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CHAPTER ONE
INTRODUCTION
1.1 INTRODUCTION
Audio amplifiers are meant to amplify signals, which
operate a part or the entire audio frequency spectrum. The
regular domestic hi-fi audio amplifiers are designed to
amplifier speech signals, which form part of the audio
frequency spectrum. The amplifiers could be used for a
myriad of applications, which range from simple inter-com,
baby alarm or for filling a large concert hall, for amplifying
microphone signals or synthesizer sounds, or mainly for the
enjoyment of constructing the circuits and investigating their
properties to check avenues for technical improvement.
Audio amplifiers come in different classes and each class
with peculiar characteristics that makes it suitable for
particular application. They include; CLASS A, CLASS AB,
CLASS B, CLASS C, CLASS D and CLASS E. Details of the
various classes are explained in the literature review
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chapter, but in this project a CLASS AB amplifier is designed.
This is because the regular class B has a crossover distortion
problem which is due to the drop across the base emitter
junction of the transistors in the push pull amplifier, and
hence distortion of the ware form which causes distortion in
sound. These audio amplifiers, which are sometimes called
hi-fi (high fidelity) amplifiers must have an undistorted
representation of the input (amplified) sound at the output,
this is because the human ears response to sound distortion
is very short.
The class AB amplifier combines the quiescent bias of the
base (i.e. the base is driven to the threshold of conduction to
prevent crossover distortion) and the high efficiency of class
B to get a class AB combination. The general requirements
for a good quality audio amplifier are as follows:
(1) Match the input device characteristics input level, input
impedance and frequency response.
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(2) Match the output characteristics, usually a loud speaker
with its output power rating and impedance.
(3) Maintain an audio frequency response from 30Hz to 20
KHz on beyond.
Additional requirements vary according to the applications
but normally may include the following.
(4) Tone controls; for bass, middle and treble adjustments to
suit the loudspeakers characteristics and surroundings or
mainly for personal choice. Filters are often included for such
distortion as tape hiss, record scratches, mains hum, and the
variations of the ears response with volume (loudness
control), and so on. Every listener has preferences for the
types of sounds that are most pleasing: tone control circuits
should satisfy most different tests.
(5) A loudness control, referred to above, which connects for
the ear response. The ear hears all audio frequencies with
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equal volume only when the sound is very loud. If the
volume is reduced by 40dB,
60dB, and so on, the intensity at high and low frequency falls
until, at very low volume, the bass is almost inaudible.
(6) Scratch rumbles and tapes his filters to eliminate scratch
noise on records, rumble noise due to motor vibrators on
record player and tape recorder decks and the type hiss of
the cassette players.
(7) Stereo adjustments such as balance and left/right tone
correction to balance the two channels properly in situations
where two channels are employed.
(8) Other optional refinements, offer necessary such as:
speaker switches for left, right or lift/right through both
speakers; power VU meters; quadra switches for pseudo
quatra, clock stops on rotating controls; individual filters for
selected frequencies usually via a maze of slide controls,
monitor output volume, peak overload detector etc.
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Nowadays single units mixer, which contains equalization,
filters, stereo adjustments and all items mentioned in (8)
above are available. This project concentrates on the power
amplifier and the volume control alone.
1.2 GENERALISED BLOCK DIAGRAM
MICROPHONE
PRE-AMPLIFIER
DIFFERENTIAL
AMPLIFIER
DRIVER
STAGE
CLASS AB
AMPLIFIER
POWER
SUPPLY
8 ohms
speaker
Figure 1.2 shows the generalized block diagram of the entire
unit.
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1.3 DESIGN SPECIFICATIONS
Output power: 100watts
Output impendence: 8
Input voltage 220VAC
Supply voltage: +30 Vdc
Amplifier gain : 0-70dB
CHAPTER TWO
LITERATURE REVIEW
2.1 AMPLIFIERS
Amplifiers are one of the most common electrical building
blocks. By definition an amplifier is any circuit that provides
gain. It receives a low-power input, which controls, via an
external supply, a larger amount of power at the output.
An amplifier arrangement consists of some active device
(transistor, FET, or valve) with biasing components, a source
of power, and a load. The input signal is used to control the
current flowing through the active device. For example, with
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a FET in common source mode, the input voltage between
gate and source (VGS) will control the current flowing from
drain to source (iDS). Since the output current flows in the
load it will develop a voltage across the load so that,
Po = Voio
watts.............................................................. 2.1
While Pi =Viii
watts .................................................................2.2
Therefore
POWER GAIN Ap = Po /
PI ...................................................................2.3
In many cases an amplifier may be designed primarily for
voltage or current gain:
VOLTAGE GAIN Av = Vo /
Vi .................................................................2.4
CURRENT GAIN Ai = io / ii
.....................................................................2.5
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These are all expressions of gains as ratios; it is usually more
convenient to express gain in logarithmic units {Decibel}:
Av = 10log (Po / PI)
Av = 20log (Vo / Vi) provided that the input and output
Av = 20log (io / ii) impedances are identical.
2.2 AMPLIFIER CLASS
Classification of amplifiers
A wide variety of types exist. They are usually described
under one or a combination of the following headings.
1) Intended use: power, voltage or current gain.
2) Frequency response.
D.C. (from zero frequency).
Audio (15Hz to 20 KHz).
Tuned R.F. (narrow band with center frequency from
tens of kHz to hundreds of megahertz).
Video or pulse (wideband d.c to 10MHz).
V.H.F. (up to thousands of megahertz).
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3) Method of operation: This means the biasing
arrangement that determines the position of the
quiescent operating points.
Class A: The active device (transistor or valve) is biased so
that a current flows without any signal present. This value of
bias current is either increased or decreased about its mean
value by the input signal. This mode of operation is
commonly used for small signal low power amplifiers.
Class B: The active device is biased just to the point of cut
off so that zero current flows when no signal is present. The
device conducts on one half cycle of the input.
Class AB: This is a modified form of class B where the active
device is provided with a small amount of bias just sufficient
to allow the device to conduct slightly. This class of
operation is widely used ib audio push pull and
complementary power amplifiers to avoid non linearity at
the cross over point.
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Class C: The active device is reverse biased beyond the
point of cut off so that it only conducts when the amplitude
of one half cycle of the input exceeds a relatively large
value. This method is used in pulsed and R.F. Power
amplifiers.
2.3 AMPLIFIER COUPLING
Coupling refers to the methods used to transfer the signal
from one stage to the next. There are three basic types of
amplifier coupling (capacitive, direct and transformer).
Capacitive coupling
Capacitive coupling is useful when the signals are alternating
current. Coupling capacitors are selected to have a low
reactance at the lowest signal frequency. This gives good
performance over the frequency range of the amplifier. Any
dc component will be blocked by a coupling capacitor. Figure
2.2(a) shows a capacitor coupled amplifier.
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Coupling
capacitor3V
Q2
To next
stage
7V
+Vcc = 10V
Q1
Figure 2.2(a) Capacitor coupled amplifier
Direct coupling
Direct coupling does work at 0 Hz (direct current). A direct
coupled amplifier uses wire or some other dc path between
stages. Fig 2.2(a) shows a direct-coupled amplifier. Notice
that the emitter of Q1 is directly connected to the base of
Q2. An amplifier of this type will to be designed so that the
static terminal voltages are compatible with each other.
Temperature sensitivity can be a problem in direct-coupled
amplifiers. As temperature goes up, and leakage current11
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increases. This tends to shift the static operating point of an
amplifier.
When this happens in an early stage of a dc amplifier, all of
the following stages will amplify the temperature drift.
Q1
Direct
coupling
Direct
coupling
Direct
coupling
Q2
Signal
+Vcc
Output
Figure 2.2(b) Direct coupled amplifier
2.4 FREQUENCY
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Any signal or quality that varies regularly with time will have
a frequency which is defined as the number of complete
variations it makes in unit time-in other words, the number
of cycles per second.
The correct unit for frequency is Hertz.
Frequency is related to the periodic time of the waveform by
the formula:
F = 1 / T Hz
............................................................................2.6
Where T is measured in seconds
Angular velocity w = 2f
r/s ......................................................2.7
For a propagated wave, the velocity u is given by
V = f
.....................................................................................2.8
Where is the wavelength in meters and the velocity per
sec.
Thus for a radio signal at 30 MHz the wavelength is
V = 300 x 106 = 10m
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F 30 x 106
Frequency band
A range of frequencies, within specified limits, that are used
for particular purposes.
Frequency distortion
This type of distortion in amplifiers is caused by variations in
gain with frequency over the range of frequencies for which
the gain should be flat. The signal components of different
frequencies of complex input signal are then amplified
differently with the result that the output waveform will be
distorted.
Table 2.1 Showing frequency bands, its wavelength, types
and its typical use.
Band (f) Wavelength
(m)
Type Typical use
Below 30KHz 105 to 104 V.L.F
(very low freq)
VF telegraphy.
Radio telegraphy.30KHz to 104 to 103 L.F Carrier
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300KHz Low freq telegraphy.
A.M. radio300kHz to
3MHz
103 to 102 M.F
Medium freq
A.M radio
3MHz to
30MHz
102 to 10 H.F.
High freq
Long distance
radio30mHz to
300mHz
10 to 1 V.H.F.
Very high freq.
Mobile radio.
F.M. radio.
Television300MHz to 3
GHz
1 to 0.1 U.H.F.
Ultra high freq.
Radar.
T.V. and
communications.3GHz to 3
GHz
0.1 to 0.01 S.H.F.
Super high
freq.
Radar and
communications.
2.5 NOISE
Electrical noise is defined as any unwanted signal, which is
present at the output of a system or at any part within the
system. It is particularly important in communications
receivers that unwanted signals are kept to a minimum;
otherwise the required output information may be lost within
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the noise. Noise is a source of error in both analogue and
digital systems but the latter is much more tolerant of an
electrically noisy environment because in a digital system a
wanted signal is either logic 1 or logic 0. The difference
between these two logic states gives a barrier to noise and is
referred to as the noise margin. Fig 2.4a shows the sources
and effects of noise on a purely analogue system such as a
communications receiver.
A
E
B
L
N
Output + Noise
16
POWER
SUPPLY
SECTION
RECTIFIER
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C D
Figure 2.4 Sources of noise
A = Atmospheric noise
B = Interference from other transmitters
C = Artificial radiated noise, ie. Arcing noise
D = internally generated noise at input stage
E = Mainsbourne noise (spikes on mains)
The external noise affecting the receiver can have several
origins. Artificial or man-made sources of noise are, for
example, arcing contacts on switches or relays controlling
heavy loads such as motor. The spark will give off an
electromagnetic radiated signal, which is picked up by the
aerial. Alternatively the interference may be carried along
the mains lead since the heavy loads being switched on and
off produce large spikes n the mains which can then be
transmitted through the systems power supply. Another
source of noise is interference from radio transmitters. This
is called second channel and image channel interference.
The effects of artificial sources can be minimized either by
suppression at source (i.e. preventing arcing at switch
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contacts) or by filters and special shields at the receiver.
Second and image channel interference can be reduced by
improved selectivity in the first stage in the receiver
There are natural sources of noise, referred to as
atmospheric noise, such as static noise from space and
electrical discharges during storms.
The signal arriving at the input of the system will
therefore have a small noise superimposed. The signal
arriving at the input of the system will therefore have a small
noise superimposed. The receiver itself now adds more noise
in the process of selecting and amplifying the wanted
information. Internal noise is mostly the result of that
produced in the first stage and is caused by noise from
resistors and semiconductor devices.
INTERNAL NOISE
This is the thermal agitation or resistor noise produced
by the random motion of free electrons in a conductor.
R.M.S. noise voltage in a conductor is given by
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Vn = (4kTBR)
...............................................................2.9
Where k = Boltzmanns contact 1.38 x 10-23J/oK
T = Temperature of conductor in degrees Kelvin
B =Bandwidth in Hz over which the noise is
measured
R = Resistance of conductor in circuit.
Example The noise voltage produced by a 100 k (ohms)
resistor at a temperature of 20oC and over a bandwidth of
100 kHz is
Vn = (4x 1.38 x 10 23 x 293 x 103 x100 x 103)
= (162 x 10 12) = 12.73 V
The available noise power from any resistor is Pn = KTB
NOISE IN BIPOLAR TRANSISTORS
This has several components:
a) Thermal agitation noise, developed mostly in the
base spreading resistor r bb of the device, given by
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Vn = (4kTBr bb)
1/2 ................................................2.10
b) Partition noise resulting from the random variations
of the emitter current division between base and
collector.
c) Shot noise caused by the random arrival and
departure of charge carriers by diffusion across p-n
junction.
d) Flicker noise (1/f noise) resulting from changes in the
conductivity of the semiconductor material and
changes in its surface conductor. This noise is
inversely proportional to frequency and is usually
negligible above 1 kHz. To achieve low noise figures
from a bipolar transistor it is operated at low values
of collector current (a few micro amps) and at low
voltage.
NOISE IN FETS
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Since a FET is a unipolar device it is inherently less noisy
than a bipolar transistor. Only one type of charge carrier is
used and only one current flows. The three sources of
noise are
a) Shot noise, resulting from the changes in the small
leakage currents in the gate-to-source junction.
b) Thermal agitation noise developed in the channel
resistance of the device.
c) Flicker noise.
Signal-to-noise ratio
For a quoted input signal power, over a defined
bandwidth, the signal-to- noise ratio in an amplifier or
receiver is given by
Average wanted signal power
Ps
S/N ratio = Average noise power present
= Pn
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This is usually expressed in dB as S/N ratio =10 log10
(ps / pn)
Example At a frequency of 10 kHz the average wanted
signal power at the input is 800 mf w and the average
noise power present is 6 mf w. What is the input signal-
to-noise ratio?
Input S/N ratio = 10 log10 (800/6)
= 21.25dB at 10 kHz
In electronics, voltage ratio is also often used:
S/N ratio = 20 log10 (Vs / Vn) dB
Noise factor (B.S. 3860)
Used to specify the noisiness of an amplifier or device,
noise factor is
Total noise power out
F = power gain x Noise power due to source
resistor
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F = PN/GPn
................................................................................2.11
But since G = Ps(0)/Ps( i )where Ps is signal power,
F = PN/ (Ps(0) x Pn / Ps(i))
= Ps(i)/ Pn(i)
Ps(0)/ Pn (0)
= Signal/noise ratio at input
Signal/noise ratio at output
Noise figure = 10 log10 F dB.
Thus if the noise figure for a device at a particular
frequency is say 3 dB and the output signal-to-noise
ratio is 100: 1 (20 dB), then the resulting signal-to-
noise ratio at the output will be 3 dB less at 17 dB (a
ratio of 5:1).
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Noise in digital systems
The same sources of external noise such as arcing contact
and mains bourne spikes can affect a digital system if the
resulting noise spike on a signal lead exceeds the noise
margin. When this occurs the logic itself can generate power
line noise as gates switch and a short duration current pulse
is taken from the supply. Most logic types, apart from ECL,
suffer from this effect and therefore the power supply
decoupling and distribution is important. I.C.s should be
decoupled using 100 nF ceramic capacitors wired directly
across the IC supply pins. If possible a ground plane should
be sued to give low-inductance earth return. Other sources
of internal noise are cross-talk when the signal on one track
is coupled to an adjacent track and reflections from
mismatched lines. For cross-talk
Vin = Vs 1
.....2.12
(1.5 + Zm) ( 1 + Z1 )( Z0
Z0 )
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Where Vin is the induced voltage between the two
parallel tracks
Vs is the voltage swing of the logic
Z1 is the output impedance of gate 1
Z0 is the line impedance.
Zm is the mutual coupling impedance.
Careful design can eliminate the effects of internal logic
generated noise, and external sources can be effectively
stopped from affecting the logic by the use of mains filters,
screening and special filters on the input lies. The higher the
noise margin the better immunity of the logic to noise.
Manufacturers usually quote d.c. value of noise margin
giving typical and worst-case values. Taking TTL as an
example, the typical noise margin will be the difference
between the voltage level from the output of a gate and the
threshold of the gate input it is driving (fig. N5). Using this
criterion the best logic 1 or high-state noise margin is 1.9v,
whereas the 0 or low-state noise margin is 1.2v. However
the typical noise margin is 1 v in both cases. The worst-case
d.c. noise immunity has to take into account the minimum
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and the maximum values of output levels and input
threshold. The maximum value of logic 0 output is 400mv
and minimum value of threshold (V10) is 800 mv giving a
worst-case noise margin of 400mv.
2.6 TRANSISTORS
Transistors are active components used basically as
amplifiers and switches. The two main types of transistors
are:
The bipolar transistors whose operation depends on the flow
of both minority and majority carriers, and the unipolar or
field effect transistors (called FETs) in which current is due to
majority carriers only (either electrons or holes). The
transistor as a switch operates in class A mode. In this mode
of bias the circuit is designed such that current flows without
any signal present. The value of bias current is either
increased or decreased about its mean value by the input
signal (if operated as an amplifier), or ON and OFF by the
input signal if operated as a switch.
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IbVin
Rb
V+
IcRc
Figure 2.5 transistor as a switch
For the transistor configuration, since the transistor is biased
to saturation.
VCE =O, when the transistor is ON.
Which implies that?
V+ = Ic Rc +
VCE ...........................................................................................
.......2.13
Vin = IBRB + VBE
.................................................................................................
2.14
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IIcc = h= hfefe
..................................................................................................................................................................................................
...........2.15...........2.15
IIbb
RRbb = V= Vinin V VBEBE
................................................................................................................................................................................
...2.16...2.16
IIbb
Where,
Ic = collector current
Ib = base current
Vin = input voltage
V+ = supply voltage
VCE = collector-emitter voltage
Hfe = current gain.
2.7 OTHER PASSIVE COMPONENTS
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Passive components are components, which cannot amplify
power and require an external power source to operate.
They include resistors, capacitors, diode, indicators, and
transformers etc. their application range from potential
dividers to control of current (as in resistors), filtration of
ripples voltages and blocking of unwanted D.C voltages (as
in capacitors). They form the elements of the network circuit
oscillator stages and are also used generally for signal
conditioning in circuits.
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RESISTOR
According to ANAND, these are components which resist
the flow and hence limit the amount of current flowing
through a circuit. The resistance is measured in Ohms. The
symbol of a resistor is shown in fig. 2.1
Figure 2.7 Circuit symbol of a resistor
The resistor may be pure at low frequencies but may have
inductive or capacitive impedance at higher frequencies. The
frequency up to which it is only (pure) and has only
resistivity is called its frequency range. The resistor, while
working produces noise. This is called thermal noise and
the noise generated depends upon its resistance value and
temperature.
CLASSIFICATION OF RESISTORS
Resistor may be:
Fixed resistors
Variable resistors.
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Fixed Resistors
These are resistors whose are fixed and cannot be changed.
Examples include carbon resistors and wire wound resistors.
Carbon, binder and filter rod End caps Lead
Plastic or lacquer coating
Figure 2.8 Diagram of a carbon resistor.
SPECIFICATION OF FIXED RESISTORS
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The main specifications of fixed resistors are: The values of
resistance, its power rating, voltage rating, Temperature
coefficient voltage coefficient, Noise voltage, frequency
range, and tolerance stability and for variable resistor, e.g.
temperature, magnetic field and light intensity, the range of
the stimulus that can be applied, the range of resistance
variation and lastly the law governing the resistance
variation.
VARIABLE RESISTORS
These are resistors in which the value of the resistance
varies with the applied stimulus. From the popular equation
R = p. l/a, we can observe that stimulus has to change one
or more of these quantities to give rise to variation in the
resistance. These are four types of stimuli and
corresponding, the following four types of variable resistors;
1) Mechanically variable resistor (e.g. potentiometer,
Rheostat)
1) Thermally variable resistor (Thermistors)
2) Electrically/voltage variable resistor (Varistors)
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3) Optically/ (Light) variable resistors (photo resistors).
RESISTOR COLOUR CODE
The value of a resistor (carbon) may be obtained by looking
at the coloured rings painted on it. Each colour has a
numerical value. Below is the colour coding of a carbon
resistor given in Table 2.1.
Table 2.2 Table showing resistance colour code
COLOUR NUMERICAL VALUE MULTIPLIERBlack 0 100 =1Brown 1 101 =10Red 2 102 =100Orange 3 103 =1000
Yellow 4 104 =10000
Green 5 105 =100000Blue 6 106 =1000000Violet 7 107 =10000000Grey 8 108 =100000000White 9 109 =1000000000
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Tolerance: Each resistor has a tolerance ring. Generally they
have golden or silver rings for this purpose.
Table 2.3 Table showing colour coding tolerance
COLOUR TOLERANCE RING PERCENTAGE TOLERANCEGold + 5%Silver + 10%No ring + 20%Sometime, other colours are also used for tolerance, as
shown in Table 2.3
Table 2.4 Table showing the tolerance colour and its value
COLOUR BAND TOLERANCE
Black 20%Red 1%Yellow 2%
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Sometimes, the value and the tolerance of the resistor are
printed on the resistor itself instead of a colour code. After
the value, a letter is added to indicate the tolerance.
F = +1%
G = +2%
J = +5%, K = +10% and M = +20%
For example 1
1) 20kk is a 20k + 10% resistor.
2) 8M8M is an 8.8m 20% resistor and so on.
Example 2
Red Black Blue Gold
Figure 2.9 Resistor with colour code
Red Black Blue Gold
2 0 6 5%
= 20 106 = 2000000 Ohms
= 20Mega Ohms 5%
CAPACITOR OR CONDENSERS35
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By M. L. ANAND (2000), capacitors are the components
which have a capacity to store (condense) charge. The
capacity is measured in Farads.
1 Farad = 103 Mill farads (mf)
=106 Micro Farads (f)
= 1012 Pico Farads (pf)
A capacitor is basically made up of two metallic plates
separated by some insulting material called dielectric. The
metallic plates may be of aluminium and dielectric may be
paper, mica, ceramic, etc. A capacitor is known by itsdielectric. So we have paper capacitors, mica capacitors, and
ceramic capacitors and so on.
CLASSIFICATION OF CAPACITOR
Capacitors are of two kinds:
i. Fixed capacitors
ii. Variable capacitors.
FIXED CAPACITORS
Fixed capacitors are capacitors whose values are
fixed. Generally, the capacity and voltage are marked on
them. However, colour coding is also used to find theircapacity. On the basis of dielectric, these capacitors may
be the following types:
i) Paper capacitors
ii) Ceramic capacitors36
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iii) Mica capacitors
iv) Electrolytic capacitors
v) Aluminium electrolytic capacitors.
Non-polarized
These are made by joining two polar capacitors in
back position or both the electrode is using oxide film.
These have no polarity and therefore can be connected
without considering positive or negative terminals. These
can be used for AC appliances. Examples include
- Tantalum electrolytic capacitors
- Plastic capacitors
VARIABLE CAPACITORS
Variable capacitors are those whose capacitance can
be changed. They are used in tuning circuits to change
the operating frequency of the circuits.
Capacitance depends upon dielectric constant (), area of
plate (A) and the distance between the plates (d) i.e.
C = A/ d
Variable capacitor can be;i Rotary type
ii Concentric type.
TYPES OF VARIABLE CAPACITOR
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i Air capacitors
ii Trimmer
iii Padder
vi Varactor capacitors
Table 2.5 colour coding chart for fixed capacitors
COLOUR CAPACITANCE IN PF1ST DIGIT 2ND DIGIT MULTIPLIER TOLERAN
CE (%)Black 0 0 1 20
Brown 1 1 10 1Red 2 2 100 2
Orange 3 3 1000 30
Yellow 4 4 10000 Green 5 5 5
Blue 6 6
Violet 7 7
Grey 8 8 0.01 White 9 9 0.1 10
Gold 0.1 1
Silver 0.011 10
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TRANSFORMER
A transformer is basically two inductors placed on the same
core. One of the inductor is known as primary winding and
the other as secondary winding. A transformer is a device
that transfers electrical power from one circuit to the other.
It only transfers; therefore, input power fed at the primary is
equal to the output power obtained at the secondary in an
ideal case. Even supply frequency remains the same. If V1, I1
are the voltage and current at the primary and V2, I2 on the
secondary side, then for an ideal transformer: (As shown in
fig 2.6) V1I1 = V2I2
Transformer can also be either step up or step down.
Primary winding
Secondary winding
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Figure 2.10 Diagram of a transformer
TYPES OF TRANSFORMERS
i Power transformer
ii Output impedance transformer
iii Intermediate frequency transformer (IFT)
iv Isolation
v Instrument
vi Trigger
vii Audio
viii Video transformer
DIODE/LIGHT EMITTING DIODES
These are junctions device consisting of p type impurities
on one side and n type impurities on the other side. The
phenomenal mode of operation of this device is by diffusion
of excess carrier across the junction.
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In electronics, a diode is a two terminal component that
conduct electric current in only one direction. The term
usually refers to a semiconductor diode. The most common
function of a diode is to allow an electric current to pass in
one direction (called the diodes forward direction), while
blocking current in the opposite direction (the reverse
direction). Thus, the diode can be thought of as an electronic
version of a check valve. This unidirectional behaviour is
called rectification, and is used to convert alternating current
and to extract modulation from radio signals in radio
receivers. Below is the circuit symbol of a diode.
Figure 2.11 Circuit symbol of a diode
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Similarly light emitting diodes are p n junction as well
which emits visible light when energized (i.e. when electrons
from the n side cross the junction and recombine with
holes on the p side). Here electrons are in the higher
conduction band on the n side while holes are on the lower
valence band on the p side. During recombination this
energy difference is given up in the form heat and light
(photons).
In some semiconductors, greater percentage is given
up in the form of heat, e.g. Silicon and Germanium
semiconductors. If the semiconductor is translucent, light
is emitted and the junction becomes a light source [i.e. a
light emitting diode (LED)]. The colour of emitted light
depends on the type of material used.
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Figure 2.12 Circuit symbol of a light emitting diode
Other special types of diode include;
1. Zener diode used as voltage stabilizer.
2. Varactor diode used as a variable capacitor.
3. Shorttky diode used for AC/DC converter, detector,
mixer, application, etc.
4. Tunnel diodes used as high speed switches and high
frequency oscillators.
5. Light dependent diode (photo diode) used as photo
detector, for street lighting and for punch card reading.
CHAPTER THREE
DESIGN AND ANALYSIS
3.1 PRINCIPLE OF OPERATION
The 100watts audio amplifier is designed to give power
gain to an audio signal using class AB amplifier. The class AB
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operation is such that each transistor is biased to the
threshold of conduction in the absence of an input signal
with a small idling current flowing. The class AB amplifier
used in this project is a quasi-complementary type. The
quasi-complementary arrangement is often advantageous
because it uses identical devices in the output stage (that is
both, n-p-n or p-n-p devices rather than one n-p-n and one p-
n-pa as in the regular complimentary. arrangements). This is
very advantageous as matching of devices is made easier. It
is preferable to use n-p-n devices as they handle greater
amount of power rather than p-n-p devices.
Each output transistor has its own driver transistor as
shown in the comprehensive circuit diagram in fig. 3.5
3.2 POWER AMPLIFER STAGE
As already explained in the principle of operation, the power
amplifier uses a quasi-complementary stage where both
output transistors are of same type. The power amplifier
stage is comprised of TR4 TR7 (see comprehensive circuit
diagram).
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DESIGN CALCULATIONS
Under maximum condition (ignoring the effects of Vbe1, Vbe2
and Re of the output transistors),
Pac ~ Vcc2 .. 3.1
8RL
Where Vcc = peak to peak supply voltage
RL = load resistors
PAC = maximum ac power output.
For a power of 100watts on a load resistance of 4,
=> 100 = Vcc2 (from 3.1)
8 (4)
Vcc = 100(8) (4)
= 3200
= 56V
Since Vcc = Peak to peak voltage.
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It implies. Vcc = + 28.2
~ + 30V
Hence a power supply of + 30V was designed to power the
entire project for realization of the output power. Since the
mean d.c current drown from the supply depends on the
peak value of output voltage, which can have a maximum
value of
Vcc ignoring Vbe1, or Vbe2, then
2
Idc = 1 Vpk
........................................................................................3.2
RL
And Pdc = Vpk Vcc watts.
RL
And maximum value of pdc occurs when
Vpk = Vcc so that
2
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PdC = Vcc Watts .
..................................3.3
2
RL
Since maximum efficiency, = Pac x 100% =
Pdc
VCC2 Vcc2 X100
8RL 2 RL
X 100% =
78.5%
4
This shows that the efficiency is same as that of the
conventional class B push pull amplifier circuit.
The power dissipated in transistor Pdiss = pdc pac
And for each transistor,
Pdiss = VpkVcc V2pk
.......................................3.4
2RL 4RL
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Differentiating equation (3.4) with respect to Vpk gives
maximum power dissipation when
Vpk = Vcc/. Hence
Pdiss (max) = Vcc2 _ Vcc2
22RL 42RL
Or Pdiss (max) = Vcc2
..........................................................3.5
42RL
Therefore maximum power dissipation on the amplifier (i.e.
power wasted as heat) is,
Pdiss (max) = 602 (From 3.5)
42(4)
= 19.8 watts
~ 20 watts
The power transistors used are rated 115 watts hence can
comfortable handle the dissipated power,
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OUTPUT CONSIDERATIONS
Transistors TRI and TR2 in fig 3.5a were selected to handle
the peak current and power dissipation requirement of the
output.
Since Vce (peak) =Vcc (approx)
Vce (peak) = 30V (approx)
Hence Ic (peak) =30/ 4
= 7.5A
Hence, the 2N3055 was selected for TR1 and TR2 since it
has a max output current of 15A, and power rating of
115watts.
The hfe of the 2N3055 is 20 (from data sheets)
Since hfe = Ic / IB
IB = Ic / hfe
= 7.5A / 20
= 0.375
The emitter of TR3 and TR4 must supply this current to TR1
and TR2 base.
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TR3 and TR4 use TIP31 and TIP32 respectively, with an Hfe of
60 each.
The base current requirement for TR3 and TR4 is given by
IB = 0.375/ 60
= 6.25mA
This is the quiescent base bias required to prevent cross-
over distortion of the class AB amplifier stage. Diodes D5 and
D6 are used to drop a base voltage of 1.2V across TR3 and
TR4. Considering the quiescent bias circuit in fig 3.5b
R10
V -
D5
D6
R11
Vin
V +
Figure 3.1 Quiescent bias circuits
For symmetry of bias, R10 = R11
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R = (Vcc 1.2) / IB
= 60 1.2
(60 - 1.2) = 9.408 K
6.25mA
But R10 = R11 = R
R = 4.7k, hence R10 and R11 = 4.7K
3.3 DIFFERENTIAL AMPLIFIER STAGE
The differential amplifier stage amplifiers the difference
between the input and the output signals, hence signals
common to both input and output are cancelled out; hence
any residual noise is not amplified. The differential amplifier
has two input; one to the output of the amplifier and the
other to the signal input.
The gain of the differential amplifier determines the entire
amplifier gain. Fig.3.1a shows the differential amplifier
stage.
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BVin A
V
Input A
C1
R1
R2
R3
R5
R4
V+
Input
output
C3
C2
-
Figure 3.2 Differential Amplifier Stage
The differential amplifier stage, sometimes called the longed
tail pair gives an output, which goes to the driver stage. R3
is the feedback resistor while R2 is the input resistor. For a
power gain of 20 and letting R3 =33K
Since gain = 1+ Rf / Rin
20 = 1+ 33K / Rin
Rin = 1+ 33K / 19
= 1.73K
= 1.5K preferred value.
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C1 and C3 are decoupling capacitors.
The driver stage using TR5 feeds the complementary pair of
transistors from the differential amplifier stage.
3.4 POWER SUPPLY STAGE.
All stages in the project use +30V and 30V. The power
supply stage is a linear power supply type and involves in
step down transformer, rectifier and filter-capacitor. Voltage
regulators were not used, as there was no critical need for a
fixed stabilized voltage in a power amplifier. Fig 3.5a below
shows the circuit of the power supply stage.
C2
220V AC
C1
D1D2
D3D4
Figure 3.3 Power supply stage
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The rectifier is designed with four diodes to form a full wave
bridge network. C1 and C2 are filter capacitors and the filter
capacitor C1 is inversely proportional to the ripple gradient of
the power supply.
Vrms
dt
dv
Figure 3.4 Ripple gradient
Where dv is the ripple voltage for time dt, where dt is a
dependent in power supply frequency.
For an rms voltage of 20volts (from transformer)
Vpeak = 20 x 2 (i.e., rms x 2
= 28.2V
Hence letting a ripple voltage of 10% makes dv = 2.82V
But 1/C = dv
dt
C = dt
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dv
= 10ms (where dt = 10ms for 50Hz)
2.82
= 3546uF
= 4700uF (preferred value).
Hence C1 and C2 = 4700uF. Diodes D1-4 are 1N5404 power
rectifier diodes.
3.5 COMPREHENSIVE CIRCUIT DIAGRAM
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V-
R3
R4
R5
R6
R7
R8
R9
R10
R11
R12
R13
R14
C3
C4
C5
TR1
TR2
TR3
TR4
TR5
TR6 TR7
-30V
+30VV+
V+
V-
INPUT
C2
220
C1
D1D2
D3D4
Figure 3.5 COMPREHENSIVE CIRCUIT DIAGRAM.
3.6 COMPONENTS LIST
C1 & C2 - 470F
D1 D4 - 1N5404
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C3 - 1F / 35V
C4 - 100F
C5 - 47F
R3, R5 - 4.7K
R4 - 15K
R6 - 680
R7 - 2.7K
R8 - 1.5K
R9 - 33K
R10, R11 - 4.7K
R12 - 100 1/2watt
D5, D6 - IN4007
TR3, TR4 - TIP 31 & TIP 32
TR1, TR2 - 2N3055
TR6, TR7 - A733
TR5 - BD139
CHAPTER FOUR
CONSTRUCTION AND TESTING
4 .1 C ONSTRU CT IO N
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The circuit was first assembled on a project board
component by component according to the schematic
diagram. At each placement, a jumper copper wire was
employed to extend and make connections from one
componenet to another, and also to ensure tight
connections. Unit by unit starting with the power supply,
microphone amplifier, differential amplifier, driver stage, and
class AB amplifier, the entire circuits was completed with
testing done on each unit for comfirmations. Later on the
whole unit were tested and coupled together still on the
project board. Afterwards, the entire system was transferred
to the Vero-board or strips board where there were properly
soldiered together again component by component and unit
by unit. Figure 3.5 is the circuit diagram of a 100 wattage
amplifier.
4.2 IMPLEMENTATION
The implementation of this project was done on the
breadboard. The power supply was first derived from a
bench power supply in the school electronics lab. (To confirm
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the workability of the circuits before the power supply stage
was soldered).
Stage by stage testing was done according to the block
representation on the breadboard, before soldering of circuit
commenced on Vero board. The various circuits and stages
were soldered in tandem to meet desired workability of the
project.
4.3 TESTING
The physical realization of the project is very vital. This is
where the fantasy of the whole idea meets reality. The
designer will see his or her work not just on paper but also as
a finished hardware.
After carrying out all the paper design and analysis, the
project was constructed, implemented and tested to ensure
its working ability, to meet desired specifications. The
process of testing and implementation involved the use of
some test and measuring equipments.
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The testing of this work was done stage by stage using
available instrument stated below. The testing was done to
confirm and ensure its functionality and desired output.
POWER SUPPLY STAGE
The power supply stage was tested using the following
instruments:
Analogue and digital multimeter: This was used to measure
the output voltage, voltage drop across R1 and LED, and also
the continuity of different conducting paths and sections.
Oscilloscope: The oscilloscope was used to observe the
ripples in the power supply waveform and to ensure that all
waveforms were correct and their frequencies accurate. The
waveform of the stages was as well checked at different
stages.
Digital Multimeter: The digital multimeter basically
measures voltage, resistance, continuity, current, frequency,
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and temperature and transistor hfe. The process of
implementation of the design on the board required the
measurement of parameters like, voltage, continuity, current
and resistance values of the components and in some cases
frequency measurement. The digital multimeter was used to
check the voltage in this project.
4.4 PROBLEM ENCOUNTERED
These are some of the problems encountered.
Testing of the project on breadboard before it was
soldered on the vero board proof difficult to accomplish
due to the complementary of the circuitry.
Also in determining the frequency required.
There was some difficulty experienced while drilling on
the casing so as to get the actual size of the LED, switch
and buzzer frequency knob.
Loss of signal was experienced due to cross-talk.
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CHAPTER FIVE
CONCLUSION AND RECOMMENDATION
5.1 CONCLUSION
I have successfully constructed a suitable easy going
100 wattage amplifier, based on the effectiveness of the
discrete components in response to their linear external
dynamic variables such as voltage, current, frequency and
temperature. By employing passive and active components
this was achieved.
The design compares favourably with any standard light and
security light control in the world and has also an added
advantage of being environmental friendly and pollution
free.
5.2 RECOMMENDATION
With this device,...(FROM YOUR
ABSTRACT).....................................the. However, the degree
of perfection or operational efficiency of the system may be
improved when subjected to a high great performance test
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and evaluation in order to determine areas of modification
and to optimize the performance of the system.
Therefore I recommend that:
- Subsequent undergraduate be given this topic for
modification.
- Adequate lectures are given toward the design and
construction of circuits and the use of components.
- A timely maintenance culture.
- Funds for the execution of electronic projects should be
release to physics department to enable them carry out
research on electronics and design more projects that
will benefit the university community and the world at
large.
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