Abstract
Amplifiers,
which are devices that increase the gain of an audio signal, dominate modern
audio technologies. In this project, I will design and build an audio amplifier
from scratch in order to demonstrate that such a key device can be constructed
using basic electrical and electronics engineering principles.
After
performing major circuit calculations by hand, I modelled the circuit in
multisim and proteus, which are computer software that analyzes electrical
circuits. In particular, I studied the variance that using budget electrical
components introduced into the circuit.
1.0 INTRODUCTION
The
term amplifier refers to any device that increases the amplitude of a signal, usually
measured in voltage or current. 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.
2.0
Literature Review
Before
beginning the design process, it was necessary to understand several core
concepts of Electrical and Electronics engineering. When designing electronics,
three main specifications govern all circuit components; [voltage V; measured
in volts (V)], [current I; measured in amps (A)], and [resistance R; measured
in ohms (ῼ)]. These three concepts are connected by Ohm’s Law, Where V = I * R
For
amplifier circuits, it is also important to consider both types of current in
the design because both alternating and direct current run through the system.
Alternating current (AC) acts like a sinusoidal curve, providing the signal for
the amplifier. On the other hand, direct current (DC) runs through the circuit
as a voltage source. Used together, AC source creates the signal at the
horizontal axis, which is determined by the value of the DC source. Each is
analyzed independently of the other, but without one type of current, it is
meaningless to include the other.
2.1
Circuit Components
Some
basic components in amplification are resistors, capacitors, and transistors.
Resistors produce a voltage based on the amount of current passing through the
circuit. Capacitors consist of two metal plates separated by a weak conducting
material. At DC, these devices temporarily store the charge. However, at AC,
the frequency is high enough to complete the circuit. At this point, the
capacitors act like wires. The main advantage of these devices is the ability
to block the direct current while allowing the AC signal to flow through
Transistors are the most important part of amplifier circuits. Capable of
controlling an output signal in comparison to an input signal, a transistor can
produce gain. In other words, the transistor is responsible for the
amplification component of the audio amplifier. Although there are several
types of transistors, but the transistor used in this project is the JRC4558 is
a high performance monolithic dual operational amplifier. These devices consist
of three terminals: the base, the collector, and the emitter. Simply put, they
are terms used for labelling measurements, calculations, and schematic diagrams
2.2 Classes of Amplifiers
Assembled
in different configurations, resistors, capacitors, and transistors can create
several classes of amplifiers that can be distinguished by performance
characteristics. For this project, three economical designs were the most
essential.
Class
A amps are very linear (meaning the integrity of the signal is maintained
through the amplification process); however, this amplifier topology is known
to be very inefficient. In addition, Class A amplifiers invert the signal
(meaning the function is reflected over its axis).
Class
B amps are much less linear, leading to higher distortion of the signal, but
they are much more efficient. Since a Class B amp only amplifies half of a signal,
two Class B amps are generally used in synchronization.
The
compromise between these two topologies is the Class AB amplifier. The Class AB
is more efficient than the Class A with lower distortion than the Class B.
Often the different types of amplifiers are used in combination with other
amplifiers into order to achieve the specifications of a particular design.
2.3 Negative Feedback
Another
popular method of controlling amplifier distortion is negative feedback. A
portion of the amplifier’s output is transferred back to the input. Overall,
this method controls the gain of the amplifier even when affected by outside
factors (i.e. temperature). In addition, the recycled output signal reduces
amplifier distortion.
In
order to measure the success of an amplifier, designers use many tests for
circuit variables. One manner of representing the performance data is through a
Bode plot. A logarithmic frequency scale spans the x-axis (measured in Hertz).
The y-axis measured gain in decibels, which is also a logarithmic measurement.
Combined, the two axes present the output gain of an amplifier over a wide
range of frequencies.
After
a certain point, the gain reaches a maximum level. At even higher frequencies,
gain becomes inversely related to frequency as the performance drops off.
This
continues until the point that the gain drops with an increase in frequency.
This
point is known as the 3dB point. For optimal performance, the 3dB point of an
amplifier should fall beyond the amp’s active range of frequencies.
Audio
signals in the modern music industry are now broadcast almost exclusively in
stereo sound.
The
final output of the amplifier was fed through a low power audio speaker,
completing the circuit. Actual values for circuit components are dependent on
the DC based calculations for our 24V source.
2.4 Features
of JRC4458
No
frequency compensation required
No
latch – up : Latch-up is a failure mechanism of CMOS integrated circuits characterized
by excessive current drain coupled with functional failure, parametric failure
and/or device destruction. It may be a temporary condition that terminates upon
removal of the exciting stimulus, a catastrophic condition that requires the
shutdown of the system to clear or a fatal condition that requires replacement
of damaged parts. Regardless of the severity of the condition, latch-up is an undesirable
but controllable phenomenon. In many cases, latch-up is avoidable
Large
common mode and differential voltage range
Parameter
tracking over temperature range
Gain
and phase match between amplifiers
Internally
frequency compensated
Low noise input transistors
Pin
to pin compatible with MC1458/LM358
3.0 AMPLIFIER
An
electronic amplifier is a device for increasing the power of a signal. It does
this by taking the energy from the power supply and controlling the output to
match the input signal shape thereby, increasing its amplitude. In the sense,
an amplifier may be considered as modulating the output of the power supply.
Generally,
amplifiers can be defined as electronic equipment that increase the strength
passing through it or in a better sense is a component or devices intended to
sense a signal and produce a large version of that signal. In general, any
amplifying device is limited by available voltage, current, power, frequency
response and device maximum for voltage, current and power dissipation.
By these definitions, it means that
there are different types of amplifiers, each type with specific
characteristics and function. For example, a radio frequency amplifier is used
to improve the radio frequency received.
This project would focus on 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 signal large
enough to drive a loudspeaker.
However, the goal of using audio
amplifier is to deliver a loud, clear, audible and high quality sound for
indoor and outdoor services, such as the cinemas, stadia, lecture theatre etc.
The
amplifier could be briefly summarized into three stages:
1. The
input stage which is the microphone level
2. The
amplifying stage, the amplifier level.
3. The
output stage which is the loudspeaker level.
The
input stage involves the input of an audio signal from the voice through the
microphone, which serves as the input transducer.
The
small and weak signal is delivered to the pre-amplifier, where it is improved
and could be controlled appropriately by the tone circuit. The power amplifier
amplifies the signal and sends it to the speaker which converts it back to
sound wave. The result is a loud and clear sound which is higher in amplitude
compared to the input signal.
The loudspeakers serves
as output transducer that produces loud, clear and intelligible sound from the
electrical signal delivered to it by the amplifier.
4.0 METHODOLOGY
4.1
Design Requirements
From the very beginning of the design process, the
design specifications were crucial to the choices for topologies and
components. The amplifier had to be able to amplify a signal from a portable
music player (a 0.8V – 1.1V supply voltage load).In order to reach satisfactory
amplitude, 1.7 times gain was necessary for each section of the input stage.
Class A designs are capable of this gain, and their high inefficiency was not a
major factor in the small scale of this experiment. However, these amplifiers
invert the input signal. The DC voltage remains the same, but the AC signal
reflects over its x-axis. As a result, two Class A amps were used to correctly
orient the output signal and provide the necessary gain. Each Class A amplifier
was a common emitter BJT. Together these two amplifiers constitute the input
stage of the audio amplifier. Considering a starting voltage of around1VPP
(1 volt peak to peak) , two amplifiers with approximately 1.7 times gain
brought the output gain up to around 3 times after the input stage. As the
current flows into the output stage, the voltage becomes irrelevant.
Instead, the designer needs to increase the power
gain in order to drive the speakers at the output. Class AB amplifiers are
capable of producing power gain (at the slight expense of the previous voltage
gain). A Class AB amplifier consists of two PNP common emitters and two NPN
common emitters in a loop.
4.2 List
of Components
·
Vero board
·
0.47ῼ choke
resistor
·
100ῼ resistor
·
47Kῼ resistor
·
4.7Kῼ resistor
·
10Kῼ resistor
·
1Kῼ resistor
·
680ῼ resistor
·
D718
transistor
·
B688
transistor
·
C1061
transistor
·
T1P32
transistor
·
JRC 4558IC
·
1µF, 50V
capacitor
·
47µF, 50V
capacitor
·
24V step-down
transformer
·
Power
capacitor, 6800µF, 63V
·
Diode
·
Heat Sink
·
Connecting
wires
5.0 Conclusion
In order to design an amplifier with a quality sound (which would have
minimal disturbances), the best way is to give proper consideration to the
components. Low noise input transistors must be used, no
frequency compensation should be required, there should be No latch – up, the
transistors to be used should have large common mode and differential voltage
range. Parameter tracking over temperature range Gain and phase match between
amplifiers and the transistor should be internally frequency compensated
Furthermore, power stage of an audio amplifier plays an important
role in determining the quality and performance of each stages of amplification
especially the output (loudness) of the audio amplifier.
REFERENCES
Understanding Latch-up
in transistors
Theraja B.S and Theraja A.K (2005); A textbook on Electrical
Technology, S.Chand and Company Limited.
Singmin, Davis,
Patronis, Watkinson,Self, Brice, Duncan, Hood, Sinclair.
Audio Engineering –
Know It All. 2009,MA
Douglas Self. Audio
Power Amplifier Design Handbook Third Edition. 2002,MA.
John Linsley Hood.
Audio Electronics.1999, MA.
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