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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.