DESIGN AND CONSTRUCTION OF A 100W POWER

AMPLIFIER WITH PREAMPLIFIER AND AUDIO MIXER

BY

SARKI IKO ISHAYA

FPTB/EET/07/63146

A PROJECT REPORT SUBMITTED IN PARTIAL FULFILMENT

OF THE REQUIREMENT OF THE AWARD OF HIGHER
NATIONAL DIPLOMA (HND IN ELECTRICAL ELECTRONIC
ENGINEERING) (TELECOMMUNICATION AND
ELECTRONIC) IN ELECTRICL ELECTRONIC ENGINEERING
DEPARTMENT, SCHOOL OF ENGINEERING TECHNOLOGY,
FEDERAL POLYTECHNIC, BAUCHI

SEPTEMBER 2009.

1
 DECLARATION

This is to declare that this project work on the design and

construction of a power amplifier with pre-amplifier and a mixer was

solely carried out by sarki Iko Ishaya in the laboratory by the electrical

electronic engineering department, Federal Polytechnic, Bauchi, under

the supervision of Mallam Aliyu Mohammed Gadam.

---------------------------- ---------------------------
Sarki Iko Ishaya Date

2
 APPROVAL PAGE

This project work has been approved for acceptance by the

undersigned persons on behalf of the department, federal polytechnic,

Bauchi as meeting the requirement for the award of higher national

diploma (HND) in electrical electronic engineering (Telecommunication

and Electronic).

Project supervisor

Name: ______________________________

Signature____________________________

Date: _______________________________

Head of department;

Name: ______________________________

Signature: ___________________________

Date: _______________________________

External examiner:

Name: _______________________________

Signature: ____________________________

Date: ________________________________

3
 DEDICATION

I dedicate this project work to God Almighty for His sustaining

grace and faithfulness upon my life through the entire period of my

learning process.

4
 ACKNOWLEDGEMENT

I am indeed most grateful unto God Almighty for His divine

favour, grace, mercy, faithfulness and goodness upon my life through the

learning process.

My most sincere gratitude goes to my able supervisor, Mallam

Aliyu Mohammed Gadam for his contributions, advice, and pains, taking

efforts in going through the manuscripts. All of these put together made

this project a reality, keep up the good work sir for you shall be rewarded.

The fact remains true that nobody can single handedly write a text

or complete a worthy task as this without the caring hands of some caring

persons. In line with this I would like to acknowledge on this page the

efforts of my beloved family beginning at my parents Mr. and Mrs.

Ishaya Sarki, My elder and younger sisters Blessing, Kassang, Godiya,

Gift and the entire family for their unflinching support and

encouragement given to me throughout the course. Thank you all for your

moral and financial support.

Mention will also be made of my very special friends Zarah, Zaks,

Jerry, Yakubu, Yusuf and every one who has in one way or the other

contributed to success of this laudable project, meeting you did shape my

life. Thank a million.

5
 ABSTRACT

This project is aimed at achieving an amplified signal output with a three

(3) input signals fed to the device through an electromagnetic transducer.
Three (3) microphones are placed some distance apart in order to get the
stereo effect. Each microphone hears a slightly different version of the
music and three separate recordings are made. The researcher resolved in
using a linear log potentiometer (10k variable resistors). The recordings
are then finally combined in a single tape for the user. An output is taken
from the loudspeaker.

6
 LIST OF SYMBOLS

n nano (x 10-9)

m micro (x 10-6)

p peco (x 10-12)

Ω ohm

W watts

F farad

7
 TABLE OF CONTENTS

TITLE PAGE----------------------------------------------------------------I

DELCARATION------------------------------------------------------------II

APPROVAL PAGE-------------------------------------------------------III

DEDICATION--------------------------------------------------------------IV

ACKNOWLEDGEMENT-------------------------------------------------V

ABSTRACT-----------------------------------------------------------------VI

LIST OF SYMBOLS-----------------------------------------------------VII

TABLE OF CONTENTS -----------------------------------------------VIII

CHAPTER ONE

1.0 INTRODUCTION----------------------------------------------------1

1.1 BACKGOUND--------------------------------------------------------1

1.2 MOTIVATION-------------------------------------------------------2

1.3 OBJECTIVE----------------------------------------------------------3

1.4 SCOPE OF WORK--------------------------------------------------3

CHAPTER TWO

2.0 LITERATURE REVIEW-------------------------------------------5

2.1 INTRODUCTION----------------------------------------------------5

2.2 PAST WORK ON AUDIO POWER AMPLIFIER------------5

2.3 DESIRED IMPROVEMENT---------------------------------------6

2.4 BASIC ELECTRONIC COMPONENT OF THE

AMPLIFIER-------------------------------------------------------------------7

8
CHAPTER THREE

3.1 INTRODUCTION------------------------------------------------------8

3.2 POWER SUPPLY------------------------------------------------------8

3.2.1 TRANSFORMER------------------------------------------------------8

3.2.2 RECTIFIER AND SMOOTHING CIRCUITE-----------------9

3.3 TONE CONTROL AND PRE-AMPLIFIER--------------------13

3.4 POWER AMPLIFIER----------------------------------------------- 18

3.4.1 DIFFERENCIAL AMPLIFIER-----------------------------------18

3.4.2 AUDIO MIXER-------------------------------------------------------22

3.4.3 DRIVER AND OUTPUT STAGE---------------------------------23

CHAPTER FOUR

4.0 TEST AND RESULT-------------------------------------------------27

4.1 INTRODUCTION-----------------------------------------------------27

4.2 PROCEDURE FOR TEST------------------------------------------27

4.3 VOLTAGE GAIN-----------------------------------------------------29

4.3.1 OUTPUT POWER----------------------------------------------------30

4.3.2 TOTAL HARMONIC DISTORTION-----------------------------30

4.3.3 INPUT AND OUTPUT IMPEDENCE----------------------------31

4.4 DISCUSION------------------------------------------------------------ 32

9
CHAPTER FIVE

5.0 CONCLUSION--------------------------------------------------------33

RECOMMEDATION--------------------------------------------------------33

REFERENCES----------------------------------------------------------------34

10
 CHAPTER ONE

1.0 INTRODUCTION

1.1 BACKGROUND

The present sophistication in sound system and the compensates in

music productions and the need to keep pace with the advancing

technology in this field brought about the need to design an effective,

economical and reliable sound system.

This project uses a scientific and engineering approach to design a

simplified and effective system.

Among the basic elements of information dissemination, communication

is vital. The rapid growth of human civilization can be corrected with

their ability to communicate. The guttural sound and sign languages of

the early civilization could not support a technically advanced society.

History depicts that technical advancement began as soon as people could

effectively communicate their thought and ideas.

One very important means of communication is the human voice.

As the complex muscles of the vocal chords work, they cause waves of

changing air pressures, to produce.

These sound waves travel in the surrounding air spreading in all

directions. When they hit the human eardrum, they are converted into

what the brain interprets as sound. For this, all we have to do to

11
communicate is to just talk. However, it is not practicable, or possible for

must persons to talk loudly enough to communicate over long distances

as the vocal range of individual vary from one another. Besides, sounds

waves die putting as they travel through the air.

1.2 MOTIVATION

This project is based on the stereo phonic system type of audio

amplification.

A stereophonic system employs two monaural devices in its operation i.e.

double channel audio amplifier is utilized, we can locate a source of

sound quite easily with the minimum body movement. This is made

possible because our ears have separation and are very sensitive; sounds

arrive at slightly different times and loudness on each ear. For example, if

we look at one of our ears, we shall discover that locating a source of

sound became more difficult.

The functions of the ear can be likened to the stereo microphones

used in a recording studio to record music. Two microphones are placed

some distance apart from one another to get a stereo effect. Each

microphone hears a slightly different version of the music and two

separate recordings are made.

The recordings are then finally combined into a single disc or tape

for the user. When played back, each channel is a gain separated. The left

12
channel feeds the left speaker while the right channel feeds the right

speaker. If the speakers are placed far enough apart, the recording sounds

just like the music in the recording studio.

1.3 OBJECTIVE

This project is design to show that many signals can be processed

and one signal out of several present in the input and can be selected and

send to the output as an output signal. This can be done or achieved by

providing control to all input signal circuit through the variable resistor.

INPUT OUTPUT
CONTROLLER
TRANSDUCER TRANSDUCER

SOURCE

Fig 1.1 Simplify Block Diagram of the System.

1.4 SCOPE OF WORK

The system presented will be made up of “dual power supply” this

involves the use of both positive DC voltages and ground. Precisely this

allows equal positive and negative voltage swings in the power amplifier

and also for the effective utilization of the power transformer.

However a regulated voltage will be used for the tone control.

13
The final stage of the system is the loudspeaker stage. It function is to

interpret from electrical signal into audio signal.

It is from the loudspeaker that the massage goes to the audience for their

consumption.

14
 CHAPTER TWO

2.0 LITERATURE REVIEW

2.1 INTRODUCTION

In early days of electronics precisely early sixties, seventies

discrete component like values were used to build up audio amplifier

circuits, this made it to be bulky and heavy. Since the discovery of the

transistor in1947, it has played an important role in active circuit element

in amplifier. The great majority of amplifier employs the use of transistor

because of their low cost and high reliability. By mid 1970s inexpensive

general purpose integrated operational amplifier with excellent

characteristic were widely available, and must be use when the required

general purpose device cannot be used to attain the desired specifically.

2.2 PAST WORK ON AUDIO POWR AMPLIFIERS

Ahmed Bello a final student of ABU Zaria designed and

constructed a 75 watts audio power amplifier with a mixer. He was fairly

successful in his attempt. But I discovered the component he used were

purely discrete. Also the heat sink he used of the power amplifier stage

had low capacity. As we know that transistors are the back bone of the

electronic components in amplifier, but their proximity to heat affect their

output, which brings about distortion. Also the final year students of

Electrical Electronics Engineering student of the Federal Polytechnic

15
Bauchi, Joakim Othman 2003/2004 session in his project work of design

and construction of an audio power amplifier rated 50watts, though most

of the components he used were discrete components, the input stage

consists of the common emitter transistor amplifier which is the first

stage in the process of amplification, while the intermediate stage is made

of multistage amplifier controlled by quiescent current setting transistor.

But what made his work fascinating is that he was able to use the proper

heat sink for the transistors.

2.3 DESIRED IMPROVEMENT

From the various research and consultations of the past project of

my predecessors, I discovered that most of them used discrete

components in achieving their various results of different power ratings

of the audio amplifier (for audio signal amplification) from this case

study. I now decide to make an improvement to design and construct a

100watts power amplifier with a pre-amplifier and a mixer console used

for instruments using a high fidelity, reliable and low cost power

transistor, A1015 and A733. So as to meet up with the requirement of

high power output and for the pre-amplifier stage C945 transistors were

used. Though I also make use of discrete component but I work towards

improving the power rating and the provision of three input channel.

16
2.4 BASIC ELECTRONICS COMPONENT OF THE AMPLIFIER

The electronic component of the amplifier includes transistors,

resistors, Darlington pair transistor. Resistors are used in the circuit as

current limiter (limiting the amount of current passing through the circuit)

and also used with capacitor in implementing filter network (low pass,

high pass, band pass). Diodes can rectify a.c voltage to its d.c equivalent

as in the case of power supply. Transistors are the backbone of

electronics components in amplifiers, for the amplification of signals. It

can also be used as switches or drivers when saturated. In this project it is

used for amplification of signals in each stage.

17
 CHAPTER THREE: DESIGN AND IMPLEMENTATION

3.1 INTRODUCTION

This chapter is aimed at designing a 100 Watt power amplifier with

a mixer console. The system consists of: power supply (transformer),

rectifier and smoothing circuit, tone control stage, mixer, power

amplification stage and pre-amplification stage.

3.2POWER SUPPLY

3.2.1 TRANSFORMER

Considering equation 3.1 below; this is usually referred to as the basic

transformer equation (Theraja B. L, A. K, 1999)

V = 4FfaBN X 10-8………………………………..3.1

Where:

V = the r.m.s voltage across a considered winding in volts.

F = form factor (normally 1.11 for sine wave)

f = input frequency in Hertz (50Hz)

B = flux density in lines per square inch

N = number of turns on a considered winding

a = core area in square inches

A conservative figure for B is 75000 lines per square inch of core area.

V = 4 X 1.11 X 50 X a X 75X103 X N X 10-8

18
V = 16650X10-5 X N X a

N/V = 6/a…………….3.2

The term N/V is known as the turns per-volt figure for a

transformer- that is to say, the number of turns on the winding for each

volt across them. This ratio is the same for each winding on a

transformer.

3.2.2 RECTIFIER AND SMOOTHENING CIRCUIT

The rectifier circuit is needed for ac signal rectification. A full

wave bridge rectifier is the more commonly used rectifier circuit. Its

arrangement is shown in figure 3.1 below

D4 D1
Vdc

Vac D2
+

D3 C1
1uF

Fig 3.1 full wave bridge rectifier.

The bridge rectifier consists of four diodes. The operation of the circuit is

that two diodes conduct during any of the half cycle of the ac input

voltage the resultant output voltage waveform is shown in fig3.1

19
 However since a dual power supply is required for this project, the

rectifier circuit is modified as shown fig3.2 where C1 and C2 are the filter

capacitors (Theraja B. L, A. K, 1999).

D4 D1

220

+
D2 C1
ac D3

+
C2

Fig 3.2 full wave rectifier with dual supply.

A circuit that converts a pulsating output signal of a rectifier into a

smooth dc voltage is known as a filter to achieve this; a capacitor is used

in parallel with the load. This type of filter is known as CAPACITOR

INPUT FILTER. The filtering action of this filter wave is shown in

fig3.3 below.

Fig3.3 filtering action of a capacitor filter.

The value of the shunt capacitor is given by

……………….3.2

20
 Where F = frequency at the main voltage

γ = Ripple factor

R2 = load resistance (Theraja B. L, A. K, 1999)

In order to achieve 100W power with 76.55% efficiency a dual voltage

+35v and -35v is needed (both dc voltage).

Since these voltages are peak voltages therefore their root mean square

(r.m.s) will be:

Vr.m.s = Vpeak (dc) ………………3.3

Volts

The conversion from r.m.s value to peak value is practically done using

smoothening capacitors.

Since the r.m.s value is pulsating and supplied by a bridge rectifier then

the r.m.s voltage will be transformer r.m.s voltage (ac) less two diode

drops since for each half-cycle at the ac voltage two out of the four diodes

of a bridge rectifier conducts.

Transformer r.m.s voltage Trms is given by:

Trms = 24.75 + 1.4 = 26.15v (ac)

Hence, since the power supply is intended to be dual, then transformer

has to supply twice the voltage in the equation. This invariably means the

21
transformer will be a centre tapped type. This is transformer secondary

voltage.

Vs = 52.3v

Recall, the turn per voltage ratio is N/V= 6/a …………………..3.4

Where:

N = number of turns on a considered winding

V = voltage across a considered winding in Volts

a = core area in square meter which has been chosen to be 18cm2

for the primary winding

V = 4.444fΦNP (Theraja B.L, A.K 1999)

Φ = BA

Where B = 1.2T

A = 4.0 X 4.5cm

A = 18 X 10-4m

NP= 2.085 X 220

NP = 458.8

 Primary turns = 459 turns

For the secondary winding

Vs = 52.3v

22
 2.085

Ns = 2.085 X 52.3 = 109.05

 Secondary turns = 110 turns

3.3 TONE CONTROL AND PRE-AMPLIFIER

Fig3.4 Pre-amplifier and tone control

C2,C3,C4,C5,R1,R2,R3,R4,R5 and R6 constitute the Maxwell tone control

circuit. The configuration of this tone circuit is standard as such only the

component value will differ for designs done by different designers R 1

and R3 are related by (Bruce A. C 1984)

6.5 ≤ R3 ≤ 12 ………………………………..3.5

Where R4 and R6 are individually less than R5 practically a good choice is

R3/R1 = 10 ………………………………….3.6

Choosing R1 = 10KΩ then equation 3.6 becomes

R3 = 10 R1 = 10 X 10K = 100KΩ

23
The ratio R3/R1 gives the maximum gain at the bass control section of the

tone control. Hence, the voltage gain is 10(or 20dB).

Similarly

6.5 ≤ R4/R5 ≤ 12 …………………………..3.7(a)

But R4 < R1/3 …………………………..3.7(b)

R5 < R3/3 …………………………..3.7(c)

If R4/R5 = 10

And R5 = 680Ω then

R4 = 10 R5 = 6.8KΩ

The ratio R4/R5 is the voltage gain of the treble control section of the tone

control circuit.

R2 and R6 are linear taper 100K potentiometers.

For roll off at 33Hz, the reactance Xc 3 of C3 must be equal to R2,

that is 100KΩ

…………………………….3.8

C3 = 0.048μf

For break point and around 600Hz-730Hz, the reactance Xc2 of C2 must

be equal R1 that is 10K.

……………………………3.9 (Mischa S. 1981)

24
 Where fe break point frequency = 730Hz

C2= 0.022μf

R2 provides full bass cut when the slider is moved towards R 3; while full

bass boost is obtained when the slider is moved towards R1.

For a roll off at upper frequency of 20 KHz the reactance Xc 4 of C4 must

be equal to R5 that is:

……………………..3.10

C4 = 0.0111μf ≈ 0.01μf

High frequency response starts when the reactance Xc 5 of C5 = R4

that is 6.8K

……………………..3.11

Where f = 1 KHz

C5 = 0.023μf

C5 = 0.022μf

Depending on its position R8 will produce treble boost or treble cut. C6 is

a feedback capacitor which presents low resistance path to all frequencies

within the audio range. The practical value of C 6 is 10μf. C7 is about half

25
of C6 hence C7 is approximately 2.2μf C7 provides the interaction of the

tone control section and the amplifier hence it is a coupling capacitor.

The small signal amplifier-voltage divider bias will be designed below:

Vcc = 12v

Choosing RE = 1.2KΩ

IE = VE/RE ………………………3.12

But VE = 1/10 Vcc = 1/ 10(12) = 1.2v

 IE = 1.2/1.2K = 1mA

If IE ≈ IC; Vcc = 6v

……………………..3.13

VB = VBE +VE …………………………3.14

VB = 0.7 + 1.2

VB = 1.9v

Since

R2 ≤ 10βRE

R2 ≤ 1/10(120) (1.2K)

R2 = 14.4KΩ

Also

26
 ……………………….3.15

1.9R1 + 27.36K = 172.8K

R1 = 76.55KΩ

 at f = 16Hz

≈ 1μf

27
3.4 POWER AMPLIFIER

3.4.1 DIFFERENTIAL AMPLIFIER

+35V
D1
R4
D2

Q3

C1 R1 R8
V2 Vo2
+ Q1 Q2
C2 R2 Vo1

R3
R6
R7
R5
D3

+
C3

-35V

Fig 3.5 The differential amplifier circuit

The two diodes D1 and D2 hold the base of transistor Q3 at 1.4 volts

below the positive supply voltage. The emitter of Q 3 is thus at 0.7v below

the supply voltage.Q3 is a current source.

For low noise, performance is stable. The collector current of Q 3

has been chosen to be 2mA. Since Q 1 and Q2 are matched pairs with large

hfe then IB3 can be neglected and so therefore- IC3 = IE3

Hence,

R4 = VE/IE = 0.7/2X10-3 = 350Ω

However

hfe/hie = gm

28
Where hfe = current gain of the transistor

hie = input impedance

But gm = 40IC3

gm = 40 X 2 X 10-3

gm = 80mA/v

hie = hfe/gm

hfe for Q3 = 100

hoe for Q3 = 25 X 10-6 or 40K

hie = 100/80m = 1.25K

The open circuit loading of hoe on the transistor Q 3 is 25μs the voltage

gain:

Av = -gmRL ……………………….3.16

Hence

Av = - (80 X10.3) X 40 X 10.3

Av = -3200

The output resistance Rout is given by

Rout = hoe + R2 (1+ Av)

= 40 X 103 + 47 (1+3200)

= 40 X 103 + 150447000

= 15084700

Rout = 1.50mΩ

Hence the output impedance of the current source is 1.50mΩ

29
Since Q1 and Q2 are matched pairs, then;

IC1 = IC2 = ½ IC3

IC1 = IC2 = ½(2 X 10.3) = 1mA

Practically the voltage drop VR3 across R3 is not supposed to exceed two

diode voltage drops. A good choice is VR3 = 0.73v

Therefore;

R3 = VR3/IC1 = 0.7/1m = 700

Because of standardization R3 = 680Ω

To achieve a balance in the operation of the differential amplifier R 5 has

to be equal to R3

R5 = Vcc – 2VBE – VEE …………………………..3.17

To ensure sufficient base drive of Q3, IR7 has to be at least 100IB3

But IB3 = IC3/hfe

IR7 = 100 IB3

…………………….3.18

R7 = 34.3KΩ

Resistor, R2 is chosen based on the resistance value that forces the base

voltage of Q1 to zero volt. A good value of R2 is 33K.

30
Considering Q1;

but hfe of Q1 = 100

The input impedance of the differential amplifier is:

………………………..3.19

So,

Where f = lower 3dB frequency = 16Hz

C1 = 1.99μf

It is of practical importance to increase the upper 3dB point of the

differential amplifier so as to reduce the rise time. For an upper 3dB

frequency at 33 KHz, the rise time is:

Rise time

But rise time is also equal to 10R1C2; therefore:

10R1C2 = 10.6 X 10-6

31
C2 = 10 x 10-6
10 x 33 x 103

C2 = 30pf

3.4.2 AUDIO MIXER

The virtual Earth circuit configuration in fig.3.6 was used. Inputs at

A, B, C, ------------N can be selected and combine with potentiometers

RA, RB, RC-----RN if the feedback current:

IF = IA + IB + IC + --------IN, then

-Vo = VA + VB + VC + ----- + VN----------------------3.20

RF RA RB RC RN

Vo = - (RFVA + RFVB + RFVC + ------- + RFVN)

RA RB RC RN

If RA = RB = RC = -------RN = RF

Then;

Vo = - (VA + VB + VC +----------VN)

In order to avoid output offset voltage problems an upper limit was set for
Rx and expressed as: 1 = 1 + 1 + 1 + ----- + 1 + 1 --------3.20
Rx RA RB RC R RF

Now: for unit gain we choose

RA = RB = RC = --------- RN = RF = 10 K Ω

Rx = 1 + 1 + 1 + 1_ + ---------- = 2.5KΩ
10K 10K 10K 10K

32
Rx = 5KΩ as a preferred value.

Fig. 3.6: The Audio Mixer

3.4.3 DRIVER AND OUTPUT STAGE

+35V

Q7
Q9

R11 R13 R15

R16
R12 R14
L1
C4
Q8

V2 Q6
-35V

FIG3.7 The Driver and output stage of the power amplifier

The diodes D1, D5 arrangement ensures Q6, Q7, Q8 and Q9 are at threshold

point at conduction so as to delimitate cross over distortion. Point V 1 is

maintained at 1.4v (Mischa. S. 1981)

VR11 = VBE7 = 0.7v

For TR11 = 2mA, then

R11 = 330Ω

R11 =R12= 330Ω

33
R13 and R14 ensure that Q6, Q7 and Q8, Q9 do not conduct simultaneously.

Under signal condition and a good value is 0.33Ω

That is R13 = R14 = 0.33Ω

R13 and R14 also ensure thermal stability due to the heat dissipated

by Q8 and Q9 are placed on the heat sink to ensure thermal equilibrium.

R15 is a current limiter and in conjunction with L 1, R16 and C4 helps

to remove parasitic oscillation. The values of R 15, R16, L1 and C4 have

been standardized to the following:

R15 = 10Ω

L1 = 10μh

R16 = 4.7Ω

C4 = 0.1μf

Now to have an ac voltage swing of 33.9v peak at the output with

0.33v peak at the differential amplifier, a closed loop gain G at 100 is

needed.

The average supply current is:

I supply

Where Vo is the output voltage swing

The average power drain from the supply is:

Psupply = Isupply X 2Vcc

34
 = 1.349 X 35 = 94.406W

The average power delivered to R2 is

Efficiency, η =

35
36
 CHAPTER FOUR

4.0 TEST AND RESULT

4.1 INTRODUCTION

The circuit design was implemented and tested to ensure

compliance with the design specifications. The methodology in carrying

out the test and results obtained are also contained in this chapter.

4.2 PROCEDURE FOR TEST

In order to measure the input resistance of the circuit, the voltage

divider method was used as shown in fig4.1 below. The box represents

the circuit under test.

BOX

Fig4.1 Resistance Box

The value at the resistance box, R was adjusted to obtain a

convenient value at V1 for a known value at V1 for a known value at Vs

……………………………4.1

……………………………4.2

37
For Vin = ½ Vs and Rin = R

The input frequency was 1 KHz

The gain was measured over a frequency range with a fixed input

voltage for all the measurements. The frequency response of plot of gain

against frequency (in dB) was obtained.

The current was measured by connecting an ammeter in the supply

line. The power dissipated was calculated from the product of supply

voltage current drawn by a circuit at no load from the supply.

Pd = IsVcc …………………………………..4.3

All measurements except frequency were done at 1KHz structure.

All values were ascertained based on the oscilloscope and determined

within limits for all circuits.

Quiet a number of amplifier parameters can be measured using a

signal generator, an oscilloscope and other equipment. The most

important parameter measurements carried out on the amplifiers are:

a) Voltage gain

b) Input and output impedance

c) The output power

d) Distortion of the output waveform

38
4.3 VOLTAGE GAIN

To determine the voltage which when applied to the input terminal

as an amplifier, distortion of the output waveform

Fig4.2 Circuit for measurement of power amplifier voltage gain

The frequency of the generator was set to a test value of 1KHz the

generator output voltage (sinusoid) was increased steadily from zero unto

the onset of distortion noticed. The input signal to the amplifier was then

reduced a little while the distortion at the scope disappears. The voltage

gain (that is the closed loop gain of the amplifier is the ratio of the output

voltage to the amplifier input voltage.

Fig4.3 Circuit for measurement of output Impedance of the amplifier

39
 The output impedance of the amplifier was measured using a

similar technique, the arrangement being shown in fig4.5

Rout = R – R2

R obtained to be 8.040 Ω

Therefore Rout = 8.040 – R2

However R2 – 8 Ω

Rout = 8.040 – 8 = 0.04 Ω

4.3.1 OUTPUT POWER

The signal generator was set to a test frequency of 1KHz and its

output voltage was steadily increased until distortion of the output sine

wave displayed on the oscilloscope screen was obtained. The voltage of

the signal generator was then reduced slightly until no distortion of the

signal was observed. The peak voltage across the dummy load resistance

was then measured and found to be 33.9v

4.3.2 TOTAL HARMONIC DISTORTION

To determine the total harmonic distortion of the designed and

constructed amplifier a sinusoidal signal at peak value 0.24 at 20Hz was

generated using a function generator. This was fed into the power

amplifier, the output of which was further applied to a spectrum analyzer.

The harmonic components of the fundamental frequency were then

observed on the oscilloscope.

40
 In this way the amplitude of each harmonic component relative to

the fundamental amplitude was determined. The percentage harmonic

distortion was then calculated using the relation of equation 4.4

THD = …………………….4.4

Where V2', V3' etc are the relative amplitude of each harmonic

component.

4.3.3 INPUT AND OUTPUT IMPEDANCE

The voltage delivered by the signal generator to the amplifier was

set to some convenient nature (less than the value which causes distortion

of the output waveform that is 0.4v). This is illustrated in fig4.4

Fig4. Circuit for measurement of input impedance of an amplifier

V= ………………………….4.5

Where Rs = output impedance

Es = emf of the signal generator

Rin = amplifier input impedance to be determined.

41
 A variable resistance R was then connected in series with the input

impedance until the input voltage (0.339v) has fallen to one half its

original value (0.2v) therefore

……………………………4.6

From 4.5 and 4.6

2(Rs + Rin) = Rs + R + Rin

Hence;

Rin = R – Rs

R3 = 0.6KΩ

R = 20.8KΩ

Therefore Rin = 20.8 – 0.6 = 20.2KΩ

There was a variation between the calculated value and measured

value by 0.2; this was due to error in measurement of R.

4.4 DISCUSSION

For a good design, the results obtained are most often very close to

the theoretical values, exceptions may be used in certain circuits due to

some impractical assumptions. The maximum theoretical efficiency of a

power amplifier was gotten to be 76%. The gain of the various circuits

agreed with the designed values.

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 CHAPTER FIVE

CONCLUSION

The design of power has been undertaken in this project and the

performance of the individual units that make up the system showed an

appreciable level of success in the implementation of the design. It can

therefore be asserted that the function of the system as given by the

specification has been achieved by the coupled system. The complete

circuit diagram of the 100W power amplifier with a pre-amplifier and a

mixer console is presented in fig3.22. all sections of the audio amplifier

were designed and complemented.

RECOMMENDATION

Improving the power of the amplification which will improve the

distance between the receiver and transmitter also the function of the

amplifier could be increased from the mono to stereo, muting, wireless

microphones and more channels of operations.

43
REFERENCES

1) BRUCE AC (1984) Communication System An Introduction To

Signals And Noise In Electrical Communication, Mc Graw-
Hill book company, Singapore.

2) Theraja B.L A.K(1999): A Textbook Of Electronics Technology

3) Mischa (1981): Information, Transmission modulation and noise Mc

Graw-Hill book company Singapore

4) Stephen R Fleeman (1990): Electronic Discrete And Integrated

Preventive New York

5) Caholts A (1978) Electronic Circuits, Digital And Analogue John

Wiley and Sons inc USA.

6) Horowitz P. W. (1980) The Art Of Electronics Cambridge

University press New York.

7) Millman ICC Integrated Electronics Mc Graw-Hill book company

Japan

8) Mallam Ladan Maijama’a Lecturer Analoque Electronics Note

Book, Electrical Electronics Engineering Department
Federal Polytechnic Bauchi

9) D.C. Kulshreshtha: Electronics Devices And Circuits.

44