JIMMA UNIVERSITY

JIMMA INSTITUTE OF TECHNOLOGY

FACULITY OF ELECTRICAL AND COMPUTER ENGINEERING

COURSE NAME: COMMUNICATION SYSTEM

TARGET: 4TH YEAR COMMUNICATION STREAM SUB- GROUP 2 AND 4

TITLE: FREQUENCY MODULATED (FM) TRANSMITTER

SUBMITTED TO: Mr. GETACHEW A.

SUBMISSIONDATE: MARCH 10, 2020

 GROUP NAME ID NUMBER

1) KITILA TESFA RU2877/09

2) AMAUEL TSEGAYE RU2609/09

3) GETAHUN HASSEN RU4939/09

4) SABONA DAME RU1364/09

5) HAWI HAILU RU0988/09

6) TEDDY GEBEYEHU RU1865/09

7) FESHA TEMESGEN RU4770/09

8) TARIKU MULUGETA RU4629/09

9) SABA TIKU RU0576/09

10) BALEW YITAYEW RU0046/09

11) BISRAT GELANA RU0866/09

12) ESRAEL BEKELE 02655/06

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ACKNOWLEDGEMENT
We are delighted to express our heartfelt appreciation to Mr. Getachew A. for giving us this
opportunity to do our project on frequency modulated (FM) transmitter. We then extend our
sincere gratitude to all of our friends who were involving and helping us in doing this project.

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ABSTRACT
An information in electronic communication systems influence most of human activities. A
rapidly growing demand for the use of FM transmitter exists among individuals and institutions.
The FM transmitters are however complex equipment demanding high power supply, system
design and high price. These problems of the transmitter constitute major impediments or hinders
to individuals and institution that may wish to adopt radio broadcast as means of electronic
media. The aim of this project is to design and implement an FM transmitter that is cheap in
price, simple in maintenance, efficient in use yet operate on low power supply. This design of
FM transmitter contains amplifier (preamplifier and power amplifier), tank or modulator circuit
and oscillator. The successful completion of this design has indicated that practical frequency
modulated (FM) transmitter transmit information at 91.3MHz frequency with 133mW
transmitted power.

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Table of Contents
ACKNOWLEDGEMENT ............................................................................................................... i

ABSTRACT.................................................................................................................................... ii

List of Figures ................................................................................................................................. v

ACRONOMY ................................................................................................................................ vi

CHAPTER 1 ................................................................................................................................... 1

INTRODUCTION ....................................................................................................................... 1

1.1 Background ........................................................................................................................... 1

1.2 Problem Statement ................................................................................................................ 1

1.3 Objective ............................................................................................................................... 1

1.3.1 General Objective ........................................................................................................... 1

1.3.2 Specific Objectives ......................................................................................................... 1

1.4 Project Scope ......................................................................................................................... 2

1.5 Significance of Project .......................................................................................................... 2

1.6 Methodology ......................................................................................................................... 2

CHAPTER 2 ................................................................................................................................... 3

METHODOLOGY AND SYSTEM DESIGN ............................................................................ 3

2.1 Components Description ....................................................................................................... 3

2.2 General Block Diagram ......................................................................................................... 4

2.3 Design Procedures ................................................................................................................. 5

2.3.1 Pre-amplifier or Audio Amplifier Design ...................................................................... 5

2.3.2 Oscillator Design ............................................................................................................ 7

2.3.3 Power Amplifier Design ............................................................................................... 10

CHAPTER 3 ................................................................................................................................. 13

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RESULT AND DISCUSSION.................................................................................................. 13

5.1 Simulation of Preamplifier (Audio Amplifier).................................................................... 13

5.2 Simulation of Oscillator Circuit .......................................................................................... 14

5.3 Simulation of Power Amplifier ........................................................................................... 15

5.4 Overall Circuit Diagram ...................................................................................................... 16

CHAPTER 4 ................................................................................................................................. 17

CONCLUSION AND RECOMMENDATIONS ......................................................................... 17

6.1 Conclusion........................................................................................................................... 17

6.2 Recommendations ............................................................................................................... 17

REFERANCE ............................................................................................................................... 18

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List of Figures
Figure 1.1 Methodology of FM Transmitter design ....................................................................... 2
Figure 2.1 Block Diagram............................................................................................................... 5
Figure 2.2 Pre-amplifier circuit diagram ........................................................................................ 5
Figure 2.3 Oscillator circuit diagram .............................................................................................. 8
Figure 2.4 Power amplifier circuit diagram .................................................................................. 11
Figure 3.1 Audio Amplifier Circuit .............................................................................................. 13
Figure 3.2 Output Signal of audio amplifier ................................................................................. 13
Figure 3.3 Schematic Diagram of Oscillator ................................................................................ 14
Figure 3.4 Output of Oscillator ..................................................................................................... 14
Figure 3.5 Power amplifier circuit ................................................................................................ 15
Figure 3.6 Simulation result of power circuit ............................................................................... 15
Figure 3.7 FM Circuit on Multisim............................................................................................... 16
Figure 3.8 Simulation result of overall circuit .............................................................................. 16

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ACRONOMY
AC – Alternate Current

FM – Frequency Modulation

IF – Intermediate Frequency

PCB – Printed Circuit Board

KVL – Kirchhoff’s Voltage Law

SI – Standard International

AM – Amplitude Modulation

DC – Direct Current

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CHAPTER 1
INTRODUCTION
1.1 Background
The world has entered an era known as ‘information age’ when electronics communication
systems influence most of human activities. These electronics communication systems consist of
three major aspects; the source (transmitter), the channel (medium) and the receiver
(destination). In frequency modulated (FM) transmitter which is usually employed for broadcast
purposes at the medium frequency, the frequency of a carrier wave is varied in accordance with
the amplitude and frequency of modulating wave which is very selective. In FM transmitter the
message signal is not highly affected by noise since only frequency is varying and amplitude is
constant.

1.2 Problem Statement

A rapidly growing demand for the use of FM transmitter exists among individuals and
institutions. Private individual also wishes to provide a view of local and community news that is
independent government stand. These plants are however complex equipment demanding, high
power supply, high voltage system design and high price.

1.3 Objective
1.3.1 General Objective
The general objective of this project is to design and implements an FM transmitter.

1.3.2 Specific Objectives

In specifically the objectives of this project is:

✓ To analyze pre-amplifier or audio amplifier circuit.

✓ To design FM oscillator circuit.
✓ To evaluate tank circuit of modulator using capacitor and inductor.
✓ To design the power amplifier.

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1.4 Project Scope

To design and implement FM transmitter in the area of universities and other tertiary institutions
need to broadcast educational, entertainment and even news program that are particularly
designed for campus audience. For rural area this is inefficient because there many obstacles
related to power supply.

1.5 Significance of Project

To convey information by using low power and low cost and also, to transmit an information to a
receiver with simple circuit design.

1.6 Methodology
In our design and implementation of FM transmitter we follow a method of identification
components which are very helpful for the design procedure and then in order to get the accurate
value of designed parameters we have to do a certain mathematical calculation. After that we
simulate the designed part to identify the result by using multisim and oscilloscope (output
display). And finally the result is discussed briefly. The block diagram is shown below.

Figure 1.1 Methodology of FM Transmitter design

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CHAPTER 2
METHODOLOGY AND SYSTEM DESIGN
In FM design also active electronic devices like transistor, microphone and the passive electronic
devices such as resistor, capacitor and inductor are used. The complete description of those
components is listed in this chapter.

2.1 Components Description

The various components to be used for each part of the project are listed. To facilitate adequate
understanding of their operations the major components in this circuit with respects to the
characteristics were discussed briefly.

1. Microphone

Microphones a device that translates sound vibrations in the air into electronic signals to a
recording medium. It converts sound energy to electrical energy for further processing by the
circuit. In our project we use dynamic microphones since it have quite low output impedance,
and therefore have built in step up transformer match line impedance.

2. Resistors

A resistor is a passive two-terminal electrical component that opposes electrical flow in

electronic circuits. It used to reduce current flow, to adjust the signal levels, to divide voltages, to
bias active elements, and to terminate transmission lines. Its SI unit is ohm (Ω).

3. Transistor

A transistor is a semiconductor device used to amplify or switch electrical signals and power. It
is composed of semiconductor material usually with at least three terminals for connection to an
external circuit which are emitter (E), base (B) and collector (C) terminals.

A transistor can have used as an amplifier and switch. It amplifies a signal only in active region
and as a switch in cutoff and saturation region. Today, some transistors are packaged
individually, but most are found embedded in integrated. In this project we use transistors for
amplification (audio amplifier and power amplifier) and Colpitts oscillator.

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4. Capacitor

A capacitors a device that stores electrical energy in an electric field. It is a passive electronic
with two terminals. The effect of a capacitor is known as capacitance and it’s SI unit is farad (F).
The importance of capacitor in this project is for coupling of one stage with the other stage,
bypass emitter resistance to increase the gain of the amplifier and to produce the resonant
frequency of the colpitts oscillator.

5. Inductor

An inductor is a passive two-terminal electrical component that stores energy in a magnetic field
when an electric current flows through it. It consists of an insulated wire wound into a coil
around a core and it’s SI unit is Henry (H). The inductor is used to produce the resonant
frequency of the colpitts oscillator by connecting parallel with capacitor.

6. Lead-Acid Battery

Lead-Acid Battery: is the most common batteries in the world today which can take a fair
amount of abuse, high discharge rates and fast charging. But for long term storage it has the most
favorable characteristics, losing 40% of their charge over one year as compared to 6 months with
others batteries. We use this battery to bias the overall circuit.

7. Transmitter Antenna

In radio engineering, an antenna is the interface between radio waves propagating through space
and electric currents moving in a metal conductor, used with transmitter or receiver. In
transmission a radio transmitter supplies an electric current to the antenna’s terminals and the
antenna radiates the energy from the current as electromagnetic waves (radio waves).
Omnidirectional antenna is used to transmit in all direction.

2.2 General Block Diagram

Designing is the process of finding each and every component used in the circuit by taking some
assumptions which depends on specification and operating characteristic of materials. This FM
transmitter consists of five stages, which are microphone, amplifier (audio and power), oscillator
and antenna.

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Figure 2.1 Block Diagram

2.3 Design Procedures

2.3.1 Pre-amplifier or Audio Amplifier Design
The pre-amplifier design is needed because of the microphone has an operating voltage (5mV –
50mV) which is very low for FM transmitter and this operating voltage has to be amplified for
the next stage. For this design process,𝑉𝑐𝑐 = 12𝑉.

Figure 2.2 Pre-amplifier circuit diagram

In DC biasing of transistor, the capacitor is acting as open circuit since the frequency of DC
supply is zero.

For Imax is given, 𝐼𝑠𝑎𝑡 = 𝐼𝑚𝑎𝑥 = 10𝑚𝐴.

𝐼𝑚𝑎𝑥 10𝑚𝐴
In this case 𝐼𝑐 = = = 5𝑚𝐴 ≅ 𝐼𝐸
2 2

By using the rule of Thumb 𝑉𝐸 is approximated to 10% of 𝑉𝑐𝑐 . Then

𝑉𝐸 = 0.1𝑥𝑉𝑐𝑐 = 0.1𝑥12𝑉 = 1.2𝑉.

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𝑉𝐸 1.2𝑉
The emitter resistor is obtained by applying ohm’s law; 𝑅𝐸 = 𝑅3 = = = 𝟐𝟒𝟎𝛀
𝐼𝐸 5𝑚𝐴

Since the transistor is only used as an amplifier only in active region, the transistor collector-
emitter voltage (𝑉𝐶𝐸 ) is approximated to 50% of 𝑉𝑐𝑐 .

𝑉𝑐𝑐 12𝑉
𝑉𝐶𝐸 = = = 𝟔𝑽.
2 2

The collector voltage 𝑉𝐶 will be, 𝑉𝐶 = 𝑉𝐸 + 𝑉𝐶𝐸 = 1.2𝑉 + 6𝑉 = 𝟕. 𝟐𝑽.

𝑉𝑐𝑐 − 𝑉𝐶 12𝑉−7.2𝑉
The collector resistance 𝑅𝐶 will be, 𝑅𝐶 = 𝑅4 = = = 𝟗𝟔𝟎𝛀.
𝐼𝐶 5𝑚𝐴

In the design of common emitter DC biasing and using rule of Thumb,

𝛽𝑥𝑅𝐸
𝑅1 ≤ = 0.1𝑥100𝑥240Ω = 𝟐. 𝟒𝒌𝛀.
10

To find 𝑅3 we apply voltage divider rule and first we obtain base voltage and base current. The
base voltage 𝑉𝐵 is, 𝑉𝐵 = 𝑉𝐸 + 𝑉𝐵𝐸 , Since 𝑉𝐵𝐸 = 0.7𝑉 for silicon.

𝑉𝐵 = 1.2𝑉 + 0.7𝑉 = 𝟏. 𝟗𝑽.

𝐼𝑐 5𝑚𝐴
The base current 𝐼𝐵 is, 𝐼𝐵 = = = 0.05𝑚𝐴 = 𝟓𝟎𝝁𝑨.
𝛽 100

𝑉𝐵 1.9𝑉
Also, the current that pass-through resistor 𝑅1 is calculated as, 𝐼2 = = = 𝟕𝟗𝝁𝑨.
𝑅2 2.4𝑘Ω

The total current through 𝑅2 is, 𝐼2 = 𝐼𝐵 + 𝐼1 = 50𝜇𝐴 + 792𝜇𝐴 = 842𝝁𝑨.

𝑉𝑐𝑐 − 𝑉𝐵 12𝑉−1.9𝑉 10.1𝑉
Applying ohm’s law resistor 𝑅2 will be, 𝑅2 = = = 842𝜇Α = 𝟏𝟐𝒌𝛀.
𝐼2 84𝜇𝐴

In approximating the input impedance and input coupling capacitor, we use ac biasing or
analysis of the transistor. In this case first we find the ac equivalent resistance (𝑟𝑒 ) of the emitter
𝑉𝑇
of transistor. 𝑟𝑒 = , where 𝑉𝑇 the threshold or operating voltage of transistor which is 26mV.
𝐼𝐸

25𝑚𝑉
𝑟𝑒 = = 𝟓𝛀.
5𝑚𝐴

The input impedance 𝑧𝑖𝑛 of the amplifier will be calculated by,

𝑧𝑖𝑛 = 𝑅2 ||𝑅1 ||𝛽𝑟𝑒

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1 1 1 1
= + + = 50.32𝑥10−5 𝑆
𝑧𝑖𝑛 12000Ω 2400Ω 100𝑥5Ω

1
𝑧𝑖𝑛 = = 1.99𝑘Ω = 𝟐𝒌𝛀.
50.32𝑥10−5 𝑆

Since we want design low noise amplifier we use the lowest audio frequency which is 20Hz.
1
Then the input coupling capacitor 𝐶2 will be, 𝐶2 = where 𝑋𝐶 = 𝑧𝑖𝑛
2𝜋𝑓𝑋𝐶

1 1
𝐶2 = = = 0.0039788𝑥10−3 𝐹 = 4𝜇𝐹.
2𝜋𝑓𝑧𝑖𝑛 2𝜋(20𝐻𝑧)(2𝑘Ω)

The estimated bypass capacitor 𝐶3 for emitter 𝑅𝐸 ( 𝑅3 ) is,

1
𝐶3 = where 𝑋𝐶 = 𝑅𝐸 = 240Ω = 𝟐𝟒𝟎𝛀.
2𝜋𝑓𝑋𝐶

1 1
𝐶3 = = = 3.32𝑥10−5 𝐹 = 33.2𝜇𝐹.
2𝜋𝑓𝑋𝐶 2𝜋(20𝐻𝑧)(240Ω)

2.3.2 Oscillator Design

Oscillator is an electronic device which is used to generate sinusoidal wave form from DC input
signal. The tank circuit in Colpitts oscillator consists of two series capacitors with parallel
inductor. Inductor is made from a coil. The two capacitors in tank circuit are in series and the
total or equivalent capacitance is given by:

𝐶1 𝐶2
𝐶𝑒𝑞 =
𝐶1 + 𝐶2
1
The reactance of a capacitor and an inductor is expressed as 𝑋𝐶 = 2𝜋𝑓 𝐶 𝑎𝑛𝑑 𝑋𝐿 = 2𝜋𝑓𝑟 𝐿1 ,
𝑟 𝑒𝑞

where 𝑓𝑟 = 𝑓𝑐 is the resonant or carrier frequency of oscillation. Colpitts oscillator is used as an

oscillator if and only if the reactance of a capacitor and an inductor is equal. i.e. 𝑋𝐶 = 𝑋𝐿

1
= 2𝜋𝑓𝑐 𝐿1 𝑖𝑚𝑝𝑙𝑖𝑒𝑠 𝑡ℎ𝑎𝑡 (2𝜋𝑓𝐶 )2 𝐿1 𝐶𝑒𝑞 = 1
2𝜋𝑓𝐶 𝐶𝑒𝑞

1
𝑓𝑐 =
2𝜋√𝐿1 𝐶𝑒𝑞

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In FM design the carrier frequency is between 88MHz and 108MHz. but in this case both
capacitor and inductor are unknown. So, we assume an inductor and calculate for capacitor.

𝐿1 = 0.2𝜇𝐻

Then the capacitance for lower carrier frequency band (𝑓𝐶 = 88𝑀𝐻𝑧) is:

1 1
𝐶𝑒𝑞 = = = 16.4𝑝𝐹.
(2𝜋𝑓𝐶 ) 𝐿1 (2𝑥3.14𝑥88𝑀𝐻𝑧)2 𝑥(0.2𝜇𝐻)
2

For higher carrier frequency band (𝑓𝐶 = 108𝑀𝐻𝑧) is:

1 1
𝐶𝑒𝑞 = = = 10.87𝑝𝐹.
(2𝜋𝑓𝐶 )2 𝐿1 (2𝑥3.14𝑥108𝑀𝐻𝑧)2 𝑥(0.2𝜇𝐻)

The best variable capacitor which is between 10pF and 20pF is selected to obtain the frequency
of frequency modulated (FM) transmitter. In this FM design we choose a frequency of 91.3MHz
depending on the availability and universal selection of FM station design and an FM transmitter
has 200kHz bandwidth.

Figure 2.3 Oscillator circuit diagram

In DC analysis the capacitor is act as open circuit and inductor as a short circuit since they are
frequency dependent. By using the specification of transistor BC337 characteristics, assume β =
200 and 𝐼𝐶 = 2mA. Let the collector-emitter voltage will be half of voltage supply 𝑉𝑐𝑐 .

𝑉𝑐𝑐 12𝑉
𝑉𝐶𝐸 = = = 𝟔𝑽.
2 2

By the rule of collector-emitter voltage drop, the emitter terminal voltage 𝑉𝐸 = 1𝑉 is:

𝑉𝐶 = 𝑉𝐶𝐸 + 𝑉𝐸 = 6𝑉 + 1𝑉 = 𝟕𝑽.

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From characteristics of transistor the emitter and collector current are relatively the same.

𝐼𝐸 ≅ 𝐼𝐶 = 2𝑚𝐴.

Emitter resistance 𝑅𝐸 (𝑅7 ) will be obtained by applying ohm’s law.

𝑉𝐸 1𝑉
𝑅𝐸 = 𝑅7 = = = 500Ω
𝐼𝐸 2𝑚𝐴

In the design of common emitter DC biasing and using rule of Thumb,

𝛽𝑥𝑅𝐸
𝑅5 ≤ = 0.1𝑥200𝑥500Ω = 𝟏𝟎𝒌𝛀.
10

To find 𝑅6 we apply voltage divider rule and first we obtain base voltage and base current. The
base voltage 𝑉𝐵 is, 𝑉𝐵 = 𝑉𝐸 + 𝑉𝐵𝐸 , Since 𝑉𝐵𝐸 = 0.7𝑉 for silicon.

𝑉𝐵 = 1𝑉 + 0.7𝑉 = 𝟏. 𝟕𝑽.
𝐼𝑐 2𝑚𝐴
The base current 𝐼𝐵 is, 𝐼𝐵 = = = 0.01𝑚𝐴 = 𝟏𝟎𝝁𝑨.
𝛽 200

𝑉𝐵 1.7𝑉
Also, the current that pass-through resistor 𝑅5 is calculated as, 𝐼5 = = = 𝟏𝟕𝟎𝝁𝑨.
𝑅5 10𝑘Ω

The total current through 𝑅6 is, 𝐼6 = 𝐼𝐵 + 𝐼5 = 5𝜇𝐴 + 170𝜇𝐴 = 𝟏𝟕𝟓𝝁𝑨.

𝑉𝑐𝑐 − 𝑉𝐵 12𝑉−1.7𝑉 10.3𝑉
Applying ohm’s law resistor 𝑅6 will be, 𝑅6 = = = 175𝜇Α = 𝟓𝟖. 𝟗𝒌𝛀.
𝐼6 175𝜇𝐴

In finding of the input coupling capacitor to the modulator circuit the equivalent ac resistance 𝑟𝑒
𝑉𝑇 25𝑚𝑉
is: 𝑟𝑒 = , but 𝑉𝑇 is threshold voltage which is 26mV. Therefore 𝑟𝑒 = = 𝟏𝟐. 𝟓𝛀.
𝐼𝐸 2𝑚𝐴

Input impedance to the Colpitts oscillator or modulator circuit is the parallel combination of 𝑅6 ,
𝑅5 , and 𝛽𝑟𝑒 from modulator circuit and 𝑅4 from preamplifier circuit. Then,

𝑧𝑖𝑛 = 𝑅4 //𝑅5 ||𝑅6 ||𝛽𝑟𝑒

1 1 1 1 1
= + + +
𝑧𝑖𝑛 960Ω 10000Ω 58900Ω 200𝑥12.5Ω
= 10.42𝑥10−4 𝑆 + 1𝑥10−4 𝑆 + 0.17𝑥10−4 𝑆 + 4𝑥10−4 𝑆
1 1
𝑧𝑖𝑛
= 1.559𝑚𝑆 implies that 𝑧𝑖𝑛 = 1.559𝑚𝑆 = 𝟔𝟒𝟏. 𝟒𝟒𝛀.

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The input coupling capacitor to the modulator circuit is given as:

1
𝐶5 = , where 𝑋𝐶 = 𝑧𝑖𝑛 = 641.44Ω.
2𝜋𝑓𝑚 𝑋𝐶

1
𝐶5 = = 𝟎. 𝟐𝟓𝝁𝑭.
2𝑥3.14𝑥(1000𝐻𝑧)(641.44Ω)

To find capacitance 𝐶5 let we use thumb rule which is:

𝛽𝑅𝐸 200𝑥500Ω
𝑋𝐶 ≤ = = 𝟏𝟎𝒌𝛀.
10 10

But if the reactance of the capacitance is equal with the resistance 𝑅𝐵2 : 𝑋𝐶 = 𝑅𝐸 = 10𝑘Ω, then,

1 1
𝐶6 = = = 𝟏𝟔𝒏𝑭.
2𝜋𝑓𝑚 𝑋𝐶 2𝑥3.14𝑥(1000𝐻𝑧)(10𝑘Ω)

2.3.3 Power Amplifier Design

For power amplifier circuit transistor 2N3860 which specification is taken from appendix A. It
has Imax = 2mA and β = 200 and also 𝑉𝑐𝑐 = 12𝑉. For this FM transmitter design, we select class
D FM transmitter which is used for non-commercial educational purpose. The power transmitted
in FM transmitter is calculated as

𝐸 2 𝑥𝑑2
𝑃𝑡 = Where 𝑃𝑡 – the required power transmitted
30

E – sensitivity of available radio receivers

d – the range or distance to be covered by the transmitter.

In most cases we use the lower values of sensitivity (E) for computation which is 10𝜇𝑉/𝑐𝑚. In
this project we only need to design an FM transmitter which cover the range of 2km (d = 2000m)
only for evaluation purpose. Therefore, the transmitted power will be:

(10𝜇𝑉/𝑐𝑚)2 𝑥(200000𝑐𝑚)2 4𝑊
𝑃𝑡 = = = 0.133𝑊 = 𝟏𝟑𝟑𝒎𝑾.
30 30

Power transmitted in dBm is calculated as:

𝑃𝑡 133𝑚𝑊
𝑃𝑡 (𝑑𝐵𝑚) = 10 log10 ( ) = 10 log10 ( ) = 21.24𝑑𝐵𝑚 ≅ 𝟐𝟐𝒅𝑩𝒎.
1𝑚𝑊 1𝑚𝑊

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This FM transmitter, the transmitted power of 22dBm is transmitted to an antenna which has the
characteristic impedance of 50Ω as a load.

The transmitted power is expressed as: 𝑃 = (𝐼𝐶 )2 𝑍𝐿

𝑃 0.133𝑊
(𝐼𝐶 )2 = = = 0.00266𝐴2
𝑍𝐿 50Ω

𝐼𝐸 ≅ 𝐼𝐶 = √0.00266𝐴2 = 0.05157𝐴 ≅ 𝟓𝟐𝒎𝑨.

Figure 2.4 Power amplifier circuit diagram

𝑉𝐶𝐸 2
The transmitted power is also expressed as: 𝑃 = 𝑍𝐿

𝑉𝐶𝐸 2 = 𝑃𝑥𝑍𝐿 = 0.133𝑊𝑥50Ω = 6.65𝑉 2

𝑉𝐶𝐸 = √6.65𝑉 2 = 𝟐. 𝟓𝟖𝑽.

Since the transistor act as an amplifier only in active region, let we choose the quiescent point
𝑉𝑐𝑐 12𝑉
will be at 50% of 𝑉𝑐𝑐 . 𝑉𝐶 = = = 𝟔𝑽.
2 2

The emitter voltage 𝑉𝐸 is found to be, 𝑉𝐶𝐸 = 𝑉𝐶 − 𝑉𝐸

𝑉𝐸 = 𝑉𝐶 − 𝑉𝐶𝐸 = 6𝑉 − 2.58𝑉 = 𝟑. 𝟒𝟐𝑽.

The base voltage 𝑉𝐵 is, 𝑉𝐵 = 𝑉𝐸 + 𝑉𝐵𝐸 , Since 𝑉𝐵𝐸 = 0.7𝑉 for silicon.

𝑉𝐵 = 3.42𝑉 + 0.7𝑉 = 𝟒. 𝟏𝟐𝑽.

𝐼𝑐 52𝑚𝐴
The base current 𝐼𝐵 is, 𝐼𝐵 = = = 0.26𝑚𝐴 = 𝟐𝟔𝟎𝝁𝑨.
𝛽 200

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By following the same procedure as preamplifier stage the resistance 𝑅9 , 𝑅10 , 𝑅11 and 𝑅12 will
be determined. Therefore,

𝑉𝑐𝑐 − 𝑉𝐶 12𝑉 − 6𝑉 6𝑉
𝑅𝐶 = 𝑅12 = = = = 𝟏𝟏𝟓. 𝟑𝟖𝛀.
𝐼𝐶 52𝑚𝐴 52𝑚𝐴

𝑉𝐸 3.42𝑉
𝑅𝐸 = 𝑅11 = = = 𝟔𝟓. 𝟕𝟕𝛀.
𝑉𝐸 52𝑚𝐴

𝑅9 = 𝟏. 𝟑𝒌𝛀. and 𝑅10 = 𝟐. 𝟐𝟕𝒌𝛀.

Also, using the procedure as preamplifier circuit to find input coupling capacitor (𝐶7 ), bypass
capacitor (𝐶8 ) but the frequency is changed to 91.3MHz which is not used in preamplifier circuit.
Then, 𝐶7 = 22µ𝐹 and 𝐶8 = 265𝑝𝐹.

The coupling capacitor 𝐶9 which connect the antenna with power amplifier circuit is obtained
from the load impedance or the equivalent impedance of antenna. In this case the equivalent
impedance of antenna is 50Ω. Then,

1
𝐶9 = , 𝑏𝑢𝑡 𝑋𝐶 = 𝑅𝐿 = 50Ω
2𝜋𝑓𝑋𝐶

1
𝐶9 = = 𝟑𝟓𝒑𝑭.
2𝑥3.14𝑥(91.3𝑀𝐻𝑧)𝑥(50Ω)

To determine the length of antenna, use the following equation.

c
c = λf implies that λ = f

λ
Antenna Length =
4

Since the modulated frequency is 91.3MHz, then the length of antenna is,

3x108 m/s
λ= = 3.29m
91.3MHz

3.29
Antenna Length = = 0.82m
4

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CHAPTER 3
RESULT AND DISCUSSION
5.1 Simulation of Preamplifier (Audio Amplifier)
The audio amplifier is used to amplifies the input signal from microphone to the amplifier,
because the output signal of microphone is very small (5mV – 50mV) which not enough for FM
transmitter and the output of the audio amplifier is 625mvpeak which is greater than the input
from microphone.

Figure 3.1 Audio Amplifier Circuit

Figure 3.2 Output Signal of audio amplifier

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5.2 Simulation of Oscillator Circuit

Figure 3.3 Schematic Diagram of Oscillator

Figure 3.4 Output of Oscillator

This oscillator produces some distortion which also affects the output of the overall FM
transmitter. But some of this distortion is reduced by power amplifier since we use a low noise
power amplifier transistor.

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5.3 Simulation of Power Amplifier

Figure 3.5 Power amplifier circuit

Figure 3.6 Simulation result of power circuit

The power amplifier is used to amplifies the output of the audio amplifier stage. We use
resistors, capacitors and transistors and amplifies to the required point. The input is 625mVpeak
and the output of the power amplifier is 2.023Vpeak which is greater than the input.

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5.4 Overall Circuit Diagram

Figure 3.7 FM Circuit on Multisim

Figure 3.8 Simulation result of overall circuit

The simulation of overall circuit implies that the input from the source which is 5mVpeak is
amplified to 2.055Vpeak voltage by using audio amplifier and power amplifier. The frequency of
the final output is also increased by the colpitts oscillator used. The overall output has a very
small noise due the component used in the circuit and the colpitts oscillator.

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CHAPTER 4

CONCLUSION AND RECOMMENDATIONS

6.1 Conclusion
The Design and Implementation of an FM Transmitter which transmits at a frequency of 91.3
MHz with an output power of 22dBm which with a turnstile antenna at 10 meters above the
ground was able to transmit to a distance of 2 Kilometers. This shows that Locally Designing
and implementing of FM transmitters is very feasible and much cheaper than importing the
transmitters.

6.2 Recommendations
The design used for this project is essentially quite the difficult one, and it is using the last effort
of which partially brings it down when it comes to the overall reliable performance. The main
area of to do this project is that, the design parts and collecting of the necessary components of
materials that have used to done the suitable ways. In order to doing this project we are
early expected to simulated and implemented of the final achievements. In this case when we
started from the beginning to the end the members /teams are participated actively in order to
collecting the data and gathering information from different areas/ parts to fulfill the necessary
documents. However, since we did not get all materials such as dynamic microphone in the
laboratory, we cannot implement the hardware.

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REFERANCE
[1] Robert Boylestad, Louis Nashelsky “Electronic Devices and Circuit Theory”, (7th Edition),
(ISBN 978-0-13-262226-4)

[2] Donald A. Neamen, “Microelectronics: Circuit Analysis and Design”, (4th Edition), (ISBN
978-0-07-338064-3—ISBN 0-07-338064-4 (alk. paper))

[3] Behzad Razavi, “RF Microelectronics”, (2nd Edition (Prentice Hall Communications
Engineering and Emerging Technologies Series)-Prentice Hall (2011))

[4] Simon Haykin, “Communication System”, (4th Edition)

[5] http//: www.Wikipedia.Org (December 1,2019 - January 10, 2020)

[6] http//:m.google.com/electrical circuit (December 1,2019 - January 10, 2020)

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