1 | P a g e

PHYSICS PROJECT

ON

TRANSISTOR AMPLIFIER

NAME: NAMIDA .M

XII F2

ROLL NO: 20

2

Table of Contents

1 INTRODUCTION TO SEMICONDUCTOR ................................................................... 3

1.1 INTRINSIC SEMICONDUCTOR ............................................................................................................ 3

1.2 EXTRINSIC SEMICONDUCTOR........................................................................................................... 3

2 COMPONENTS ....................................................................................................... 4

2.1 P-N JUNCTION ................................................................................................................................ 4

2.1 BIPOLAR JUNCTION TRANSISTOR (BJT) ....................................................................................... 5

2.1.1 Description of a transistor ............................................................................................ 5

2.1.2 General Operation .................................................................................................................. 6

2.1.3 Uses ............................................................................................................................................ 6

2.1.4 Operation of a transistor in Active state .............................................................. 6

2.1.5 Transistor as an amplifier ............................................................................................. 9

2.2 RESISTOR .................................................................................................................................. 10

2.2.1 Resistivity ............................................................................................................................. 10

2.2.2 Basic Applications Of Resistors .................................................................................. 12

2.3 CAPACITORS ............................................................................................................................ 12

2.3.1 HOW CAPACITORS WORK ....................................................................................................... 13

2.3.2 TYPES ........................................................................................................................................ 14

2.4 CATHODE RAY OSCILLOSCOPE ............................................................................................ 15

2.4.1 Uses of CRO............................................................................................................................. 16

2.5 AUDIO FREQUENCY OSCILLATOR (AFO) ........................................................................... 16

2.5.1 Applications of AF Oscillators ................................................................................... 17

3 AMPLIFIER......................................................................................................... 18

3.1 POWER AMPLIFIER ....................................................................................................................... 18

3.2 TRANSISTOR AMPLIFIERS ............................................................................................................ 18

3.3 OSCILLOSCOPE VERTICAL AMPLIFIERS ........................................................................................ 18

3.4 AMPLIFIER TYPES ........................................................................................................................ 19

3.5 TRANSISTOR AS AN AMPLIFIER .................................................................................................. 20

4 EXPERIMENT ..................................................................................................... 23

4.1 OBSERVATION AND CALCULATION ................................................................................... 23

4.2 RESULT ....................................................................................................................................... 24

4.3 GRAPH ...................................................................................................................................... 25

5 APPLICATIONS OF AMPLIFIERS ........................................................................ 26

5.1 PURPOSE ..................................................................................................................................... 26

5.2 APPLICATIONS ............................................................................................................................ 26

6 BIBLIOGRAPHY ................................................................................................. 28

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1 INTRODUCTION TO SEMICONDUCTOR A semiconductor is a material which has electrical conductivity between that of a

conductor such as copper and that of an insulator such as glass. Semiconductors are

the foundation of modern electronics, including transistors, solar cells, light-emitting

diodes (LEDs). The modern understanding of the properties of a semiconductor relies

on quantum physics to explain the movement of electrons inside a lattice of atoms.

The conductivity of a semiconductor material increases with increasing temperature,

behaviour opposite to that of a metal. Semiconductors can display a range of useful

properties such as passing current more easily in one direction than the other, variable

resistance, and sensitivity to light or heat. Because the conductive properties of a

semiconductor material can be modified by controlled addition of impurities or by the

application of electrical fields or light, devices made with semiconductors are very

useful for amplification of signals, switching, and energy conversion.

Semiconductor is broadly classified as Intrinsic and Extrinsic semiconductor.

1.1 Intrinsic semiconductor

An intrinsic semiconductor, also called an undoped semiconductor or i-type

semiconductor, is a pure semiconductor without any significant dopant species

present. The number of charge carriers is therefore determined by the

properties of the material itself instead of the amount of impurities. In

intrinsic semiconductors the number of excited electrons and the number of

holes are equal: n = p.

The electrical conductivity of intrinsic semiconductors can be due to

crystallographic defects or electron excitation. In an intrinsic semiconductor

the number of electrons in the conduction band is equal to the number of holes

in the valence band. An example is Hg 0.8Cd 0.2Te at room temperature.

An indirect band gap intrinsic semiconductor is one in which the maximum

energy of the valence band occurs at a different k (k-space wave vector) than

the minimum energy of the conduction band. Examples include silicon and

germanium. A direct band gap intrinsic semiconductor is one where the maximum

energy of the valence band occurs at the same k as the minimum energy of the

conduction band. Examples include gallium arsenide.

1.2 Extrinsic semiconductor

An extrinsic semiconductor is a semiconductor that has been doped, that is, into

which a doping agent has been introduced, giving it different electrical

properties than the intrinsic (pure) semiconductor.

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Doping involves adding dopant atoms to an intrinsic semiconductor, which

changes the electron and hole carrier concentrations of the semiconductor at

thermal equilibrium. Dominant carrier concentrations in an extrinsic

semiconductor classify it as either an n-type or p-type semiconductor. The

electrical properties of extrinsic semiconductors make them essential

components of many electronic devices.

2 COMPONENTS

Components are listed below:

Transistor

Resistor

Capacitor

Cathode Ray Oscilloscope (CRO)

Audio Frequency Oscillator (AFO)

2.1 P-N Junction

Two important processes occur during the formation of a p-n junction:

Diffusion:

Due to the concentration gradient across p-, and n- sides, holes diffuse from p-

side to n-side (p → n) and electrons diffuse from n-side to p-side (n → p giving

rise to diffusion current across the junction. When an electron diffuses from n

→ p, it leaves behind an ionised donor on n-side. This ionised donor (positive

charge) is immobile as it is bonded to the surrounding atoms. As the electrons

continue to diffuse from n → p, a layer of positive charge (or positive space-

charge region) on n-side of the junction is developed. Similarly, when a hole

diffuses from p → n, a layer of negative charge (or negative space-charge

region) on the p-side of the junction is developed. This space-charge region on

either side of the junction together is known as depletion region as the

electrons and holes taking part in the initial movement across the junction

depleted the region of its free charges.

Drift:

Due to the positive space-charge region on n-side of the junction and a negative

space charge region on p-side of the junction, a electric field directed from

positive charge towards negative charge develops. Due to this field, an electron

on p-side of the junction moves to n-side and a hole on n side of the junction

moves to p-side. The motion of charge carriers due to the electric field is called

drift. Thus a drift current, which is opposite in direction to the diffusion

current, starts to flow.

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2.1 Bipolar Junction Transistor (BJT)

The first junction transistor consisting of two back-to-back p-n junctions

was invented by William Schockley in 1951.

Types of transistors

There are two types of transistors,

(i) n-p-n transistor: two segments of n-type semiconductor (emitter

and collector) are separated by a segment of p-type semiconductor

(base).

(ii) p-n-p transistor: two segments of p-type semiconductor ( emitter

and collector) are separated by a segment of n-type semiconductor

(base).

2.1.1 Description of a transistor

A transistor has three doped regions forming two p-n junctions between

them.

All the three segments of a transistor have different thickness and their

doping levels are also different.

(i) Emitter: This is the segment on one side of the transistor shown

in. It is of moderate size and heavily doped. It supplies a large

number of majority carriers for the current flow through the

transistor.

(ii) Base: This is the central segment. It is very thin and lightly doped.

Fig: p-n junction formation

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(iii) Collector: This segment collects a major portion of the majority

carriers supplied by the emitter. The collector side is moderately

doped and larger in size as compared to the emitter.

Symbol of a transistor

The arrowhead shows the direction of conventional current in the

transistor.

2.1.2 General Operation

Depletion regions are formed at the emitter base-junction and the base-

collector junction. The charge carriers move across different regions of the

transistor when proper voltages are applied across its terminals. The biasing of

the transistor is done differently for different uses.

2.1.3 Uses

(i) As an amplifier, a device which produces an enlarged copy of a

signal. For the project, the transistor BC107 is used.

(ii) As a switch.

2.1.4 Operation of a transistor in Active state

Fig: Transistor and its symbol

Fig: Transistor as an amplifer

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Emitter-base junction is forward biased and the base-collector junction

is reverse biased

.

The VCC and VEE represent the two power supplies with base as a common

terminal and other terminals are connected to emitter and collector. The

voltage between emitter and base is represented as VEB and that between the

collector and the base as VCB. As emitter is the common terminal, the power

supply between the base and the emitter is represented as VBB and that

between collector and emitter as VCC.

The heavily doped emitter has a high concentration of majority carriers,

which will be electrons in an n-p-n transistor. These majority carriers enter the

thin and lightly doped base region in large numbers swamps the small number of

holes in the base as the majority carriers in the base would be few. As the base

collector-junction is reverse-biased, these electrons, which appear as minority

carriers at the junction, can easily cross the junction and enter the collector.

The base is made thin so that most of the electrons find themselves near the

reverse-biased base-collector junction and so cross the junction instead of

moving to the base terminal.

The current entering into the emitter from outside is equal to the

emitter’s current IE. The current emerging from the base terminal IB is a small

fraction of the emitter current and the current from collector terminal is IC.

The emitter current is the sum of collector current and base current.

Fig: Bias voltage applied to a transistor

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IE = IC + IB

Also IC IE.

As the direction of motion of electrons is just opposite to that of the

current, in a n-p-n transistor current enters from the base into the emitter.

In the active state of the transistor the emitter-base junction acts as a

low resistance while the base collector acts as a high resistance.

Three configurations

The transistor can be connected in either of the following three configurations:

(i) Common Emitter (CE),

(ii) Common Base (CB),

(iii) Common Collector (CC)

Common emitter transistor characteristics

When a transistor is used in CE configuration, the input is between the

base and the emitter and the output is between the collector and the emitter.

The variation of the base current IB with the base-emitter voltage VBE is

called the input characteristic. Since the increase in VCE appears as increase in

VCB, its effect on IB is negligible. As a consequence, input characteristics for

various values of VCE will give almost identical curves. Hence, it is enough to

determine only one input characteristics.

The variation of the collector current IC with the collector-emitter

voltage VCE is called the output characteristic. There will be different output

characteristics corresponding to different values of IB .

Fig: Transistor currents

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2.1.5 Transistor as an amplifier

In general, amplifiers are used to amplify alternating signals.

Fixing the bias:

To operate the transistor as an amplifier it is necessary to fix its

operating point somewhere in the middle of its active region. If we fix

the value of VBB corresponding to a point in the middle of the linear part

of the transfer curve then the dc base current IB would be constant and

corresponding collector current IC will also be constant. The dc voltage

VCE = VCC – ICRC would also remain constant. The operating values of VCE and IB determine the operating point, of the amplifier.

Applying the input:

If a small sinusoidal voltage with amplitude vs is superposed on the

dc base bias by connecting the source of that signal in series with the VBB supply, then the base current will have sinusoidal variations superimposed

on the value of IB. As a consequence the collector current also will have

sinusoidal variations superimposed on the value of IC, producing

in turn corresponding change in the value of VO.

Measuring the output:

We can measure the ac variations across the input and output

terminals by blocking the dc voltages by large capacitors. The output is

taken between the collector and the ground.

Fig: CE Transistor amplifier

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2.2 RESISTOR

A resistor is an electrical component designed to have an electrical

resistance that is independent of the current flowing through it. The common

type of resistor is also designed to be independent of temperature and other

factors. Resistors may be fixed or variable. Variable resistors are also

called potentiometers or rheostats

Commercially produced resistors for domestic use or in laboratories are of two

major types: wire bound resistors and carbon resistors. Wire bound resistors

are made by winding the wires of an alloy, viz., manganin, constantan, nichrome

or similar ones. The choice of these materials is dictated mostly by the fact

that their resistivities are relatively insensitive to temperature. These

resistances are typically in the range of a fraction of an ohm to a few hundred

ohms

Carbon Resistors Wire Wound Resistor

2.2.1 Resistivity

The resistivity of a material is found to be dependent on the temperature.

Different materials do not exhibit the same dependence on temperatures. Over

a limited range of temperatures, that is not too large, the resistivity of a

metallic conductor is approximately given by,

= 0 [1 + α (T–T0)]

where is the resistivity at a temperature T and 0 is the same at a

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reference temperature T0. α is called the temperature co-efficient of resistivity, and the dimension of α is (Temperature)–1.

Resistors in the higher range are made mostly from carbon. Carbon resistors

are compact, inexpensive and thus find extensive use in electronic circuits.

Carbon resistors are small in size and hence their values are given using a colour

code

Some materials like Nichrome (which is an alloy of nickel,

iron and chromium) exhibit a very weak dependence of

resistivity with temperature. Manganin and constantan

have similar properties. These materials are thus widely

used in wire bound standard resistors since their

resistance values would change very little with

temperatures. Unlike metals, the resistivities of

semiconductors decrease with increasing temperatures. A

typical dependence is shown in the graph

An ohmic resistor is made ohmic because its function follows Ohm's law. Ohm's

law essentially says that current is equal to voltage divided by resistance.

Additionally, resistance is equal to voltage divided by current, and voltage is

equal to current times resistance. Therefore, in a circuit, if a resistor's

resistance is equal to voltage divided by current, the resistor is ohmic.

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Ohmic resistors are used in most cases where a standard resistor is required in

a circuit. For example, if you wanted to power a one-ampere LED in a circuit

running on two amperes, you would use an ohmic resistor.

Ohmic Resistor

2.2.2 Basic Applications Of Resistors

Heaters

Joule heating is the heat created as current passes through a resistor.

The heat is generated by the interaction with the electrons flowing

through a conductor, impacting its atoms and ions, essentially generating

heat through friction.

Fuses

Specially designed resistors are commonly used as single use fuses. The

conductive element in a fuse is designed to destroy itself once a certain

current threshold is reached, essentially sacrificing itself to prevent

damage to more expensive electronics.

Sensors

Resistors are often used as sensors for a wide range of applications

from gas sensors to lie detectors. A change in resistance can be caused

by a large number of factors including water and other liquids,

moisture, strain or flexing, and absorption of gas in to the resistive

material.

2.3 CAPACITORS

A capacitor is a two terminal device consisting of two bodies known as plates

which store equal and opposite charge, such that the amount of charge stored in

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each plate of the capacitor is directly proportional to the potential difference

across the plates (terminals) of the capacitor, and the proportionality constant

is called capacitance.

Capacitors vary in shape and size, but the basic configuration is two conductors

carrying equal but opposite charges. Capacitors have many important

applications in electronics. Some examples include storing electric potential

energy, delaying voltage changes when coupled with resistors, filtering out

unwanted frequency signals, forming resonant circuits and making frequency-

dependent and independent voltage dividers when combined with resistors.

The conductors may be so charged by connecting them to the two terminals of a

battery. Q is called the charge of the capacitor, though this, in fact, is the

charge on one of the conductors – the total charge of the capacitor is zero. The

electric field in the region between the conductors is proportional to the charge

Q. That is, if the charge on the capacitor is, say doubled, the electric field will

also be doubled at every point. (This follows from the direct proportionality

between field and charge implied by Coulomb’s law and the superposition

principle.)

2.3.1 HOW CAPACITORS WORK

When the voltage from a source such as a battery is connected, the negative

side of the battery voltage immediately begins to push negative charges toward

one of the plates. Simultaneously, the positive side of the battery voltage

begins to pull electrons (negative charges) away from the second plate.

What permits current to flow is the electric field that quickly builds up

between the two plates. As the plate on the negative side of the circuit fills

with electrons, the electric field created by those electrons begins to push

electrons away from the plate on the other side of the insulator, toward the

positive side of the battery voltage.

As this current flows, the negative plate of the capacitor builds up an excess of

electrons, whereas the positive side develops a corresponding deficiency of

electrons. Thus, voltage is developed between the two plates of the capacitor.

But there's a catch: This current flows only for a brief time. As the electrons

build up on the negative plate and are depleted from the positive plate, the

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voltage between the two plates increases because the difference in charge

between the two plates increases.

The voltage continues to increase until the capacitor voltage equals the battery

voltage. Once the voltages are the same, current stops flowing through the

circuit, and the capacitor is said to be charged.

Also, once a capacitor has been charged, you can disconnect the battery from

the capacitor, and the voltage will remain in the capacitor. In other words,

although the voltage in the capacitor is created by the battery, this voltage

isn't dependent on the battery for its continued existence. Disconnect the

battery, and the voltage remains across the two plates of the capacitor.

Thus, capacitors have the ability to store charge — an ability known as

capacitance.

2.3.2 TYPES

The capacitance of a capacitor purely depends on the geometry of the capacitor

and the medium in which it is placed. A few common types are 9based on

geometry:

Cylindrical capacitors

Parallel plate capacitor

Spherical capacitors

Based on material they are classified as:

Air - Often used in radio tuning circuits

Mylar - Most commonly used for timer circuits like clocks, alarms and

counters

Glass - Good for high voltage applications

Ceramic - Used for high frequency purposes like antennas, X-ray and MRI

machines

Super capacitor - Powers electric and hybrid cars

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2.4 CATHODE RAY OSCILLOSCOPE

The German physicist Ferdinand Braun. Cathode Ray Oscilloscope is an

electronic-display device containing a cathode ray tube that generates an

electron beam that is used to display visible patterns or graphs on a

phosphorescent screen. The graphs plot relationships between two variables

with the x-axis generally being a function of time and the y-axis a function of

the voltage generated by the input signal to the oscilloscope. A CRO contains a

cathode ray tube with three main parts

Electron gun

This consists of a heater, a cathode, a grid (brightness control) and anodes. The

grid is at a lower potential than the cathode and is used to control number of

electrons passing through central hole of the gun. The anodes are at a high

potential compared to the cathode. They accelerate electrons and focus them

into a narrow beam.

Phosphorescent screen

A bright spot of light is produced on the screen where the electron beam hits

it. Beams are deflected to produce luminous patterns on the screen such that

these patterns visually capture the voltages applied to the deflection plates.

Deflecting system

Beyond the anodes are two pairs of deflecting plates to which potential

differences can be applied. The Y-plates are horizontal and cause vertical

deflection. The potential difference to create the electric field between the Y-

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plates is applied to the Y-input terminal. The input is usually amplified to give

suitable vertical deflection. The vertical plates are called the X-plates and they

produce horizontal deflection. The potential difference applied to the X-plates

is also amplified by an amplifier – the X-amplifier, and is commonly from the

time base circuit of the CRO. The time base circuit of the CRO applies a wave

saw-tooth waveform to the X-plates. The beam is moved from the left hand side

to the right in the time that the voltage rises to a maximum, and then is

returned rapidly to the left as the voltage returns to zero. Alternatively the

electron beam may also be deflected magnetically through externally mounted

deflection coils.

2.4.1 Uses of CRO

1) Voltage Measurements

The oscilloscope must first be calibrated with by connecting a dc source of

known emf to the Y-plates and measuring the deflection of the spot on the

screen. By repeating this process by altering the known emf, the linearity of the

deflections, in Volts per meter can be found.

2) Displaying waveforms

In this role, the time base is on. The CRO acts as a graph-plotter to show the

waveform i.e. the variation of potential difference applied across the Y-plates

with time.

3) Frequency Measurements

Measurement of frequency is enabled by a calibrated time base. If the time

base is set at a certain value in time per length for a waveform, one complete

waveform occupies two units of length. Therefore twice the value at which the

time base is set is the time period of the waveform. The reciprocal of the time-

period is the frequency of the waveform.

2.5 AUDIO FREQUENCY OSCILLATOR (AFO)

Audio Frequency Oscillator (AF Oscillator), also known as audio oscillator, is a

useful test instrument capable of generating sine, and also square & triangle

waveforms at any required frequency within audible range of average human

being. The generally accepted standard range of audible frequency of human is

20Hz – 20KHz.

Oscillators come with different waveforms, which determine how it varies.

Some, such as sine wave oscillators are smooth whereas others are sharp and

sudden, like square wave oscillators.

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An AFO merely produces an oscillation in voltage, not air pressure. Hence we

cannot hear it directly. In order to hear it, the AFO needs to be connected to a

speaker through the output terminals.

An AFO uses an electron tube, transistor, or other nonrotating device to

produce an audio-frequency alternating current. The instrument is provided with

output banana sockets and generally gives a maximum wave output of 20 volts

peak to peak approximately, which may be adjusted by an amplitude control

potentiometer. The frequency of the signal source is set on a dial (1 to 11)

provided with the AFO, along with a multiplier range switch (x1, x10, x100, x1K,

x10K, x100K).

General specifications of an AFO may be stated as,

Frequency range : 20Hz to 20KHz (audible range)

Wave forms : Sine, Square, Triangle waveforms

Output Amplitude : 0-20V peak to peak variable ± 3% approx.

Power Supply : 220 Volts

2.5.1 Applications of AF Oscillators

AFO’s are widely used in electronic industry and educational institutions

AFO's are often used as test signal sources in laboratories and research

facilities.

Variable frequency oscillators are used in audio signal generators that are

essential for testing amplifiers and fault tracing in many electronics

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3 AMPLIFIER

An electronic amplifier is an electronic device that increases the power of

a signal. It does this by taking energy from a power supply and controlling the

output to match the input signal shape but with a larger amplitude. In this

sense, an amplifier modulates the output of the power supply.

There are four basic types of electronic amplifier: the voltage amplifier,

the current amplifier, the transconductance amplifier, and the transresistance

amplifier.

3.1 Power amplifier

The term power amplifier is a relative term with respect to the amount of

power delivered to the load and/or provided by the power supply circuit. In

general the power amplifier is the last 'amplifier' or actual circuit in a signal

chain (the output stage) and is the amplifier stage that requires attention to

power efficiency.

3.2 Transistor amplifiers

The essential role of this active element is to magnify an input signal to yield a

significantly larger output signal. The amount of magnification (the "forward

gain") is determined by the external circuit design as well as the active device.

Applications are numerous, some common examples are audio amplifiers in a

home stereo or PA system, RF high power generation for semiconductor

equipment, to RF and Microwave applications such as radio transmitters

3.3 Oscilloscope vertical amplifiers

These deal with video signals that drive an oscilloscope display tube, and can

have bandwidths of about 500 MHz.

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3.4 Amplifier types

Amplifiers are described according to their input and output

properties.[2] They exhibit the property of gain, or multiplication factor that

relates the magnitude of the output signal to the input signal. The gain may be

specified as the ratio of output voltage to input voltage (voltage gain), output

power to input power (power gain), or some combination of current, voltage, and

power. In many cases, with input and output in the same unit, gain is unitless

(though often expressed in decibels (dB)).

The four basic types of amplifiers are as follows: Voltage amplifier – This is the

most common type of amplifier. An input voltage is amplified to a larger output

voltage. The amplifier's input impedance is high and the output impedance is low.

Current amplifier – This amplifier changes an input current to a larger

output current. The amplifier's input impedance is low and the output

impedance is high.

Transconductance amplifier – This amplifier responds to a changing input

voltage by delivering a related changing output current.

Transresistance amplifier – This amplifier responds to a changing input

current by delivering a related changing output voltage. Other names for

the device are transimpedance amplifier and current-to-voltage

converter.

Transfer characteristic

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3.5 Transistor as an amplifier

For using the transistor as an amplifier we will use the active region of the Vo

versus Vi curve. The slope of the linear part of the curve represents the rate of

change of the output with the input. It is negative because the output is VCC –

ICRC and not ICRC. That is why as input voltage of the CE amplifier increases

its output voltage decreases and the output is said to be out of phase with the

input. If we consider ΔVo and ΔVi as small changes in the output and input

voltages then ΔVo/ΔVi is called the small signal voltage gain AV of the

amplifier. If the VBB voltage has a fixed value corresponding to the mid point

of the active region, the circuit will behave as a CE amplifier with voltage gain

ΔVo/ ΔVi. We can express the voltage gain AV in terms of the resistors in the

circuit and the current gain of the transistor as follows.

We have, Vo = VCC – ICRC

Therefore, ΔVO = 0 – RCΔ Ic

Similarly, from Vi = IBRB + VBE

ΔVi= RB ΔIB + ΔVBE

But ΔVBE is negligibly small in comparison to ΔIBRB in this circuit.

So, the voltage gain of this CE amplifier is given by

AV = – RC Δ IC / RB ΔIB

= –βac(RC/RB )

where βac is equal to Δ IC/ΔIB . Thus the linear portion of the active region of

the transistor can be exploited for the use in amplifiers. Transistor as an

amplifier (CE configuration) is discussed in detail in the next section.

The most common way to use a transistor as an amplifier is in an electronic

circuit sometimes called a common-emitter circuit because the emitter is

connected to ground, which means that both the input signal and the output

signal share the emitter connection.

Below circuit uses a pair of resistors as a voltage divider to control exactly how

much voltage is placed across the base and emitter of the transistor. The AC

signal from the input is then superimposed on this bias voltage to vary the bias

current. Then, the amplified output is taken from the collector and emitter.

Variations in the bias current are amplified in the output current.

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A voltage divider is simply a pair of resistors. The voltage across both resistors

equals the sum of the voltages across each resistor individually. we can divide

the voltage any way we want by picking the correct values for the resistors. If

the resistors are identical, the voltage divider cuts the voltage in half.

Otherwise, we can use a simple formula to determine the ratio at which the

voltage is divided.

If we look at the schematic diagram, we see that there are actually two voltage

dividers in the circuit. The first is the combination of resistors R1 and R2,

which provide the bias voltage to the transistor's base. The second is the

combination of resistors R3 and R4, which provide the voltage for the output.

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This second voltage divider is a variable voltage divider: The ratio of the

resistances changes based on the bias voltage, which means the voltage at the

collector varies as well. The amplification occurs because very small variations in

an input signal are reflected in much larger variations in the output signal.

Looking at this circuit more closely:

The input arrives at the left side of the circuit in the form of a signal,

which usually has both a DC and an AC component. In other words, the

voltage fluctuates but never goes negative.

One side of the input is connected to ground, to which the battery's

negative terminal is also connected. The transistor's emitter is also

connected to ground (through a resistor), as is one side of the output.

The purpose of C1 is to block the DC component of the input signal. Only

pure AC gets past the capacitor. Without this capacitor, any DC voltage in

the input signal would be added to the bias voltage applied to the

transistor, which could spoil the transistor's ability to faithfully amplify

the AC part of the input signal.

R1 and R2 form a voltage divider that determines how much DC voltage is

applied to the transistor base. The AC portion of the signal that gets

past C1 is combined with this DC voltage, which causes the transistor's

base current to vary with the voltage.

R3, R4, and the variable resistance of the collector-emitter circuit form

a voltage divider on the output side of the amplifier. Amplification occurs

because the full power supply voltage is applied across the output circuit.

The varying resistance of the collector-emitter path reflects the small

AC input signal on the much larger output signal.

C2 blocks the DC component of the output signal so that only pure AC is

passed on to the next stage of the amplifier circuit.

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4 EXPERIMENT

4.1 OBSERVATION AND CALCULATION

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4.2 Result

The transistor amplifier in CE mode is constructed. Values of

Input

Peak voltage : 1 V

Half wave time: 2ms

Full wave time: 4ms

Output

Peak voltage : 4.4V

Half wave time: 5.2ms

Full wave time: 5x2ms

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4.3 GRAPH

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5 Applications of Amplifiers

5.1 Purpose

Used for amplification i.e. the process of increasing the amplitude (and

consequently the strength) of a signal. Amplifiers are capable of increasing the

power, voltage or current of a signal. It comprises of an electronic circuit and is

necessary for the compensation of the attenuation of signals in communication

using a DC power source.

The amplifier plays a role at locations between the source and the

destination wherever the signal strength becomes weaker than the

required strength.

5.2 Applications

Amplifiers are generally seen in audio systems where signals are

repeatedly converted from electrical signals to audio signals and vice

versa. During the conversion, amplifiers set the signal to the frequency

required for the specific device.

The common emitter amplifier is commonly used and is used in transistor

circuits where voltage gain is required.

The common-emitter amplifier is designed so that a small change in the

input voltage (Vin) changes the small current through the base of the

transistor; the transistor's current amplification combined with the

properties of the circuit mean that small swings in Vin produce large

changes in Vout (output voltage).

Amplifiers are used in devices that transmit signals as well as devices

that receive signals.

Amplifiers are used in devices like mobile phones and televisions for

sound reproduction, radio transmission, and signal processing.

27

When the power dissipated across the resistor in the amplifier circuit is

more than the supply power, the amplifier is a power amplifier.

In the common emitter amplifier, the voltage across the resistor in the

amplifier circuit is more than the input voltage.

28

6 BIBLIOGRAPHY

www.howstuffworks.com

www.electronicstutorials

hyperphysics.phy-astr.gsu.edu/hbase/solids/trans.html

www.britannica.com/

www.ncert.nic.in