amplified more than others.
As a result:
Signal shape changes.
Harmonics are produced.
Audio quality becomes poor.
Communication signals become inaccurate.
How Does Negative Feedback Reduce Non-linear Distortion?
This is one of the biggest advantages of negative feedback.
Step 1
The amplifier produces an output.
Step 2
A small part of this output is returned to the input.
Step 3
If distortion appears in the output, that distorted portion is also fed back.
Step 4
Since the feedback is opposite in phase, it cancels much of the distortion.
Step 5
The amplifier automatically corrects its own errors.
As a result:
Harmonic distortion decreases.
Output becomes cleaner.
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Sound quality improves.
Signal becomes more accurate.
Real-Life Example
Think about writing with a pencil.
Without checking your work, mistakes remain.
If you keep looking back and correcting each mistake while writing, the final answer
becomes much better.
Negative feedback works in the same way.
It continuously checks the output and corrects errors.
How Does Negative Feedback Improve Gain Stability?
The gain of an amplifier tells us how much it amplifies the input signal.
Gain
Output
Input
Sometimes amplifier gain changes because of:
Temperature changes
Aging of components
Supply voltage variations
Manufacturing differences
Without feedback:
Morning Gain = 100
Afternoon Gain = 90
Night Gain = 110
The amplifier becomes unreliable.
With Negative Feedback
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The feedback automatically adjusts the input whenever the gain tries to change.
Actual Gain tries to change
│
▼
Negative Feedback detects change
│
▼
Corrects the input
│
▼
Gain remains almost constant
Thus, even if internal components change slightly, the overall gain remains nearly constant.
This is called gain stability.
Advantages of Negative Feedback
Reduces non-linear distortion.
Improves gain stability.
Produces cleaner output signals.
Reduces unwanted noise.
Increases bandwidth.
Improves frequency response.
Makes the amplifier more reliable.
Enhances overall performance.
Disadvantages of Negative Feedback
Reduces overall gain.
Makes circuit design slightly more complex.
Requires additional components.
Despite these drawbacks, its advantages are far more important, which is why it is widely
used.
Conclusion
Feedback is a technique in which a part of the amplifier's output is returned to its input. It is
mainly of two types: positive feedback and negative feedback, and based on input-output
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connections it is classified into Voltage-Series, Voltage-Shunt, Current-Series, and Current-
Shunt feedback.
Among these, negative feedback is the most commonly used because it greatly improves
amplifier performance. It reduces non-linear distortion by canceling unwanted signal errors
and keeps the gain stable even when temperature, power supply, or component values
change. Although it slightly lowers the amplifier's gain, it provides a cleaner output, higher
reliability, better frequency response, reduced noise, and improved overall efficiency. For
these reasons, negative feedback is considered an essential feature in modern electronic
amplifier circuits.
6. Draw and explain working of common emitter amplifier circuit. How it amplifies input
signal and produces 180° phase difference in input and output signals ?
Ans: Common Emitter (CE) Amplifier Circuit, Working and 180° Phase Difference (Simple
Explanation)
A Common Emitter (CE) Amplifier is one of the most widely used transistor amplifier
circuits. It is called a common emitter because the emitter terminal is common to both the
input and output circuits. This amplifier is mainly used to increase the strength (amplitude)
of weak electrical signals without changing their original shape. It provides high voltage
gain, good current gain, and produces an output signal that is 180° out of phase with the
input signal.
Simple Circuit Diagram
+VCC
|
RC
|
+---------- Output (Vout)
|
Collector (C)
|
|\
Input (Vin) --C1---| > NPN Transistor
|/
Base (B)
|
R1 R2
+VCC --------/\/\/\----+----/\/\/\------ GND
|
Base
|
Emitter (E)
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|
RE
|
GND
C1 = Input Coupling Capacitor
RC = Collector Resistor
RE = Emitter Resistor
R1 & R2 = Voltage Divider Bias Resistors
What is an Amplifier?
Before understanding the Common Emitter amplifier, let us first understand what an
amplifier is.
Imagine you are speaking very softly into a microphone. Your voice is too weak to be heard
by a large audience. A microphone amplifier takes your weak voice signal and makes it
much stronger so that everyone can hear it through a loudspeaker.
Similarly, in electronics, many devices produce very small electrical signals. An amplifier
increases the strength of these signals without changing the information they carry.
Why is it called "Common Emitter"?
A transistor has three terminals:
Emitter (E)
Base (B)
Collector (C)
In this circuit:
The input signal is applied between the Base and Emitter.
The output signal is taken between the Collector and Emitter.
Since the Emitter is common to both input and output circuits, this arrangement is known
as the Common Emitter Amplifier.
Components Used in the Circuit
Each component has an important role:
1. Transistor
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The transistor acts as the main amplifying device. A very small current entering the base
controls a much larger current flowing through the collector.
2. Collector Resistor (RC)
This resistor converts changes in collector current into changes in output voltage. It is mainly
responsible for obtaining voltage amplification.
3. Emitter Resistor (RE)
It provides thermal stability and prevents excessive current from flowing through the
transistor.
4. Biasing Resistors (R1 and R2)
These resistors provide the correct DC voltage to the transistor so that it operates
continuously in the active region, where proper amplification takes place.
5. Coupling Capacitor (C1)
The capacitor allows the AC input signal to enter the transistor while blocking unwanted DC
voltage from the previous circuit.
Working of the Common Emitter Amplifier
The working can be understood step by step.
Step 1: Applying the Input Signal
A small AC signal is applied to the base of the transistor through the coupling capacitor.
At this stage, the transistor is already properly biased by R1 and R2, so it is ready to amplify.
Step 2: Small Change at Base
Suppose the input signal increases slightly.
This causes:
Base current (IB) to increase.
Collector current (IC) increases much more because
IC = β × IB
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where β (beta) is the transistor current gain.
Even a tiny increase in base current produces a much larger increase in collector current.
This is the basic principle of amplification.
Step 3: Voltage Drop Across RC
When collector current increases, a larger voltage is dropped across the collector resistor
RC.
Since
Collector Voltage = VCC − IC × RC
an increase in collector current causes the collector voltage to decrease.
This changing collector voltage becomes the output signal.
Thus, a small input signal produces a much larger output voltage variation.
How Does Amplification Occur?
Think of a water tap.
Your finger turns the tap only a little.
But a large amount of water flows out.
Similarly,
A small base current controls
A much larger collector current.
Therefore,
Small Input Signal → Large Collector Current Change → Large Output Voltage
This is called signal amplification.
Why is There a 180° Phase Difference?
This is the most important property of the Common Emitter amplifier.
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Suppose the input signal is increasing.
Case 1: Input Signal Increases
Base current increases.
Collector current increases.
Voltage drop across RC increases.
Collector voltage decreases.
So,
Input goes UP → Output goes DOWN
Case 2: Input Signal Decreases
Base current decreases.
Collector current decreases.
Voltage drop across RC decreases.
Collector voltage increases.
So,
Input goes DOWN → Output goes UP
Thus, the output always moves in the opposite direction to the input.
This opposite movement means the output waveform is shifted by 180°, or half a cycle.
Waveform Representation
Input Signal
/ \ / \
/ \ / \
____/ \__/ \____
Output Signal
____ ____ ____
\ / \ /
\___/ \___/
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Whenever the input reaches its highest value, the output reaches its lowest value, and vice
versa. Therefore, the input and output are 180° out of phase.
Advantages of Common Emitter Amplifier
Provides high voltage gain.
Offers good current gain.
Gives high power gain.
Simple and inexpensive circuit.
Widely used in audio amplifiers, communication systems, radio receivers, and
electronic devices.
Applications
The Common Emitter amplifier is commonly used in:
Audio amplifier circuits
Radio receivers
Television circuits
Microphone amplifiers
Signal processing circuits
Communication systems
Conclusion
A Common Emitter (CE) amplifier is one of the most important transistor amplifier circuits.
In this configuration, the emitter is common to both the input and output. A small AC signal
applied at the base controls a much larger collector current, resulting in a much larger
output voltage. Thus, it effectively amplifies weak electrical signals. Since an increase in
input causes the collector voltage to decrease and a decrease in input causes the collector
voltage to increase, the output signal is always opposite to the input, producing a 180°
phase difference. Because of its high gain, simple design, and reliable performance, the
Common Emitter amplifier is extensively used in electronic and communication systems.
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SECTION-D
7. Draw circuit diagram of Phase shift oscillator, explain its working and find an expression
for its frequency of oscillations.
Ans: Introduction
A Phase Shift Oscillator is an electronic circuit that produces a continuous sine wave (AC
signal) without requiring any external input signal. It is widely used in audio frequency
generators, musical instruments, communication systems, and laboratory testing
equipment.
Think of it like this:
Imagine you push a child on a swing only once. Normally, the swing will gradually stop
because of air resistance and friction. But if someone keeps giving the swing a gentle push
at exactly the right time, it will continue swinging forever.
A Phase Shift Oscillator works in exactly the same way. Instead of a person pushing the
swing, the circuit continuously feeds a small portion of its own output back to its input. This
process is called positive feedback, which keeps the oscillations going continuously.
Circuit Diagram
+VCC
|
Rc
|
Output (Vo)
|
C
Q (Transistor)
E
|
Re
|
GND
Feedback Network
Vo ── C1 ── R1 ── C2 ── R2 ── C3 ── R3 ──► Base of Transistor
Main Parts of the Circuit
1. Transistor Amplifier
o Acts as the amplifier.
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o Increases the strength of the signal.
o Provides a phase shift of 180°.
2. RC Phase Shift Network
o Contains three resistor-capacitor (RC) sections.
o Each RC section produces approximately 60° phase shift.
o Total phase shift produced is:
What is Phase Shift?
Before understanding the oscillator, we must know what phase shift means.
Suppose two friends start running together.
If both run together, they are in phase.
If one starts later, they are out of phase.
Similarly, in electronics, when one signal is delayed compared to another, it is called phase
shift.
The phase is measured in degrees (°).
One complete wave = 360°
Working of Phase Shift Oscillator
The working can be understood step by step.
Step 1: Power Supply
When the DC power supply is switched ON, a very small electrical noise is naturally
produced inside the transistor.
This tiny signal becomes the starting signal for oscillations.
Step 2: Signal Amplification
The transistor amplifies this small signal.
During amplification, the transistor changes the signal phase by
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180°
This means the output wave becomes exactly opposite to the input wave.
Step 3: RC Network Produces Additional Phase Shift
The output now passes through three RC sections.
Each RC section delays the signal by approximately
60°
Therefore,
First RC = 60°
Second RC = 60°
Third RC = 60°
-------------------
Total = 180°
Step 4: Total Phase Shift
Now combine both phase shifts.
Transistor = 180°
RC Network = 180°
Total phase shift
180° + 180° = 360°
A 360° phase shift means the signal comes back in phase with the original input.
This satisfies the Barkhausen Criterion, which states that sustained oscillations occur when:
The total phase shift around the loop is 360° (or 0°).
The loop gain is equal to or slightly greater than 1.
Because this condition is met, the oscillator keeps producing a continuous sine wave
without any external input.
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Frequency of Oscillation
The oscillation frequency depends on the values of the resistors and capacitors in the RC
network.
For a Phase Shift Oscillator with three identical RC sections:
Where:
f = Frequency of oscillation (Hz)
R = Resistance (Ω)
C = Capacitance (F)
Derivation (Simplified Explanation)
The mathematical derivation is based on the RC phase shift network.
1. Each RC section contributes part of the required phase shift.
2. Solving the network equations shows that oscillation occurs when the total RC phase
shift equals 180°.
3. From this condition, the frequency of oscillation is obtained as:
Thus, the oscillation frequency depends only on R and C.
Important Conditions for Oscillation
For continuous oscillations:
Total phase shift = 360°
Loop gain = 1
RC network provides 180°
Transistor provides 180°
Advantages
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Produces a pure sine wave with low distortion.
Simple and inexpensive circuit.
Does not require transformers or inductors.
Suitable for low and medium frequencies.
Easy to design and maintain.
Disadvantages
Limited to low-frequency operation (typically audio frequencies).
Output power is relatively low.
The RC network attenuates the signal, so sufficient amplifier gain is required.
Frequency stability is lower than crystal oscillators.
Applications
Phase Shift Oscillators are commonly used in:
Audio signal generators
Function generators
Laboratory testing equipment
Musical instruments
Communication circuits
Electronic measurement systems
Conclusion
A Phase Shift Oscillator is a simple RC-based oscillator that generates a continuous sine
wave without needing an external input signal. It works by using a transistor amplifier,
which provides 180° phase shift, and a three-stage RC network, which provides another
180° phase shift. Together they produce a total phase shift of 360°, satisfying the
Barkhausen criterion and allowing continuous oscillations. The frequency of oscillation is
determined by the resistor and capacitor values and is given by:
Because of its simple design, low cost, and ability to produce stable audio-frequency sine
waves, the Phase Shift Oscillator is widely used in electronic circuits and laboratory
equipment.
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8. Explain, with circuit diagram working of Tuned oscillator. How it can produce oscillation
in output signal and find its frequency of oscillations.
Ans: Introduction
A Tuned Oscillator is an electronic circuit that generates a continuous AC (Alternating
Current) signal without requiring any external input signal. It is widely used in radio
transmitters, radio receivers, televisions, signal generators, communication systems, and
wireless devices.
The word "tuned" means that the circuit is adjusted to produce only one particular
frequency. This is done using a tuned circuit, also called an LC Tank Circuit, which contains
an Inductor (L) and a Capacitor (C).
Think of a tuned oscillator like a swing in a playground.
If you push the swing once, it starts moving.
After every swing, if you give it a small push at the correct time, it keeps moving
continuously.
Similarly, in a tuned oscillator, a small part of the output is sent back to the input
(called positive feedback). This continuous feedback keeps the electrical oscillations
alive.
Basic Circuit Diagram of a Tuned Oscillator
+VCC
|
RC
|
Output ----+--------------------
|
Transistor
|
|
+-----+------+
| |
L C
(Inductor) (Capacitor)
| |
+------------+
LC Tank Circuit
|
Feedback Network
|
Transistor Input
Main Components
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Transistor → Acts as an amplifier.
Inductor (L) → Stores energy in the form of a magnetic field.
Capacitor (C) → Stores energy in the form of an electric field.
Feedback Network → Returns a small portion of the output back to the input.
Power Supply (VCC) → Provides DC power to the circuit.
Working of a Tuned Oscillator
The working of a tuned oscillator can be understood in simple steps.
Step 1: DC Power is Applied
When the power supply is switched ON, the transistor starts conducting. At this time, the
capacitor begins charging.
Step 2: Energy Exchange Between L and C
Once the capacitor becomes fully charged, it starts discharging through the inductor.
The capacitor loses electrical energy.
The inductor stores this energy as magnetic energy.
After that,
The magnetic field of the inductor collapses.
It sends energy back to the capacitor.
The capacitor charges again with opposite polarity.
This process repeats continuously.
So,
Electrical Energy → Magnetic Energy → Electrical Energy
This continuous exchange produces alternating current (AC) oscillations.
Step 3: Feedback Maintains Oscillation
In every practical circuit, a little energy is lost due to resistance.
If nothing is done, the oscillations gradually become smaller and finally stop.
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To overcome this loss, the transistor amplifies the signal.
A small portion of the output is fed back to the input through the feedback network.
This is called Positive Feedback.
Positive feedback replaces the lost energy, allowing the oscillator to produce a constant
output signal.
Step 4: Continuous Output
As long as
power supply is available,
positive feedback is maintained, and
the Barkhausen criterion is satisfied (overall gain is 1 and total phase shift is 0° or
360°),
the tuned oscillator keeps generating a continuous sine wave.
How Does the Oscillator Produce Oscillations?
Oscillations are produced because of two important actions happening together:
1. LC Tank Circuit
The capacitor and inductor continuously exchange energy.
This naturally creates oscillations.
2. Positive Feedback
Some energy is lost during every cycle.
The transistor amplifies the signal and sends a small portion back to the input.
This replaces the lost energy.
Therefore,
Oscillations never die.
The output remains continuous.
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Without positive feedback, the oscillations would slowly disappear.
Frequency of Oscillation
The frequency of a tuned oscillator depends only on the values of the inductor (L) and the
capacitor (C) in the LC tank circuit.
The frequency is given by:
Where:
f = Frequency of oscillation (Hertz)
L = Inductance (Henry)
C = Capacitance (Farad)
Understanding the Formula
If L increases, the frequency decreases.
If C increases, the frequency also decreases.
If both L and C are small, the oscillator produces a higher frequency.
Thus, by changing the values of L or C, we can tune the oscillator to generate different
frequencies.
Applications of Tuned Oscillators
Tuned oscillators are commonly used in:
Radio transmitters
Radio receivers
Television circuits
Signal generators
Wireless communication systems
Mobile communication equipment
Frequency synthesizers
Electronic testing instruments
Advantages
Produces a stable sinusoidal output.
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Frequency can be adjusted easily by changing L or C.
Simple and reliable circuit.
Widely used in communication systems.
Requires no external input signal.
Disadvantages
Frequency may change slightly due to temperature variations.
Component values must be accurate for stable operation.
High-frequency circuits require careful design.
Improper feedback can stop oscillations.
Conclusion
A tuned oscillator is an electronic circuit that generates a continuous AC signal without any
external input. It uses an LC tank circuit, where the inductor and capacitor continuously
exchange energy to create oscillations. A transistor amplifier and positive feedback
compensate for energy losses, allowing the oscillations to continue indefinitely. The output
frequency is determined by the values of the inductor and capacitor and is calculated using
the formula
. Because of its stable frequency and simple operation, the tuned
oscillator is an essential circuit in radio, television, communication, and signal generation
systems.
This paper has been carefully prepared for educational purposes. If you notice any mistakes or
have suggestions, feel free to share your feedback.
