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
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
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
