AUDIO POWER AMPLIFIER

26

CHAPTER THREE: THEORETICAL BACKGROUND
3.1 THEORY AND ANALYSIS
POWER SUPPLYINPUTTRANSDUCER(MICROPHONE)PRE-AMPLIFIER(AUDIO AMPLIFIER)MODULATOR/POWER AMPLIFIERRADIO FREQUENCY(OSCILATOR)The method used in achieving Transmission is show in the block diagram bellow in figure 3.5
POWER SUPPLY
INPUT
TRANSDUCER
(MICROPHONE)

PRE-AMPLIFIER
(AUDIO AMPLIFIER)

MODULATOR/
POWER AMPLIFIER

RADIO FREQUENCY
(OSCILATOR)













Fig 3.1
Block diagram of an fm Transmitter

The whole circuit is power by the complete power supply show in figure 3.5 above, microphone (condenser) is used as a transducer which converts the audio signal (information) into electrical signal, BC547 is used as pre-amplifier which raise the strength of a weak signal the radio frequency (RF) oscillator is a frequency generator, 2N3019 NPN transistor was used as modulator, power Amplifier. For the tank (oscillator) Hartley mode would be used. Monopole antenna would be used as output transducer.

3.1.2 Power Supply
A power supply is a circuit that provides a dc regulated out put voltage.
In order to perform function the transmitter and receiver most have to be powered by a constant DC power supply.
The power supply is constructed using the following elements are chosen.
Step down transformer,
Diode as rectifier
A capacitor as fitter
IC positive voltage regulator
The method used in achieving receiver is shown in the lock diagram in figure 3.5

RADIO FREQENCYOSCILLATORLOCAL OSACILLATORMIXERIF AMPLIFIERLIMITERDETECTORAUDIO AMPLIFIERSMOOTHING NETWORK
RADIO FREQENCY
OSCILLATOR
LOCAL OSACILLATOR
MIXER
IF AMPLIFIER
LIMITER
DETECTOR
AUDIO AMPLIFIER

SMOOTHING NETWORK






RADIO FREQENCYOSCILLATORTDA 7000LM 380LOCOL OSCILLATOR3.2 Fm receiver circuit diagram using discrete component
RADIO FREQENCY
OSCILLATOR

TDA 7000

LM 380

LOCOL OSCILLATOR

The above diagram can be reduced by using IC chips TDA 7000 and LM380





3.3 Fm receiver circuit diagram using IC chips.
3.1.3 IDA 7000 (FM Receiver)
18 17 16 15 14 13 12 11 101 2 3 4 5 6 7 8 9This simple one chip FM receiver will allow you to receiver frequencies from 88 to 108 MHZ. TDA 7000 is a great chip because it includes RF input state, mixer, local oscillator, IF (inter mediate frequency) limiter, If filter, amplifier, phase demodulator, mute doctor, and various other circuit that are essential for a receiver system. The figure below shown the data sheep of TDA 7000
18 17 16 15 14 13 12 11 10
1 2 3 4 5 6 7 8 9

Figure 3.4 TDA 7000

Pin 1 is Vec, pin 2 is out put voltage, pin 6 and 5 local oscillators, pin 14 and 13 tank circuit (oscillator), pin 16 is ground. Pin 10 corrector, pin 17 demodulator, and pin 15 IF limiter, pin 10 and 12 & 11 If filter, pin 3 noise mute, pin 4 loop filter.
3.3.3 LM 380 (Audio amplifier)
Lm 380 is an audio amplifier its high power audio amplifier. The output of the TDA 7000 receiver is connected to the input of the LM 380 (audio amplifier) the output of the TDA 7000 (receiver) is not enough to driving crystal earphones or high impendence headphones that is the reason of connecting the LM380 (audio amplifier).
From the date sheep Lm 380 has the following picture which shown below in figure 3.8

14 13 12 11 10 9 8 1 2 3 4 5 6 7
14 13 12 11 10 9 8
1 2 3 4 5 6 7


Figure 3.5 Lm 380
3.1.4 Transducer (Loudspeaker)
The out put transducer (loudspeaker) is a device which converts electrical impulses in to sound.
It consists of a coil with many turns placed in strong magnetic field produced by the polis of a permanent magnet which is joined to a piece of soft iron. The coil is attached to a cone made from thick paper, and shaped as to make it flexible varying current from an amplifier is passed into the coil through the leads. This current produces a varying electromagnetic force on the coil making it vibrate with the same frequency to the cone attached to the coil. The cone will then vibrate causing the large mass of air around it to vibrate, there by producing a loud sound. Figure 3.9 below the diagram






Figure 3.6 a moving coil loud speaker




POWER SUPPLY
3.2.1 TRANSFORMER
Considering equation 3.1 below; this is usually referred to as the basic transformer equation (Theraja B. L, A. K, 1999)
V = 4FfaBN X 10-8………………………………..3.1
Where:
V = the r.m.s voltage across a considered winding in volts.
F = form factor (normally 1.11 for sine wave)
f = input frequency in Hertz (50Hz)
B = flux density in lines per square inch
N = number of turns on a considered winding
a = core area in square inches
A conservative figure for B is 75000 lines per square inch of core area.

V = 4 X 1.11 X 50 X a X 75X103 X N X 10-8
V = 16650X10-5 X N X a
N/V = 6/a…………….3.2
The term N/V is known as the turns per-volt figure for a transformer- that is to say, the number of turns on the winding for each volt across them. This ratio is the same for each winding on a transformer.

3.2.2 RECTIFIER AND SMOOTHENING CIRCUIT
The rectifier circuit is needed for ac signal rectification. A full wave bridge rectifier is the more commonly used rectifier circuit. Its arrangement is shown in figure 3.1 below

Fig 3.1 full wave bridge rectifier.
The bridge rectifier consists of four diodes. The operation of the circuit is that two diodes conduct during any of the half cycle of the ac input voltage the resultant output voltage waveform is shown in fig3.1
However since a dual power supply is required for this project, the rectifier circuit is modified as shown fig3.2 where C1 and C2 are the filter capacitors (Theraja B. L, A. K, 1999).

Fig 3.2 full wave rectifier with dual supply.
A circuit that converts a pulsating output signal of a rectifier into a smooth dc voltage is known as a filter to achieve this; a capacitor is used in parallel with the load. This type of filter is known as CAPACITOR INPUT FILTER. The filtering action of this filter wave is shown in fig3.3 below.

Fig3.3 filtering action of a capacitor filter.
The value of the shunt capacitor is given by
……………….3.2
Where F = frequency at the main voltage
γ = Ripple factor
R2 = load resistance (Theraja B. L, A. K, 1999)
In order to achieve 100W power with 76.55% efficiency a dual voltage +35v and -35v is needed (both dc voltage).
Since these voltages are peak voltages therefore their root mean square (r.m.s) will be:
Vr.m.s = Vpeak(dc) ………………3.3

Volts
The conversion from r.m.s value to peak value is practically done using smoothening capacitors.
Since the r.m.s value is pulsating and supplied by a bridge rectifier then the r.m.s voltage will be transformer r.m.s voltage (ac) less two diode drops since for each half-cycle at the ac voltage two out of the four diodes of a bridge rectifier conducts.
Transformer r.m.s voltage Trms is given by:
Trms = 24.75 + 1.4 = 26.15v (ac)
Hence, since the power supply is intended to be dual, then transformer has to supply twice the voltage in the equation. This invariably means the transformer will be a centre tapped type. This is transformer secondary voltage.
Vs = 52.3v
Recall, the turn per voltage ratio is N/V= 6/a …………………..3.4
Where:
N = number of turns on a considered winding
V = voltage across a considered winding in Volts
a = core area in square meter which has been chosen to be 18cm2 for the primary winding
V = 4.444fΦNP (Theraja B.L, A.K 1999)
Φ = BA
Where B = 1.2T
A = 4.0 X 4.5cm
A = 18 X 10-4m


NP= 2.085 X 220
NP = 458.8
Primary turns = 459 turns
For the secondary winding
Vs = 52.3v
2.085
Ns = 2.085 X 52.3 = 109.05
Secondary turns = 110 turns

3.2.3 Transducer (microphone)
Microphone is a device which converts sound in to electrical impulse we have different types of microphone in this project condenser will be used
Condenser (electrets) microphone it contains permanently embedded static electric charge in between two parallel plates (which due to high resistance and chemical stability of the material will not decay for hundreds of years).
A voltage is applied across the metal plates, coursing a small current to flow through the charge material, when sound is applied on the condenser microphone applying a varying pressure to the two plates, which causing the plates to adjust and the voltage across the two plates varies linearly with distance which cause the voltage at sources varies which producers electrical signal across the out put resistance.
3.2.4 PRE-AMPLIFIER
Pre- amplifier circuit is a circuit design to raise the strength of a weak signal.
It is used to raise the strength of the weak signal coming from the transducer and couple to the modulator.
BC 547 is used which has the following characteristics
Voltage gain = 100
Maximum voltage= 45V
VEBO=6V
Collector maximum current=100mA
Power dissipation, Pt= 500MW
In analysis of amplifier the following have to be consider
Transistor configuration
We have three types of transistor configuration
Common emitter (CE)
Common collector (CC)
Common base (CB)
2 transistor bias; we have different types of transistor bias. The following are some
Base bias or fixed current bias
Base bias with a emitter feed back
Base bias with collector feedback
Voltage divider bias
Frequency response :- we have the following frequency response
1 Audio frequency (AF) amplifier
Intermediate frequency (IF) amplifier
Radio frequency (RF) amplifier
Transistor biasing condition
Class- A amplifier
Class-B amplifier
Class- C amplifier
Base on the above analysis I would choose the following
Common emitter (CE) configuration most of the transistor amplifier are of CE type because of large gains in voltage, current and power. Moreover, their input and out put impedance characteristics are suitable for many application
Base bias with collector feed back it is a negative feedback which has the following advantages, higher fidelity, stabilize gains increase band with (improved frequency response)and less amplitude, harmonic, frequency, Noise and phase distortion and also input and out put impedance can be modified as desired
Class- A biasing condition in class A biasing condition the circuit is bias in such away the collector current(IC) flow at all time during the full cycle of the input signal that is the transistor most work at the active region and small amount of collector current.
Vce=1/2 Vcc and small amount of collector current.
Therefore, the amplifier is as shown below
In fig. 3.2

Fig3.2 pre amplifier
3.2.5 RF AMPLIFIER
A Transistor is used to do the following function RF Amplifier, modulator and power amplifier.
3.2.6 Hartley Oscillator
For the case of this project a Hartley oscillator was chosen as in the tank circuit targeted to generate frequency between 88MHZ to 108MHZ so that to avoid capture effect the tank circuit consist of a trimmer capacitor, inductor the circuit frequency of oscillation is then given by approximately it is shown in figure 3.3 below
F = 12πLC
Where F = frequency
C = capacitance
L = inductance




Fig 3.3 tank circuit (RF oscillator)
3.2.7 MODULATION
Modulation ; it is the process of combining an audio frequency (AF) signal with a radio frequency (AF) currier wave.
During modulation, some characteristic of the currier wave is varied in time with the modulating signal (AF signal) and is accomplished by combining the two.
The method of modulation the mathematical expression for a sinusoidal carrier is
E = Ec sin (wc t + θ) = Ec sin (2πfc+θ) obviously the wave form can be varied by any of its following three factors or parameters
1 Ec – the amplitude
2. fc- the frequency
3. θ- the phase
In this project only frequency modulation is choose frequency modulation(FM) -: the information signal changes the frequency of the carrier wave without changing its amplitude or phase.
If the carrier signal is given by :-
ec= Em cos wct wmt and the information signal is given by:-
ei = Em Cos Wmt the modulated output is given by:-
ec= Ec cos Wct (Wct+ sinWmt)/Ec
= DF (max)/Fm
Where
DF= is the frequency modulation deviation
Fm = is modulation frequency.
3.2.8 Power Amplifier
For power Amplifier common emitter, voltage divider bias with emitter feed back and class-B is chose as shown in fig 3.4 below

Fig 3.4 Voltage divider
Rb can be calculate by using the following formula
Rb=R1R2/(R1+R2)
3.2.9 Antenna
An antenna is a structure usually mentally object, which provides effective sending and reception of electromagnetic waves. For efficient radiation, the impedance of the antenna and circuit has to properly match. The antenna used in this project is simple monopole.
The length of the antenna obtains using the relation given by
L= wave lenth/4
Where
L= length of the antenna

3.3.0 RECEIVER
3.3.1 Theoretical Background
After analysis, the following components used in the project were purchased in our local market; these are transistors, capacitors, resistors, transformer inductors (coil) and IC such as TDA 7000, LM 380.
Table1: Transistors parameters.
TRANSISTOR
Vce(volt)
Ic(mA)
Pmax(mw)
Fmax(MHZ)
ß(Hfe)
2N3019
30
800
800
110
180
BC547
6
100
500
120
100



Table 2: technical specifications
IC
Vcc(volt)
Fmax(MHZ)
P(mA)
TDA7000
2 – 10
70 - 120
8
LM 380
5 – 14
0.002 - 20
7




Pre- amplifier
Clas-A amplifier
VceQ= ½ Vcc
But Vcc= 9v
VceQ= ½ x9
VceQ= 4.5v
Choosing Icq very small for amplifier stability
IcQ= 0.95MA and ßQ= ½ ß
ßQ= ½ x100=50
Vce= Vcc-IcRc
Rc = Vcc-Vce/Ic
Rc= 9-4.5/0.95x103 = 4.736x103
Rc= 4.736k
The chooses value of resistor available is the market is 4.7k
Hence Rc=4.7 K
Ic = ß Ib
Ib = Ic/ ß
Ib= 0.95/50
Ib=0.019MA
Ib=19µA
Vce=Vcc-IbRb-Vce
Rb= Vcc-Vce/Ib
Rb= 9-4.5-0.3/19x10-6 = 4.2/19x10-6
=0.2211x106
The one available in the market is 220K hence RB= 220K choose C=4.7µF as coupling capacitor
3.4.7 Modulator
Vb= R2/(R1+R2)xVcc
Choosing R1=R2 =10K
Vb= 10/(10+10)x9= 90/20
Vb= 4.5v
Vb-Vbe- IeRe=0
Approximately Ie= Ic
Re=(Vb-Vbe)/Ic
Re=4.5-0.2/Ic
Taken IcQ = 40Ma
Re= (4.5-0.7)/40x10-3
Re = 0.095x103 = 95
The available resistor in the market is 100 hence Re= 100
3.4.8 Tank circuit (oscillator) for transmitter
L= inductance
From the formula
L= R2c N2r / {25 (9Rc+10h)}
Where L = inductance in µH (micro Henry)
Rc= Radius of the coil in (mm)
Nr= Number of turns
h= the overall length of the winding coil in (mm)
Coil of 0.55mm in diameter was winded on biro tube (4mm diameter) for up to 15 turns
Rc = 4mm/2 = 2mm
h = 0.55x15= 8.25mm
Nr = 15
L = 22 x 152/ 25 (9x2+10x8.25)
L = 900/2512.5
L = 0.358µH
When F = 88 MHZ by using

F = 12πLC
C = 14π2F2L
C = 143.1422*88*1062*0.358*10-6
C = 9.13 ρF
When F = 108MHZ
C = 14π2F2L
C = 143.1422*108*1062*0.358*10-6

C = 6.06 ρF
Choosing trimmer (Variable capacitor) of range 4 to 20ρF is suitable for the range of frequency 88 to 108MHZ.
3.4.7 Antenna (monopole type)
L = ¼λ
Where L = Antenna length
λ = wave length
V = f λ
V = 3.0x108 m/s/104
λ = 2.885m
L = ¼ x 2.885m
L = 0.72m
3.4.8 Receiver Tank circuit (oscillator)
Choosing C = 56 and 82ρ F in series
C = 56*8256+82
C = 4592/138
C = 33.29 ρ F
From the formula
L= R2c N2r / {25 (9Rc+10h)}
Where L = inductance in µH (micro Henry)
Rc= Radius of the coil in (mm)
Nr= Number of turns
h= the overall length of the winding coil in (mm)
Rc = 8mm/2 = 4mm
h = 8mm
Nr = 5
L = 42 x 52/ 25 (9x4+10*8)
L = 400/2900
L = 0.138µH
F = 12πLC
F = 12(3.142)*0.138-6*33.29*10122
F = 11.3*10-8
F = 78MHZ
Choosing trimmer (Variable capacitor) of range 4 to 20ρF series with 18ρF and Variable inductor 200µH
So that the maximum and minimum capacitances are
C = 18*2018+20
C = 36038

C = 9.47ρF
And
C = 18*418+4
C = 3620

C =1.8ρF
Two Channel (Band) Graphic Equalizer

Fig3.4 Pre-amplifier and tone control
C2,C3,C4,C5,R1,R2,R3,R4,R5 and R6 constitute the Maxwell tone control circuit. The configuration of this tone circuit is standard as such only the component value will differ for designs done by different designers R1 and R3 are related by
Bass Control Section
6.5 R2 12 ………………………………..3.5
Where R4 and R6 are individually less than R5 practically a good choice is
R2R1=10= ………………………………….3.6
Choosing R1 = 10K then equation 3.6 becomes
R3 = 10 R1 = 10 X 10K = 100K
The ratio R3/R1 gives the maximum gain at the bass control section of the tone control. Hence, the voltage gain is 10(or 20dB).
Treble Control Section
6.5 VR2R6 12 …………………………..3.7(a)
But R5 < R13 .............................…...........3.7(b) and
R5 < R13 …………………………..3.7(c)
If R13= 10
And R5 = 680 then
R4 = 10 R5 = 6.8K
The ratio R13 is the voltage gain of the treble control section of the tone control circuit.
VR1 and VR2 are linear taper 100K potentiometers.
For roll off at 33Hz, the reactance Xc3 of C3 must be equal to R2, which is 100K
…………………………….3.8

C3 = 0.048μf
For break point and around 600Hz-730Hz, the reactance XC2 of C2 must be equal R1 that is 10K.
……………………………3.9
Where f break point frequency = 730Hz

C2= 0.022μf
The bass cut and bass boost via R2 Which provides full bass cut when the slider is moved towards VR1, while full bass boost is obtained when the slider is moved towards R1.
For a roll off at upper frequency of 20 KHz the reactance Xc4 of C4 must be equal to R5 that is:
……………………..3.10

C4 = 0.0111μf 0.01μf
High frequency response starts when the reactance Xc5 of C5 = R4 that is 6.8K
……………………..3.11
Where f = 1 KHz

C5 = 0.023μf
C5 = 0.022μf
Depending on its position R8 will produce treble boost or treble cut. C6 is a feedback capacitor which presents low resistance path to all frequencies within the audio range. The practical value of C6 is 10μf. C7 is about half of C6 hence C7 is approximately 2.2μf C7 provides the interaction of the tone control section and the amplifier hence it is a coupling capacitor.
The small signal amplifier-voltage divider bias will be designed below:
Vcc = 12v
Choosing RE = 1.2K
IE = VE/RE ………………………3.12
But VE = 1/10 Vcc = 1/ 10(12) = 1.2v
IE = 1.2/1.2K = 1mA
If IE IC; Vcc = 6v
……………………..3.13


VB = VBE +VE …………………………3.14
VB = 0.7 + 1.2
VB = 1.9v
Since
R2 10βRE
R2 1/10(120) (1.2K)
R2 = 14.4K
Also
……………………….3.15

1.9R1 + 27.36K = 172.8K

R1 = 76.55K
at f = 16Hz

1μf

DIFFERENTIAL AMPLIFIER


Fig 3.5 The differential amplifier circuit
The two diodes D1 and D2 hold the base of transistor Q3 at 1.4 volts below the positive supply voltage. The emitter of Q3 is thus at 0.7v below the supply voltage.Q3 is a current source.
For low noise, performance is stable. The collector current of Q3 has been chosen to be 2mA. Since Q1 and Q2 are matched pairs with large hfe then IB3 can be neglected and so therefore- IC3 = IE3
Hence,
R4 = VE/IE = 0.7/2X10-3 = 350
However
hfe/hie = gm
Where hfe = current gain of the transistor
hie = input impedance
But gm = 40IC3
gm = 40 X 2 X 10-3
gm = 80mA/v
hie = hfe/gm
hfe for Q3 = 100
hoe for Q3 = 25 X 10-6 or 40K
hie = 100/80m = 1.25K
The open circuit loading of hoe on the transistor Q3 is 25μs the voltage gain:
Av = -gmRL ……………………….3.16
Hence
Av = - (80 X10.3) X 40 X 10.3
Av = -3200
The output resistance Rout is given by
Rout = hoe + R2 (1+ Av)
= 40 X 103 + 47 (1+3200)
= 40 X 103 + 150447000
= 15084700
Rout = 1.50m
Hence the output impedance of the current source is 1.50m
Since Q1 and Q2 are matched pairs, then;
IC1 = IC2 = ½ IC3
IC1 = IC2 = ½(2 X 10.3) = 1mA
Practically the voltage drop VR3 across R3 is not supposed to exceed two diode voltage drops. A good choice is VR3 = 0.73v
Therefore;
R3 = VR3/IC1 = 0.7/1m = 700
Because of standardization R3 = 680
To achieve a balance in the operation of the differential amplifier R5 has to be equal to R3
R5 = Vcc – 2VBE – VEE …………………………..3.17
To ensure sufficient base drive of Q3, IR7 has to be at least 100IB3
But IB3 = IC3/hfe

IR7 = 100 IB3

…………………….3.18

R7 = 34.3K
Resistor, R2 is chosen based on the resistance value that forces the base voltage of Q1 to zero volt. A good value of R2 is 33K.
Considering Q1;

but hfe of Q1 = 100

The input impedance of the differential amplifier is:
………………………..3.19


So,

Where f = lower 3dB frequency = 16Hz

C1 = 1.99μf
It is of practical importance to increase the upper 3dB point of the differential amplifier so as to reduce the rise time. For an upper 3dB frequency at 33 KHz, the rise time is:
Rise time
But rise time is also equal to 10R1C2; therefore:
10R1C2 = 10.6 X 10-6
C2 = 10 x 10-6
10 x 33 x 103

C2 = 30pf
3.4.3 DRIVER AND OUTPUT STAGE

FIG3.7 The Driver and output stage of the power amplifier
The diodes D1, D5 arrangement ensures Q6, Q7, Q8 and Q9 are at threshold point at conduction so as to delimitate cross over distortion. Point V1 is maintained at 1.4v (Mischa. S. 1981)
VR11 = VBE7 = 0.7v
For TR11 = 2mA, then
R11 = 330Ω
R11 =R12= 330Ω
R13 and R14 ensure that Q6, Q7 and Q8, Q9 do not conduct simultaneously. Under signal condition and a good value is 0.33Ω
That is R13 = R14 = 0.33Ω
R13 and R14 also ensure thermal stability due to the heat dissipated by Q8 and Q9 are placed on the heat sink to ensure thermal equilibrium.
R15 is a current limiter and in conjunction with L1, R16 and C4 helps to remove parasitic oscillation. The values of R15, R16, L1 and C4 have been standardized to the following:
R15 = 10Ω
L1 = 10μh
R16 = 4.7Ω
C4 = 0.1μf
Now to have an ac voltage swing of 33.9v peak at the output with 0.33v peak at the differential amplifier, a closed loop gain G at 100 is needed.
The average supply current is:
I supply

Where Vo is the output voltage swing
The average power drain from the supply is:
Psupply = Isupply X 2Vcc
= 1.349 X 35 = 94.406W
The average power delivered to R2 is

Efficiency, η =





CHAPTER FOUR
4.0 TEST AND RESULT
4.1 INTRODUCTION
The circuit design was implemented and tested to ensure compliance with the design specifications. The methodology in carrying out the test and results obtained are also contained in this chapter.

PROCEDURE FOR TEST
In order to measure the input resistance of the circuit, the voltage divider method was used as shown in fig4.1 below. The box represents the circuit under test.
BOX




BOX
Fig4.1 Resistance Box
The value at the resistance box, R was adjusted to obtain a convenient value at V1 for a known value at V1 for a known value at Vs
……………………………4.1
……………………………4.2
For Vin = ½ Vs and Rin = R
The input frequency was 1 KHz
The gain was measured over a frequency range with a fixed input voltage for all the measurements. The frequency response of plot of gain against frequency (in dB) was obtained.
The current was measured by connecting an ammeter in the supply line. The power dissipated was calculated from the product of supply voltage current drawn by a circuit at no load from the supply.
Pd = IsVcc …………………………………..4.3
All measurements except frequency were done at 1KHz structure. All values were ascertained based on the oscilloscope and determined within limits for all circuits.
Quiet a number of amplifier parameters can be measured using a signal generator, an oscilloscope and other equipment. The most important parameter measurements carried out on the amplifiers are:
Voltage gain
Input and output impedance
The output power
Distortion of the output waveform

4.3 VOLTAGE GAIN
To determine the voltage which when applied to the input terminal as an amplifier, distortion of the output waveform

Fig4.2 Circuit for measurement of power amplifier voltage gain
The frequency of the generator was set to a test value of 1KHz the generator output voltage (sinusoid) was increased steadily from zero unto the onset of distortion noticed. The input signal to the amplifier was then reduced a little while the distortion at the scope disappears. The voltage gain (that is the closed loop gain of the amplifier is the ratio of the output voltage to the amplifier input voltage.

Fig4.3 Circuit for measurement of output Impedance of the amplifier
The output impedance of the amplifier was measured using a similar technique, the arrangement being shown in fig4.5
Rout = R – R2
R obtained to be 8.040 Ω
Therefore Rout = 8.040 – R2
However R2 – 8 Ω
Rout = 8.040 – 8 = 0.04 Ω

4.3.1 OUTPUT POWER
The signal generator was set to a test frequency of 1KHz and its output voltage was steadily increased until distortion of the output sine wave displayed on the oscilloscope screen was obtained. The voltage of the signal generator was then reduced slightly until no distortion of the signal was observed. The peak voltage across the dummy load resistance was then measured and found to be 33.9v

4.3.2 TOTAL HARMONIC DISTORTION
To determine the total harmonic distortion of the designed and constructed amplifier a sinusoidal signal at peak value 0.24 at 20Hz was generated using a function generator. This was fed into the power amplifier, the output of which was further applied to a spectrum analyzer. The harmonic components of the fundamental frequency were then observed on the oscilloscope.
In this way the amplitude of each harmonic component relative to the fundamental amplitude was determined. The percentage harmonic distortion was then calculated using the relation of equation 4.4
THD = …………………….4.4
Where V2', V3' etc are the relative amplitude of each harmonic component.

4.3.3 INPUT AND OUTPUT IMPEDANCE
The voltage delivered by the signal generator to the amplifier was set to some convenient nature (less than the value which causes distortion of the output waveform that is 0.4v). This is illustrated in fig4.4

Fig4. Circuit for measurement of input impedance of an amplifier
V = ………………………….4.5
Where Rs = output impedance
Es = emf of the signal generator
Rin = amplifier input impedance to be determined.
A variable resistance R was then connected in series with the input impedance until the input voltage (0.339v) has fallen to one half its original value (0.2v) therefore
……………………………4.6
From 4.5 and 4.6
2(Rs + Rin) = Rs + R + Rin
Hence;
Rin = R – Rs
R3 = 0.6KΩ
R = 20.8KΩ
Therefore Rin = 20.8 – 0.6 = 20.2KΩ
There was a variation between the calculated value and measured value by 0.2; this was due to error in measurement of R.

4.4 DISCUSSION
For a good design, the results obtained are most often very close to the theoretical values, exceptions may be used in certain circuits due to some impractical assumptions. The maximum theoretical efficiency of a power amplifier was gotten to be 76%. The gain of the various circuits agreed with the designed values.

CHAPTER FIVE
CONCLUSION
The design of power has been undertaken in this project and the performance of the individual units that make up the system showed an appreciable level of success in the implementation of the design. It can therefore be asserted that the function of the system as given by the specification has been achieved by the coupled system. The complete circuit diagram of the 100W power amplifier with a pre-amplifier and a mixer console is presented in fig3.22. all sections of the audio amplifier were designed and complemented.

RECOMMENDATION
Improving the power of the amplification which will improve the distance between the receiver and transmitter also the function of the amplifier could be increased from the mono to stereo, muting, wireless microphones and more channels of operations.

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