Thursday, 28 March 2013

PROJECT II



CHAPTER ONE
1.1       INTRODUCTION
The term amplifier refers to any device that increases the amplitude of a signal, usually measured in voltage or current (John Linsley Hood, 1999). This versatile device is used in a variety of different electronic applications. Especially in audio technology, a wide range of amplifiers can be produced based on product specifications (i.e. power, voltage, current). Currently, there are many types of audio amplifiers available for consumers. Sound signal amplification is used for instruments, such as the guitar or the bass. They are also used commonly in home theater systems and with stereo speakers. The basic design behind all of these amplifiers is derived from the simplest concepts of circuit design.
For this project, I shall set out to design an audio amplifier. The input of this circuit is microphone.
Although I will be using a low-power speaker, I needed to achieve approximately three times gain over the entire circuit. In addition, the amplifier had to be produced at a low cost with available materials. Before building the actual amplifier, I realized that I had to design, simulate, and test the circuit. Each step was necessary to understand the concepts involved in amplification.





1.2       AMPLIFIERS
An Amplifier is a device for increasing the power of a signal by use of an external energy source.
An electronic amplifier is used for increasing the power of a signal. It does this by taking energy from a power supply and controlling the output to match the input signal shape but with larger amplitude. In this sense, an amplifier may be considered as modulating the output of the power supply.
The term amplifier is applied to anything from a circuit (or stage) using a single active device to a complete system such as a packaged audio hi-fi amplifier.
By these definitions, it means that there are different types of amplifiers, each type with specific characteristic and function. For example, a radio frequency amplifier is used to improve the radio frequency received.
The focus of this project would be power amplifier which includes current amplification and voltage amplification.
Audio amplifiers are those designed to improve low, weak and poor audio signal from microphone, to a signal large enough to drive the loudspeaker.
However, the goal of using an audio amplifier is to deliver a loud, clear, audible and high quality sound for indoor and outdoor services, such as the lecture theatres, stadia, cinemas etc.


1.3       THE MIXER
The mixer is a controlling circuit that treats the audio signal from the microphone before it is passed into amplifier for amplification. The treatment includes filtering out the audio signal from being amplified others include:
·         Amplifier gain control circuit
·         Treble tone control circuit
·         Bass cut circuit
1.4       THE SCOPE AND LIMITATION OF THE PROJECT
The scope of this project work is to design and construct an audio amplifier with power output of 200W, using 220V/50Hz voltage supply which is the nominal voltage in Nigerian household. The amplifier could be used in lecture theatres like (Education Trust Fund lecture theatre in FUTA), conference rooms, halls, and even in various household. The amplifier will consist of mixing circuit, the tone control circuit, limited to only bass and treble controls otherwise called mixer, with adjustable amplifiers gain.
1.5       THE AIM AND OBJECTIVE OF THE PROJECT
The aim of this project is to develop an audio amplifier capable of delivering 200W audio signal power into an 8 ohms loudspeaker. The amplifier should be as portable as possible. The objective of this project is to design and build a circuit that shall meet this demand.



1.6       THE JUSTIFICATION FOR THE PROJECT
Over the years, the University community had undergone a high population increase because of the incorporation of new faculties and departments which are School of Management, department of Surveying and Geoinformatics, Library Science Technology, Project Management Technology, Transport Management Technology etc.  In order to cater for this, the school management had erected new structures such as 3 in 1, 2 in 1 which would meet up with the increasing needs of students for lecture theatres and halls. Although, some of the halls built have in-built audio amplifier facility, experience had shown that there is a need for an alternative amplication device. This device must be portable,such that if the installed audio amplification facility is faulty, the students can easily pick up the device from the department. This would ensure that students have equal chances of listening to the lecturer.
The amplifier needed should have the following characteristics:
1.      It should be reasonably cheap; this would encourage more departments to buy and hence, an alternative solution to the sound amplification in halls would have been achieved.
2.      It should have an adjustable volume; this is required especially when optional courses are taken because the number of students in the hall is less.
3.      It should be cased with strong Perspex: this would ensure the designs elegant


CHAPTER TWO
LITERATURE REVIEW
2.1       BACKGROUND INFORMATION
In the olden days, voice amplification was achieved without the use of an electronic circuit. The orator or speaker has to increase the loudness and pitch of his voice in order to be heard by a large audience. This could lead to nasal and bronchial pain. To this effect men had invented different kind of systems to measure to enhance human speech, this process continues until electronic amplification was invented.
At the inception of electronics, amplification was achieved through valves for example, thermionic valve required for amplification of weak telephone and telegraph signal, this modes which was invented by Lee De Forest in 1906, after which came diode by Sir John Ambrose Fleming in 1911. With the development of transistors in 1948 by William Shockley, amplification took a relevant position in almost all human endeavours  involving audio signal. The triode vacuum amplifier was used to make the first AM radio (http://www.nobelprize.org/). With an increase in need for portable system, integrated circuit have been invented and largely used in today amplification of audio signal,
An amplifier is an electronic device that has input and output signal.    The   output signal is the relative function of input fed into the amplifier. The output of an    amplifier can be linear or non-linear function of the input. The output characteristic is made done by some component called the amplifying element. The amplifying element could be a Valve, Bipolar Junction Transistor (BJT) or Field Effect Transistor (FET) or even combinations of one or more of the three.

2.2       FURTHER DEVELOPMENTS IN AMPLIFIER DESIGN
For some years following the introduction of solid state amplifiers, their perceived sound did not have the excellent audio quality of the best valve amplifiers led audiophiles. In 1972, Matti Otala demonstrated the origin of a previously unobserved form of distortion in amplifiers: transitory intermodulation distortion (TIM), also called slew rate distortion. TIM distortion was found to occur during very rapid increases in amplifier output voltage (Matti, 1972) TIM did not appear at steady state sine tone measurements, helping to hide it from design engineers prior to 1972. Problems with TIM distortion stem from reduced open loop frequency response of solid state amplifiers. Further works of Otala and other authors found the solution for TIM distortion, including increasing slew rate, decreasing preamp frequency bandwidth, and the insertion of a lag compensation circuit in the input stage of the amplifier. (Lammasniemi et al, 1980).
For the purpose of this project work, more would be discussed on Bipolar Junction Transistor. The transistor action, biasing and configuration would be discussed briefly. Operational amplifier, which is the building block for audio mixers will also be discussed.
2.3       A SIMPLE AMPLIFIER  
An amplifier is a device, especially one using transistor or electron tubes, which produces amplification of an electrical signal.
A simple amplifier is a device that amplifies. The term ‘amplify' basically means to make strong. The strength of an audio signal (in terms of voltage) is referred to as amplitude, from which the word amplify comes out.
For better understanding of how amplifier works, there is need for understanding the two major types of amplification, which are:
1.         The voltage amplification:  This is the increase in voltage strength
2.         The current amplification: This is the increase in current strength.
3.         The Power amplification, which is a product of both voltage and current                                 amplification (P=IV).
An amplifier is fed in by a signal source called the input. An output which is proportional to the input is collected at the output of the amplifier.
Taking a voltage amplifier for example, a small input voltage like a 2V input signal might produce an output of 100V, the proportion of output to input in this case is 50. This proportion is called the amplifier gain. This is applicable to current amplifier and power amplifier as well, which is equal product of the two amplifier gains

http://t3.gstatic.com/images?q=tbn:ANd9GcT2L0wSoD1jpP1P1X8h2Ak3AUcc_76Vb9h4rlNOmFe9BZvFOT58

12V


Fig 2.1: A simple amplifier circuit
Source: (Douglas Self (2006); Audio Power Amplifier Design Hand Book, Fourth Edition)


In designing an amplifier, some parameters are essential, these are:
(a)        Input impedance        
(b)        Output impedance
(c)        Feedback
(d)       Signal inversion
  2.3.1     Input Impedance
Amplifier places a load on the preceding equipment or stage such as preamplifier. This load constitutes the input impedance of the amplifier. The load is that resistance or impedance placed on the output of an amplifier. In the case of a power amplifier, the load is most commonly the loudspeaker. Any load will require that the source (the preceding amplifier) is capable of providing sufficient voltage and current to be able to perform its task. In the case of a loudspeaker, the power amplifier must be capable of providing a voltage and current large enough to cause the loudspeaker cones to vibrate, this vibration is converted into sound.
2.3.2    Output Impedance
The output impedance of an amplifier is a measure of the impedance or resistance "looking" back into the amplifier. It has nothing to do with the loading that may be placed at the output.
2.3.3    Feedback
 Feedback in broad sense is when a certain output is "fed back'" into the input. An amplifier or an element of amplifying device is presented with the input signal, and compares it to a "small-scale replica" of the output. If there is any difference, the amplifier corrects this and ideally ensures that the output is an exact replica of the input but with greater amplitude .This is what feedback does and impresses the overall transfer function of the amplifier
2.3.4    Signal Inversion
 Most voltage amplifiers have its output as inverted form of the input that is, when a positive signal goes in, it comes out as a negative signal but with large amplitude. This does not actually matter for most part but it is convenient to make amplifiers non-inverting.
 2.4       TYPES OF AMPLIFYING DEVICES
There are three major types of amplifying elements or devices and they are:
i      Vacuum tubes (Valves)
ii      Bipolar Junction Transistors (BJT)
 iii     Field Effect Transistors (FET)
  There are some derivatives of the above, such as Insulated Gate Bipolar Junction transistors (IGBT), Metal Oxide Semiconductor Field Effect Transistor (MOSFET). All these devices are reliant on other passive components. The passive devices are resistors, inductors and capacitors. Without these, amplifiers cannot be built.
  All devices that are used for amplification have a variable current output and it is only the way that they are arranged that make voltage amplifier. Valves and FETs are voltage controlled devices. BJT are current controlled, so the input current determines the output current. Yet all these devices can be arranged together to achieve the purpose of an amplifier, either voltage controlled or current controlled devices. By using the same components such as resistor; the current output of any of the amplifying device mentioned can be converted into voltages. There are some parameters that are needed to be considered when choosing components for the construction of an amplifier, such parameters include:

1.         Maximum Voltage
2.         Maximum Current
3.          Maximum Power dissipation
4.          Heater Voltage/Current (valve devices)
5.         Maximum junction temperature (BJT Devices)
6.          Temperature de-rating (BJT & FET)
7.          Thermal Resistance (BJT & FET)
2.4.1    The Vacuum Tubes
 The vacuum tubes are amplifying valves, the amplifying valve has three terminals and they are;
1.         The Anode which collects the electron released from the cathode.
2.         The Cathode which releases electrons when healed directly or indirectly.
3.         The control grid which controls the amount of electron flow from the cathode to the anode
When a positive voltage is applied to the anode with respect to the cathode, an electron stream is emitted from the cathode and flows to the anode, completing the circuit. The grid is a fine coil of wire, placed in between the cathode and the anode. A negative voltage applied to the grid with respect to the cathode will repel some of the electron stream, causing some of the current to be reduced. If the voltage on the grid is varied, the current flowing from cathode to anode is also varied.  And thus, this phenomenon is used for the amplification in the valve. Presently, this method is no longer in use.
2.4.2    Bipolar Junction Transistor
 A bipolar junction transistor (BJT) is a three-terminal nonlinear device composed of two bipolar junctions (collector-base, base-emitter) in close proximity. In normal operation, the voltage between base and emitter terminals is used to control the emitter current. The collector current either equals this (with BC junction in reverse bias), or goes into saturation (the BC junction in forward bias). Used for medium power (700A) and medium speed (10 kHz) application. In power electronics applications, BJT’s are typically operated as switches, in either their fully on or off states, to minimize losses. The base current flowing into the middle of the device controls the on-off state where continuous base current is required to be in the on state. A disadvantage is the low current gain. The base current is generally much smaller than collector and emitter currents, but not negligible as in MOSFET’s. (Ed, 2000) Transistors were made from germanium and silicon which were doped with other materials to give a desired property required for semiconductors.
 The transistor is a three-terminal component. .
1.  The emitter - releases electrons
2.   The base - controlling terminals
3.   The collector - it receives or collects the electrons from the emitter.

2.4.3    Field Effect Transistors (FET)
Field Effect Transistors (FET) and its derivative Metal Oxide Field Effect Transistor (MOSFET) has different types depending on the channel of the transistor and its mode. These are:
1.         N-Channel junction FET
2.         P-Channel junction FET
3.         N-Channel Enhancement mode MOSFET
4.         P-Channel Enhancement mode MOSFET
5.         N-Channel Depletion mode MOSFET
6.         P-Channel Depletion
Field Effect Transistor (FET) has three major terminals and they are.
           Source - The electron source (For N-Channel).
           Gate - Controlling terminal
           Drain - The terminal from which electron is drained.
 The gate is the controlling element; it affects the flow of electrons from the source to drain. Depletion mode is the mode where there is no negative bias signal on the gate, there will be current flow between the drain and the source, while enhancement mode remains off until a threshold voltage is reached, after which the device conducts.


2.5       BIPOLAR JUNCTION TRANSISTORS (BJT) ACTIONS
Transistor has one of its terminals common to the input and the output (in this case emitter). Emitter is made 0 volts with respect to the collector voltage, which provides appropriate biasing. As the base (input) voltage is increased at initial just above 0.6 volts, the electron or holes starts to move from the emitter (depending on whether the transistor is PNP or NPN). As a result of current flowing in the base circuit therefore; a proportion of voltage input is collected at the output. Since the current flowing in the collector is always higher, about 50 or 100 times the base, hence the voltage at (the collector is about 50 or 100 times at the base. Thus, an amplifier is achieved through a transistor.
2.5.1     Biasing Transistors
Before a transistor can be used, an external DC supply must be correctly connected to its terminals. This is necessary to supply power into the circuit and to ensure that the transistor is working with correct DC bias current and voltage. Part of this DC supply is converted to AC signal energy, so the transistor is the device that makes this possible. Transistor is connected such that:
1.        The base-emitter is forward biased
2.         Its base-collector in reverse bias




2.5.2    Configuration of Transistors
There are three major configuration mode of BJT and they are:
1.                  Common-emitter: This is when emitter terminal is common to both input (base) and
the output (collector).
Fig. 2.2: Common-Emitter configuration mode
Source (B.L. THERAJA and A.K. THERAJA; Electrical Technology)


Common-base: When the base is common to both input (emitter) and output (collector).
Fig 2.3: Common-Base configuration mode
Source: (B.L. THERAJA and A.K. THERAJA; Electrical Technology)


Common-collector: This is when collector terminal is common to both input (base) and output (emitter)
Fig 2.4 Common-Collector configuration mode
Source (B.L. THERAJA and A.K. THERAJA; Electrical Technology)
There are three major methods of basing a common emitter transistor and they are:
1. Fixed biasing
2. Collector to base biasing
3. Emitter biasing



2.6 CLASSES OF AMPLIFIERS
2.6.1    Class A
Class A amplifier is the simplest form of power amplifier. The power transistor is used for output of more than a few hundred milli watts. For an amplifier of class A, the transistor must be conducting for the whole cycle. This is the most linear of the classes, meaning the output signal is a truer representation of what was imputed. Here are the characteristics of the class:
1.         The output device (transistor) conducts electricity for the entire cycle of input signal. In other words, they reproduce the entire waveform in its entirety.
2.         These amps run hot, as the transistors in the power amp are on and running at full power all the time.
3.         There is no condition where the transistor(s) is/are turned off. That doesn't mean that the amplifier is never or can never be turned off; it means the transistors doing the work inside the amplifier have a constant flow of electricity through them. This constant signal is called "bias".
4.         Class A is the most inefficient of all power amplifier designs, averaging only around 20.
Because of these factors, Class A amplifiers are very inefficient: for every watt of output power, they usually waste at least 4-5 watts as heat. They are usually very large, heavy and because of the 4-5 watts of heat energy released per watt of output, they run very hot, needing lots of ventilation (not at all ideal for a car, and rarely acceptable in a home).
Fig. 2.5 Class A amplifier
Source: (http://www.electronics-tutorials.ws/amplifier/amp_5.html)
It is biased sufficiently to keep it conducting continuously, both in the positive half cycle and negative half cycle. The amplifier allowed with a quiescent current that is sufficient to hold the output midway between the 0 volts and positive rails for maximum amplitude and minimum amplitude and swing without distortion. This will allow the output to swing without distortion in the both half cycle. This makes the overall power efficiency to be 50%. (Oloyede,1997)
2.6.2 Class B
 The large amount of power wasted about 50% class A amplifier may be eliminated by using (two transistors. One for the positive swing of the signal whiles the other for the negative going swing. They can otherwise be referred to as push-pull amplifier. By this, the Voltage bias is increased, which produce a higher quiescent current. However, the cross over distortion produced in class A can be moved to arise at higher power level, making the signal more natural. So in class B, there will be no cross over or switching distortion of lower power level where hearing is most sensitive. This is mostly used in the CD player. (Oloyede ,1997)


Fig 2.6: Class B amplifier


2.6.3    Class AB
Class AB operation has some of the best advantages of both Class A and Class B built-in. Its main benefits are sound quality comparable to that of Class A and efficiency similar to that of Class B. Most modern amplifier designs employ this topology.
Its main characteristics are:
1.         Many Class AB amplifiers operate in Class A at lower output levels, again giving the best of both class A and class B
2.         The output bias is set so that current flows in a specific output device for more than a half the signal cycle but less than the entire cycle.
3.         There is enough current flowing through each device to keep it operating so they respond instantly to input voltage demands.
4.         In the push-pull output stage, there is some overlap as each output device assists the other during the short transition, or crossover period from the positive to the negative half of the signal.
There are many implementations of the Class AB design. A benefit is that the inherent non-linearity of Class B designs is almost totally eliminated, while avoiding the heat-generating and wasteful inefficiencies of the Class A design. At some output levels, Class AB amps operate in Class A. It is this combination of good efficiency (around 50) with excellent linearity that makes class AB the most popular audio amplifier design.
There are quite a few excellent Class AB amps available. This is the design that is recommended for most general-use applications in home and car (www.hifivision.com). Class AB topology was used during the course of this project.

Fig. 2.7: Class AB Amplifier
2.6.4 Class C
Unlike Class A that conducts when the voltage between the diodes is slightly above 0 that is 0.6 and the supply is below that is -0.6volts. The transistor Ql does not conduct until the signal exceeds a certain level, so that only the peaks of the signal are amplified. This method raises the amplifier efficiency to 80% and above.
Class C is not normally used for audio or musical signal but rather in radio frequencies. At low frequency, the inductor has low impedance. It acts as a low value resistors and signals level developed across is small. As the frequency increases, the inductor is chosen to bring the quiescent signal about halfway between the supply rails.
For an audio amplifier but it can be used for amplitude modulation
Fig 2.8: A typical Class C amplifier
Source: (http://www.baitona.net/forum/baitona53/bait23109/)
2.6.4 Class D
Class D amplifier is quite different from other classes of amplifiers discussed above because it works in a suitable switching mode.
Transistor serves the purpose or off, with rapid transition between one state and the other state. The two transistors Q1 and Q2 are connected in common-emitter mode with the two connected in parallel. The emitter of each of them is connected to the remaining ends of the transistors. The output of this amplifier is taken from another transformer T2 whose primary is connected to the collector of the transistors.
The   arrangement   is   such that   the   transistors   are   driven   to   saturation alternatively producing a square wave signal to the load.
In conclusion, Class AB is chosen for a good design of audio signal amplifiers due to any of the following advantages.
(a) No cross over distortion
(b) No switching distortion
(c) Lower harmonic distortion in the voltage amplifier
(d) Lower harmonic in the current amplifier
(e) No signal dependent distortion from the power supply (1) Constant and low output impedance
(g) Simpler design
However, the negative feedback introduced gives the amplifier advantages mentioned below.
1)  Lower distortion
2)  Higher suppression of ripple and noise from the power supply
3)  Lower output impedance
4)  Better DC stability
5)  Suppression of interference from the loudspeaker
2.7       OPERATIONAL AMPLIFIER
An op amp is an active circuit element designed to perform mathematical operations of addition, subtraction, multiplication, division, differentiation and integration. It consists of a complex arrangement of resistors, capacitors and diodes. An op amp has 8 terminals. Five important terminals are listed below.
Fig. 2.9: A typical OP-amp pin configuration
1-Output 1
2-Inverting input 1
3-Non-inverting input1
4-Vcc
5-Non-inverting input 2
6-Inverting input 2
7-Output 2
8-Vcc +
The op-amp is an electronic-unit that behaves like a voltage-controlled voltage source.  It can also be used in making a voltage or current controlled current source. An op amp can sum signals, amplify a signal, integrate it, or differentiate it. The ability of the operational amplifier to perform these mathematical operations is the reason it is called an operational amplifier. It is also the reason for the widespread use of op amps in analog design. Op amps are popular in practical circuit designs because they are versatile, inexpensive, easy to use, and fun to work with. (Charles et al 2002).
Op-amp can be used as simple amplifier, differential amplifier, as instrumentation amplifier or as a current amplifier. It can be used to sum (as a Mixer), multiply, square or take logarithm of analog circuits oscillators and regulators. The characteristics of an ideal Op-amp are:
1)  The amplifier gain of an ideal Op-amp is infinite.
2) The input impedance of an ideal Op-amp is infinite.
3)  The output impedance of an ideal Op-amp is zero.
4)  An ideal Op-amp has an infinite bandwidth.
Other characteristics of Op-amp apart from those mentioned above include;
1.   Maximum peak-to-peak output.
2.   Input sensitivity.
3.   Noise figure.
4.   Slew rate.
5.   Power rejection ratio.
Fig 2.10: A typical operational amplifier
2.7.1    Types of Op-amp
            There are two main types of op-amp.
a.       FET input or BIFET Op-amp the circuit is basically similar to that of differential but the input uses the field effect transistors to give extremely high input impedance.
b.      Bipolar Op-amp built using bipolar technology. The internal circuit is similar to differential amplifier.

2.7.2    INVERTING AND NON-INVERTING OP-AMP
Another way to classify amplifier is the phase relationship of input signal to the output sign. An inverting amplifier produces an output signal which is 180 degrees out of phase with the input the signal (that is an inversion or mirror image of the input is seen on an oscilloscope). A non-inverting amplifier maintains the phase of the input signal waveforms. An example of the non-inverting amplifier is the emitter-follower. It indicates that the signal at the emitter of the transistor is following (That is matching with unity gain but perhaps an offset) the input signal. This description can apply to a single stage of an amplifier or to a complete amplifier system.
The inverting configuration is sketched in the circuit below; following the circuit is an analysis of the circuit leading to the expression for Vout /Vin, referred to as “Gain.” (Gain means the same as amplification, but is not the same as A of the operational amplifier itself.)
Fig 2.11 A Op-Amp in the inverting configuration.













The standard non-inverting configuration is sketched below:
Fig. 2.12 A Op-Amp in the non-inverting configuration
2.7.3 Applications of Op-Amp
1)   Summing
2)  Subtraction
3)  Integration
4)   Differentiation
2.8         LOUDSPEAKERS
A loudspeaker (or "speaker") is an electro acoustic transducer that produces sound in response to an electrical audio signal input. A speaker is a part of a sound system that has the fewest specifications, but the greatest effect on quality. Most listeners cannot hear the difference with a change of amplifier but can immediately sense a switch in speakers.
      

2.8.1     Parts of a Loudspeaker
    Cone with suspension
    The voice coil
    The magnet

2.9       AUDIO MIXERS
Audio mixers are circuits for combining two or more inputs, and are used for   applications in the case of this project and in other cases like in tape recording, guitar amplifiers etc.
2.9.1 AUDIO POWER AMPLIFIER
The audio power amplifier is an amplifier designed for the audio range of frequencies that has a relatively low voltage gain but a very high current gain and hence large power gain. The power amplifiers are designed to drive loudspeakers of few ohms say 4Ω and 8Ω. The term power amplifier is often used to describe a type of amplifier which is capable of delivering an appreciable power into a load which is invariably of low impedance.

Fig. 2.13: A simple 200W audio amplifier
Source: (www.elektropage.com)

2.1.0    MICROPHONE AMPLIFICATION
A Small signal amplifier is required to raise the microphone input from a microphone. Three different biasing techniques are possible in designing an amplifier. These are:
           Fixed bias
           Self (shunt) bias
           Potential divider
Fixed bias suffers from thermal runaway which is improved in the next stage.

 Fig 2.14: Potential divider biasing.
2.11             VOLUME CONTROL OF A TYPICAL AMPLIFIER
The Volume control circuitry of an audio amplifier system is normally located between the output of the preamplifier stage and the input of the tone-control circuit. It is usually a potentiometer within the circuit. However, the rapid rotation of the potentiometer knob applies DC voltage to the next circuit for brief intervals. That voltage could upset circuit bias and cause severe signal distortion. The block diagram in Figure 3.4 shows the ideal topology and location for a volume control. It is fully DC-isolated from the output of the preamplifier by capacitor C1, and from the input of the tone-control circuit by C2. As a result, variation of the wiper of control potentiometer R1 has no effect on the DC bias levels of either circuit. Potentiometer R1 should have a logarithmic taper, that is, its output should be logarithmic function rather than linear.
The stage provides the individual control to adjust the amplitude level. There are two different methods used to control the signal amplitude using attenuator and these are:
a.       Linear volume control
b.      Log volume control
The linear volume control produces output signal that is a direct function of the input signal and the transfer function is expressed as;
                                                 VO =K Vin                                                                         (2.1)
Where,                                        k≤ 1
The logarithm volume control is not a linear function and the output is expressed as;
                                               Vout =log k Vin                                                                  (2.2)
2.12         BUFFER STAGE
An emitter follower circuit is a buffer which provides impedance matching. Since the volume control impedance varies, a buffer is introduced to limit the effect on the mixer stage.
                                       
                                                Fig 2.15: Emitter Follower (Buffer)
The circuit shown in Figure 3.3 is an emitter follower and the limitation of the circuit above is that the input impedance is reduced by the parallel combination of R1 and R2. To increase this, a modified circuit using bootstrapping can be used as shown in Figure 2.16
                                      
                                               Fig 2.16: Improved Buffer Circuit
However, the voltage gain of this stage is less than 1.
2.13   MIXER STAGE
This project sets out to provide power amplification from different channels. At this stage, the four channels are combined together using a mixer circuit which is simply a summer amplifier.
                        
                            Fig 2.17:  A typical Mixer Circuit
            Vout = - ( )                                                 (2.3)
 RF = R1= R2 = R3 =R4
           Vout = - (V1+V2+V3+V4)                                                                                  (2.4)
The transistor variation of the above circuit is shown below
                                   Fig 2.18: Mixer Circuit
                         VO =   -R3/ R4 [V1 +V2+V3+V4]                                                         (2.5)
R3 =R4,
                                                  VO =-[V1 + V2 +V3 +V4]                                           (2.6)
2.14           TONE CONTROL
This stage fine tunes the input signal using the action of the bass and treble. The bass filters out low frequencies while the treble filters high frequencies.
                              
                                               Fig. 2.19:  A typical Tone Control Circuit


2.15            MASTER VOLUME CONTROL
This design is similar to the level control provided for each channels. The configuration is shown below
                               
                                               Fig 2.20: Master Volume Control
                                               Vout = Klog Vin                                                              (2.7)
2.16      POWER AMPLIFICATION
The audio power amplifier schematic includes the three stages which are;
1.                                     Transconductance stage (Differential input )
2.                  Transimpedance stage (Voltage Amplification)
3.                  Power stage (Output stage).
           
Fig 2.21: Block Diagram of Power Amplification

 2.17   INPUT STAGE
There are two options that are possible in this stage and they include using a single transistor or differential input. A single transistor introduces distortion because of the non- linearity problem while a differential pair could also settled for because it eliminates distortion.
                                         
                            Figure 2.22: Circuit Diagram of an Input Differential Pair
However, the problem with the circuit in Figure 2.23 is the difficulty of balancing the transistor pair and this problem generates 2nd harmonics distortion. An approach at balancing the current flowing through the pair becomes necessary to overcome the distortions.
                                                                                                           (2.8)
where re is the intrinsic resistor and is given by
                                               r =                                                                        (2.9)
and
                                               R=R1//RL                                                                      (2.10)
                                                                                                                      (2.11)
The amplifier acts as a transconductance at low frequencies.
2.17.1 Elimination of 2nd harmonics distortion
Since the balance pair is required to eliminate the second harmonic distortion, it is necessary to introduce a constant current source to the circuit. 
                                                     
                           Fig 2.23.  Constant current source. Vb is kept constant by D1
                                               I =                                                                   (2.12)
The constant current source ensures that the current flowing through the pair is constant and is not affected by the power supply. The improved input configuration becomes the diagram shown in Figure 2.24
                                             
                                               Fig 2.24: Constant current source
However, the above circuit only ensures constant current but could not ensure balancing of current flowing through the transistors. Hence, the second harmonic distortion is not eliminated .To ensure this, a current mirror was introduced.
                                               
                                               Fig 2.25: Current Mirror 
                                               I2 =                                                            (2.13)
Since β >>1
                                               I1 =I2                                                                                       (2.14)
The introduction of the current mirror also doubles the transconductance of the input stage.
2.17.2   Reducing Linearity.
Linearity is a major source of distortions. Linearity can be corrected by introducing the degeneration resistors. Emitter follower degeneration can be added to the input stage to improve the slew rate and reduce high frequency distortion. However, since resistors are sources of noise, the introduced noise could be controlled by reducing the biasing current. The improved input stage is shown in Figure 2.26
                
                                               Fig 2.26: Improved Input Stage
Degeneration resistor r1 (R1) and r2 (R2) are introduced and the gain thus becomes;
                                               gis =                                                           (2.15)
since
                                               r1 =r2=r
                                               gis =                                                                   (2.16)
The current mirror degeneration resistor
                                               Re1 =                                                                    (2.17)
2.17.3    Negative Feedback
The NFB ensures stability of the power amplifier. According to (Douglas et.al, 2008) 20% Global feedback is sufficient. The feedback is obtained from the resistors connected as a potential divider shown in Figure 2.27 below.
Fig 2.27: Potential divider
VNFB =                                                                             (2.18)
However, the price to pay for stability is reduction in the gain of this stage by 20%
gis =                                                                (2.19)
2.18   VOLTAGE AMPLIFICATION STAGE (VAS)
This stage handles voltage amplification. The circuit consists is made from a class A amplifier with a LFB (local feedback). The feedback capacitor called miller capacitor provides a single pole creating frequency roll – off at high frequency.
Figure 2.28 shows the VAS stage
Fig. 2.28: Voltage Amplification stage
The power stage contributes collector impedance of about 40kΩ voltage. The voltage gain thus becomes
gVAS =                                                                              (2.20)
at 10mA,
re = 2.5Ω                                                                                  (2.21)
gVAS =  = 20000                                                    (2.22)
To increase the current capacity of VAS, a Darlington pair was introduced and is used to eliminate the loading of this stage by the output power stage.
                                          
                                Fig 2.29:  VAS with elimination of loading effect
The current source serves the VAS stage.
                                            
                                               Fig. 2.30: Constant current source
Q6 and Q7 formed the feedback current source to provide a constant current for the VAS stage.Q6 forces one VBE (620mV) across R2 base of Q6
VBQ6  =                                                    (2.23)
Voltage across R2 is VBE therefore, R2 is
R2 =                                                                       (2.24)
 And                           
 R1 =                                                                        (2.25)
Complete circuit of the VAS stage is shown below.
                            
                Fig. 2.31: Improved VAS
Q4 formed the VAS while Q5 formed the VBE multiplier (biasing voltage).
Emitter degeneration introduced to Q4 at 10mA current flowing through Q4 re which is given by
re =  =2.5
The input impedance of VAS is β (RE +re).
the gain of the input stage is
g =                                                                  (2.26)
where, 
R = RC // β (RE +re)                                           (2.27)
The low frequency gain of VAS is
gVAS =                                        (2.28)
The VAS collector load impedance is dominated by the loading of the output stage since the output impedance of the current source is quite high.
2.19     THE OUTPUT STAGE
The output stage is a simple circuit made up of three complementary pair power transistors. The number is supposed to reduce the heat dissipation.











CHAPTER THREE
MATERIALS AND METHODOLOGY
3.1            INTRODUCTION
     The amplifier circuit can be broadly divided into three stages which include:
  • Power Supply Stage
  • Pre-amplification Stage
  • Power Amplification Stage
Power supply is an electronic module that coverts power from some source (usually AC) to another form (dc in this project) which is needed by the equipments to which power is being supplied.
3.2              POWER SUPPLY STAGE
The power supply takes the large AC signals (usually between 220-230V) and reduces it to the DC signal required to operate the circuit in this project.
The first step is to pass the signal through the transformer with a specific ratio. The secondary of the transformer outputs an AC signal with 35V peak waves. The signal is then fully rectified by a rectifier. For this project, a centre-tapped transformer was used.
Centre tap transformers are useful when making symmetrical positive and negative power supplies. (Since the centre-tap can be tied on ground)Centre tap transformers are rugged compared to other types of transformers.
The waves generated were smoothened by placing a large capacitor (6800µF, 63V) between the positive/negative outputs and ground. The capacitors charge during the output peaks and discharge when the waves are low. This smoothed the signal into almost DC with very little ripples.
Fig 3.1 Picture showing the project
The major problem envisaged during circuit connections was short circuit during the course of soldering. The transformer was first connected. This can be achieved by connecting large capacitors from the positive and negative nodes to ground at multiple sites of the circuitry.
The large capacitors acts as a short to AC signals and so many distortions propagating in the supply lines are grounded through them.
3.3            THE PRE-AMPLIFICATION AND TONE CONTROL STAGE
The function of a pre-amplifier is to amplify the input signal in the microphone enough to be finally amplified by the power amplifier stage. This primarily provides stability in the operation of the circuit. The microphone converts speech or input signal into electrical current of few milliamperes. When the signal reaches the pre-amp stage, the treble and bass controls are used and help to regulate the frequency of the current to the desired value before it is then “pre-amplified”. The volume control which is just before the power amplifier circuit reduces the Amplitude of the input signal into the power amplifier circuit as desired. The design of a pre-amplifier must consider all potential signal degradation from sources of noise.







The pre-amp circuit is shown below:
Fig 3.2 Circuit Diagram Showing the tone Control Stage
Source: (www.lankatronic.blogspot.com/2010/06/amp-circuit.html?m=1)







Table 3.1 Showing the components used
P1,P2
10K Linear Potentiometer
R1
100K ¼ Resistor
R2,R6
18K ¼ Resistor
R3,3K3
¼ Resistors
R4,R5
1K8 ¼ W Resistor
R7
560R ¼ W Resistor
C1
1µF 63V Polyester Capacitor
C2
473nF 63V Polyester Capacitor
C4
1µ5 63V Polyester Capacitor
C4,C7
100nF 63V Polyester Capacitor
C5,C8
22µF 25V Electrolytic Capacitor
C6,C9
2200µF 25V Electrolytic Capacitor
IC1
T1P32 Dual Bi-FET OpAmp
IC2
C1061 15V 100mA Positive Regulator IC
IC3
D718 15V 100mA Negative Regulator IC
D1,D2
IN4002 200V 1A Diodes
J1,J2
RCA Audio Input Sockets
J3
Mini DC Power Socket

Due to unsatisfactory performance and availability of some components, a few things were modified and adjusted at some stages. The wires shown above were used to connect the pre-amplifier to the main amplifier circuit and the power supply respectively.
3.4 THE POWER AMPLIFIER STAGE
An audio power amplifier can be defined as a circuit that is able to deliver audio power into an external load without generating significant distortion, generating excessive heat or consuming quiescent current. A class AB amplifier was chosen because of its sound quality it derived from Class A and efficiency from Class B.
In the design of the circuit for the power amplification stage, a three stage amplifier structure was used. (Self, 2006) suggested that a three-stage amplifier which is the vast majority of audio amplifiers use has the following advantages:
1.                  It is easy to arrange things so that interaction between stages is negligible.
2.                  The compensation capacitor reduces the second stage output impedance, so that the non-linear loading on it due to the input impedance of the third stage generates less distortion than might be expected.
3.                  Compensating a three-stage amplifier is relatively simple and constitutes over 95% of the solid state amplifiers built.
4.                  The three-stage architecture always has a unity-gain output stage.
5.                  There is very little signal voltage at the input to the second stage, due to its current-input (virtual-earth) nature, and therefore very little on the first stage output; this minimizes Miller phase shift and possible Early effect in the input devices.
There are three stages, the first being a transconductance stage (differential voltage in, current out), the second a transimpedance stage (current in, voltage out), and the third a unity-voltage gain output stage. The second stage clearly has to provide all the voltage gain and I have therefore called it the voltage-amplifier stage or VAS.
Fig 3.3 The three-stage amplifier structure. There is a transconductance stage, a transadmittance stage (the VAS), and a unity-gain buffer output stage
Source: (Douglas Self (2006); Audio Power Amplifier Design Hand Book, Fourth Edition)
A power amplifier operates over a wide range than the pre-amplifier. This necessitates the use of a circuitry that reduces distortion to an insignificant level in the power amplifier. For this project, the incorporated power amplifier is made up of three stages namely:
  • The Differential amplifier stage (Trans Conductance Stage)
  • The Voltage Amplification Stage (VAS) or (Trans Impedance Stage)
  • The Power output Stage.
The differential Amplifier is a type of electronic amplifier that amplifies the difference between two voltages but does not amplify that particular voltage (www.wikipedia.org/differentialamplifier). The differential amplifier used for the power amplification stage consists of two PNP transistors Q1 and Q2 (2N5401). Some of the characteristics of this transistor include:
Maximum collector power dissipation (Pc): 60mW
Maximum collector-base voltage (Ucb): 15V
Maximum collector-emitter voltage (Uce): 12V
Maximum emitter-base voltage (Ueb): 2V
Maximum collector current (Ic max): 50mA
Maximum junction temperature (Tj): 85°C
Transition frequency (ft): 80MHz
Application: VHF, Low Power  (www.alltransistors.com)
The driver amplifier is essentially a current booster capable of raising the signal levels from the voltage amplifier stage to a value adequate to drive the power output stage. The driver stage in this amplifier consists of an arrangement of NPN transistors.
The power output stage is used to drive the loudspeaker loads. It is required to multiply the amplified output current of the first stage with amplified voltage current of the second stage. The output of the power stage is connected to an 8ῼ loudspeaker.
Fig 3.4 Circuit Diagram Showing the Schematics diagram of JRC 4458









Fig 3.5 Showing the complete circuit diagram of the project
Source: (www.elektropage.com)





3.5       TESTING AND OBSERVATION
After the connection of the pre-amp stage to the power amplifier and the heat sink, the audio jack was connected with the pre-amplifier. The audio jack has two ports with only one port active. The 9V battery was also connected to the wireless transmitter. It was observed that the amplifier was making some noise. When I checked books on amplifiers, I discovered that the amplifier was sensitive to minor sounds from the environment which may not hear. When the transmitter was connected to the microphone, the noise stopped.
The microphone was tested at different ranges and a high quality sound was produced when the treble was turned high and the bass was turned low. However, I discovered that on moving too close to the loudspeaker, It produces high whistling or howling noise that is it picks not only the speaker voice but also the amplified noise. This phenomenon is known as feedback. In order to avoid this phenomenon, it is advisable that the one doesn’t move too close to the speaker while using the microphone.
Fig. 3.6 The complete circuit placed inside Perspex
3.6       CASING CONSTRUCTION
After the circuit was tested, the next thing was to decide on which kinds of casing to use.
            3.6.1    Common Types of Amplifier Casing
                        1.         Metallic casing
                        2.         Perspex casing
                        3.         Wooden casing
The consideration for casing was based on the physical design recommended by my supervisor, its elegance, and durability, thickness, low maintenance easy to clean, sheet sizes, clear, tinted or coloureds. It is ideal for architectural decorations, advertisement signboard, and art illumination equipment, materials of door, window, lampshade and corrugated roof, mechanical cover, scale board for electric appliance, insulation material, acoustic material, optical instrument and electronics project cover. With the above properties considered, the best was to use Perspex. Although, it has disadvantages such as being expensive and not readily available in Akure.

Bass                             Treble                          Mixer                                       Volume knob
           
Fig. 3.7 Project after encasing with Perspex


3.6.2    SCREWING THE BOARD TO THE PERSPEX
On completion of the casing, the Vero board was drilled and holes were also made for the legs of transformer and heat sink. The circuit board was screwed to the Perspex. An improvised washer (small wood that was cut into a reasonable size) was used to fasten the head cover of this project.
3.6.3 CASING PARTS
The following can be viewed from the casing
            1. Toggle switch (On and off)
            2. Volume control knob
            3. Treble control and knob
            4. Bass control knob
            5. Microphone jack







CHAPTER FOUR
RESULTS
4.1       RESULTS
After the completion of the project, the quality of the sound produced was tested and I observed that the output sound was pleasant over the range of the volume, treble and bass knobs.
When the signal generator was set to 1 KHz, the peak-to-peak voltage was adjusted until the distortion is set in. At the maximum peak-to-peak input without a distortion at output is 150mVp-p and the output signal voltage is 22V
Power = V2 / RL
Power = 222/ 8
Power  = 60.5
Supply voltage Requirement for the Amplifier
For a power amplifier of an output average of about PAV = 200W to a load RL of 8 Ω
PAV = Vp2 /2RL
Vp2 = 2PAV RL
Vp = √2RLPAV
Vp = √2 * 8 * 200
Vp = √3200
Vp = 56.57V
Where Vp is known as the peak voltage
And Vp is given as Vrms/√2
(Vp) 2 = (Vrms) 2/ 2
P = Vp /2R
= 572/ (2 * 8)
= 203.06W
4.1.1    Output stage
The output stage of the amplifier must be able to supply the base current of the driver stage. For an output power of 200 Watts, the minimum power capability of each amplifier is:
P = 200/4
= 50 Watts
For 50W to be delivered into an 8 Ω load, the peak voltage and peak current across the load are given by the expression:
Vp = √2PR * R
Ip = √2PR/R
Where PR = Power Received and R = Resistance
Thus,
Vp = √2*50*8 
Vp  = 28.28V
CHAPTER FIVE
CONCLUSION AND RECOMMENDATION
5.1       CONCLUSION
The aim of this project is to design an audio watt amplifier capable of delivering 200W into an 8 ohm load (Loudspeaker) from a supply voltage of about 220V/50Hz and this has been accomplished. What makes this task very unique is that it was an attempt to implement a standard class AB amplifier circuit at a very minimal cost which was eventually achieved with less than #15,000. The need for a very durable casing also contributed to the eventual cost of the project (about #20,000 after casing with Perspex). This is considerably cheap compared to buying ready-made amplifiers in the market. Also, as an Electronics Engineering student in the final year, it gave me an opportunity to explore my imagination to achieve something comparable to market standard. I also had the opportunity to ask questions from technicians, specialist in amplifier casing and my supervisor which gave me beautiful suggestions concerning the design. It is very interesting to know that by understanding what transistors and amplifiers can do, an Electronics Engineer can do at length in designing a circuit of his wish.






5.2       RECOMMENDATION
The following recommendations could be made from the experiences and challenges I faced during the course of this project work.
1.      Students of Electrical and Electronics Engineering should be taken through practical classes especially EEE322 (Electronics Engineering II practical). This will ensure that students spend more time on electronic circuit analysis.
2.      Before the end of Industrial training, the department should forward at least four different topics to the e-mails of incoming final year student. This would make the completion of project easier and faster.
3.      The maximum time duration for a lecture is three hours for a three unit course. This amplifier should not be used beyond three hours at a stretch so as to ensure that it works without troubleshooting for as long as required.








REFERENCES
Adefolalu Adefolaju John (2001); Design and construction of 150Watt stereo Amplifier.
Anjorin Abiodun O (1999); Design and construction of Public Address System (PAS) with two microphone inputs.
Benjamin/Cummings, Second Edition(2009); Electronics principles and Application
Ben Ruppel (2008); Final Project Report on Audio Amplifier submitted to Proffessor Korman of George Washington University; School of Engineering and applied Science, Department of Electronics and Computer Engineering.
B.L. THERAJA and A.K. THERAJA (2008); Electrical Technology)
Dave Cutcher (2005); Electronic Circuits for the Evil Genius, McGraw Hill publishing company.
Douglas Self (2006); Audio Power Amplifier Design Hand Book, Fourth Edition.
Ed. Philip. A. Laplante (2000); Book/Definitions” Electrical Engineering Dictionary Boca Raton CRC Press LLC.
Savant C. J., Roden S. Martin and Carpenter Gorden,"Electronic Design Circuits and System
Matthew N.O Sadiku and Charles. K Alexandra (2002); Fundamental of Electric Circuits Second Edition.
Matti Otala (2002); Circuit Design Modifications for Minimizing Transient Intermodulation
Mark Horenstein (1999); Introduction to Microelectronics Circuit and Devices

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