CHAPTER ONE
INTRODUCTION
1.1 GENERAL INTRODUCTION
Amplifier
is an electronic device that increase the power of the signal, the
amplification of weak signal into stronger signal it
is
of fundamental importance
in
almost any electronic system. Sometimes this is obvious, as in a
radio receiver,
where the objective is to select and amplify the weak signal from the
antenna and to present the detected signal via loudspeaker. An
amplifier can either be a piece of equipment or an electrical circuit
contain with another device. Amplification is fundamental to modern
electronics and they are widely used in almost all electronic
equipment.
According
to the history, the first practical device that could amplify was the
triode
vacuum
tube,
which
invented in 1906 by lee de forest which lead the first amplifier
around the world in 1912. vacuum tubes were used in almost all
amplifiers until the 1960s-1970s when transistors is introduced at
1947 and vacuum was replaced by the transistor. Nowadays almost all
of amplifiers was used by transistor, but vacuum tubes continue to be
use in some application, the
development of audio communication in form of telephone was first
invented in 1876, created the need to increase the amplitude of
electrical signals to extend the transmission of signals to
a
long distances. The shreeve mechanical repeater and the vacuum tube
were the only amplifying devices, other than specialized power
devices such as the magnetic
amplifier and
amplidyne,
for 40 years. Power control circuitry used magnetic amplifiers until
the latter half of the twentieth century when power semiconductor
devices became more economical with higher operating speeds. Shreeve
repeaters were used as adjustable amplifiers in telephone subscriber
sets for the hearing impaired until the transistor provided smaller
and higher quality amplifiers in the 1950s. The replacement of bulky
electron tubes with transistors during the 1960s and 1970s created
another revolution in electronics, making possible a large class of
portable electronic devices, such as the transistor
radio developed
in 1954. Today, use of vacuum tubes is limited for some high power
applications, such as radio transmitters.
The
terms amplifier
and amplification,
derived from the Latin amplificare, (to enlarge or expand),
were first used for this new capability around 1915 when triodes
became widespread.
After
many centuries it was found that negative resistance mercy lamp could
amplify
and were also tired in repeater, the actual development
of thermionic valves starting around ca.1902, which provided an
entirely electronic method of amplifying signals. The first practical
version of such devices was the audio triode, invented in 1906 by lee
De forest which lead to the first amplifier around 1912. in analogy
to previous types of relays in telegraphy and telephony, the
amplifying vacuum tube was first called an electron relays. In the
first extensive commercial use of vacuum tube, such as repeaters
powered the first transcontinental telephone line for commercial
services in 1915, much of the mathematical theory of amplifiers was
developed at BELL TELEPHONE LABORATORIES during 1920s to 1940s.
Beginning
of 1970s more and more transistors were connected on a single chip by
creating higher scales of
integration (small-scale, medium-scale, large-scale). Many
commercially available today are base on integrated
circuits.
In the early 20
th century,
advance digital electronic were introduce such as D-amplifier.
1.2 AIM AND OBJECTION
1.2.1 AIM
The
aim of this work is to design and study of the feedback amplifiers.
1.2.2 OBJECTIVES
The
objectives of this work are:
-
To study how amplifiers work.
-
To design and study the feedback amplifiers.
-
To know the application of the feedback amplifiers.
-
To know the types and uses of feedback amplifiers.
1.3 SCOPE AND LIMITATION
1.3.1 SCOPE
This literature was
based on the study of the working principle of the feedback
amplifiers.
1.3.2 LIMITATION
This literature was
limited to study the various types, classification, application of
the feedback amplifiers.
1.4 MOTIVATION
The most important part
of this work is to see how actually amplifier is, how they work and
how is possible to transmit audio from low frequency to a higher
frequency using electronic component.
CHAPTER TWO
FEEDBACK AMPLIFIERS
2.1 INTRODUCTION
An
amplifier is an electronic device that can amplify sound from low
frequency response to the high frequency response, an amplifier can
either be a separate piece of equipment or an electronic circuit
contain within another device. Amplification is a fundamental to
modern electronic and amplifier are widely used in almost all
electronic devices.
Amplifier
can be
categorized in different ways, one
is
by the frequency of electronic signal being amplified. For example,
audio amplifier, amplify signals. In the audio (sound) range of less
than 20KHZ, RF amplifiers amplify frequencies in the radio frequency
range between 20KHZ and 300GHZ and servo amplifiers and instrumental
amplifiers may work with very low frequency down to direct current.
It can also be categorized by their physical placement in the signal
chain.
Feedback
amplifiers is one of the fundamental process in electronic also. It
defined as process whereby a portion of the output signal is fed to
the input signal in order to form a part of the system-output
concentration.
Feedback
is used in
operating
point of transistors insensitive to the both manufacture
variation as well as temperature. there is another types of feedback
called positive or regenerative feedback in which the overall gain is
increased, positive feedback is useful in oscillators and while
establishing the two stable state of flip-flop.
2.2 TYPES OF FEEDBACK AMPLIFIER
feedback
amplifier are classified into four(4) groups :-
1-
voltage-shunt feedback.
In
voltage-shunt feedback the feedback consist of single resistance RF.
Voltage developed across and feedbackRL. It sample to
input. Through RF. The shunt connection at input and
output terminal reduces input and output impedance. The amplifier
works as trans-resistance type voltage amplifier.
(2.1)
(2.1)
2-
voltage-series feedback.
In
voltage-series feedback, the voltage across the load resistance is
sampled and feedback to input through resistance RF and RE
(potential divider). Sample voltage is proportional to output voltage
and feedback series with the input voltage.
Series
connection at input is increases input resistance and shunt
connection at output reduces output resistance. The resulting
amplifier is true voltage amplifier.
Thus
(2.2)
(2.3)
so
(2.4)
(2.4)
3-
current-shunt feedback
In
current-shunt feedback, the feedback signal is proportional to the
output current and feed to input in shunt. The series connection at
input, decrease input resistance and the amplifier works as true
current amplifier.
Thus
(2.5)
(2.5)
4-
current-series feedback
In
current-series feedback, the signal is proportional to the load
current and fed to the output through a resistance RE in
series with the input signal. The series connection at the input and
the output increase the input and output impedance. The amplifier
circuit works as trans-conductance type amplifier.
Thus,
(2.6)
(2.7)
(2.8)
(2.9)2.3 CLASSIFICATION OF FEEDBACK AMPLIFIER
Amplifier
are
classified into several classes, they can be classified into:
Depending
on the portion of wave amplified, these are classified into four (4)
classes. They are
Class A
AMPLIFIER
Class B
AMPLIFIER
Class AB
AMPLIFIER
Class C
AMPLIFIER
2.3.1 CLASS A AMPLIFIER
Class
A Amplifiers
are the most common type of amplifier class due to their simple
design. Class A, literally means “the best class” of amplifier
due mainly to their low signal distortion levels and are probably the
best sounding of all the amplifier classes mentioned here. The class
A amplifier has the highest linearity over the other amplifier
classes and as such operates in the linear portion of the
characteristics curve.
Generally
class A amplifiers use the same single transistor (Bipolar, FET,
IGBT, etc) connected in a common emitter configuration for both
halves of the waveform with the transistor always having current
flowing through it, even if it has no base signal. This means that
the output stage whether using a Bipolar, MOSFET or IGBT device, is
never driven fully into its cut-off or saturation regions but instead
has a base biasing Q-point in the middle of its load line, Then the
transistor never turns “OFF” which is one of its main
disadvantages.
fig(2.1)
Class A Amplifier
To
achieve high linearity and gain, the output stage of a class A
amplifier is biased “ON” (conducting) all the time. Then for an
amplifier to be classified as “Class A” the zero signal idle
current in the output stage must be equal to or greater than the
maximum load current (usually a loudspeaker) required to produce the
largest output signal. As a class A amplifier operates in the linear
portion of its characteristic curves, the single output device
conducts through a full 360 degrees of the output waveform. Then the
class A amplifier is equivalent to a current source.
Since a class A amplifier
operates in the linear region, the transistors base (or gate) DC
biasing voltage should by chosen properly to ensure correct operation
and low distortion. However, as the output device is “ON” at all
times, it is constantly carrying current, which represents a
continuous loss of power in the amplifier.
Due to this continuous loss of
power class A amplifiers create tremendous amounts of heat adding to
their very low efficiency at around 30%, making them impractical for
high-power amplifications. Also due to the high idling current of the
amplifier, the power supply must be sized accordingly and be well
filtered to avoid any amplifier hum and noise. Therefore, due to the
low efficiency and over heating problems of Class A amplifiers, more
efficient amplifier classes have been developed.
2..3.2 CLASS B AMPLIFIER
Class
B amplifiers
were invented as a solution to the efficiency and heating problems
associated with the previous class A amplifier. The basic class B
amplifier uses two complimentary transistors either bipolar of FET
for each half of the waveform with its output stage configured in a
“push-pull” type arrangement, so that each transistor device
amplifies only half of the output waveform.
In
the class B amplifier, there is no DC base bias current as its
quiescent current is zero, so that the dc power is small and
therefore its efficiency is much higher than that of the class A
amplifier. However, the price paid for the improvement in the
efficiency is in the linearity of the switching device.
fig(2.2)
CLASS B AMPLIFIER.
When
the input signal goes positive, the positive biased transistor
conducts while the negative transistor is switched “OFF”.
Likewise, when the input signal goes negative, the positive
transistor switches “OFF” while the negative biased transistor
turns “ON” and conducts the negative portion of the signal. Thus
the transistor conducts only half of the time, either on positive or
negative half cycle of the input signal.
Then
we can see that each transistor device of the class B amplifier only
conducts through one half or 180 degrees of the output waveform in
strict time alternation, but as the output stage has devices for both
halves of the signal waveform the two halves are combined together to
produce the full linear output waveform.
This
push-pull design of amplifier is obviously more efficient than Class
A, at about 50%, but the problem with the class B amplifier design is
that it can create distortion at the zero-crossing point of the
waveform due to the transistors dead band of input base voltages from
-0.7V to +0.7.
We
remember from the transistors that it takes a base-emitter voltage
of about 0.7 volts to get a bipolar transistor to start conducting.
Then in a class B amplifier, the output transistor is not “biased”
to an “ON” state of operation until this voltage is exceeded.
This means that the the part of the waveform which falls within this
0.7 volt window will not be reproduced accurately making the class B
amplifier unsuitable for precision audio amplifier applications.
To
overcome this zero-crossing distortion (also known as crossover
distortion) class AB amplifiers were developed.
2.3.3 CLASS AB AMPLIFIER
As
its name implies,
the Class
AB Amplifier
is a combination of the “Class A” and the “Class B” type
amplifiers that stated
above. The AB
classification of amplifier is currently one of the most common used
types of audio power amplifier design. The class AB amplifier is a
variation of a class B amplifier as described above, except that both
devices are allowed to conduct at the same time around the waveforms
crossover point eliminating the crossover distortion problems of the
previous class B amplifier.
The
two transistors have a very small bias voltage, typically at 5 to 10%
of the quiescent current to bias the transistors just above its
cut-off point. Then the conducting device, either bipolar of FET,
will be “ON” for more than one half cycle, but much less than one
full cycle of the input signal. Therefore, in a class AB amplifier
design each of the push-pull transistors is conducting for slightly
more than the half cycle of conduction in class B, but much less than
the full cycle of conduction of class A. In other words, the
conduction angle of a class AB amplifier is somewhere between 180o
and 360o depending upon the chosen bias point as shown in
fig(3).
fig(2.3) Class AB Amplifier
The advantage of this small
bias voltage, provided by series diodes or resistors, is that the
crossover distortion created by the class B amplifier characteristics
is overcome, without the inefficiencies of the class A amplifier
design. So the class AB amplifier is a good compromise between class
A and class B in terms of efficiency and linearity, with conversion
efficiencies reaching about 50% to 60%.
2.3.4 CLASS C AMPLIFIER
The
Class C Amplifier design has the greatest efficiency but the poorest
linearity of the classes of amplifiers mentioned here.
The previous classes, A, B and AB are considered linear amplifiers,
as the output signals amplitude and phase are linearly related to the
input signals amplitude and phase.
However,
the class C amplifier is heavily biased so that the output current is
zero for more than one half of an input sinusoidal signal cycle with
the transistor idling at its cut-off point. In other words, the
conduction angle for the transistor is significantly less than 180
degrees, and is generally around the 90 degrees area. While this form
of transistor biasing gives a much improved efficiency of around 80%
to the amplifier, it introduces a very heavy distortion of the output
signal. Therefore, class C amplifiers are not suitable for use as
audio amplifiers.
fig(2.4)
Class C Amplifier
Due
to its heavy audio distortion, class C amplifiers are commonly used
in high frequency sine wave oscillators and certain types of radio
frequency amplifiers, where the pulses of current produced at the
amplifiers output can be converted to complete sine waves of a
particular frequency by the use of LC resonant circuits in its
collector circuit.
Other
classes of amplifier are ;
2.3.5 Class D Amplifier
A
Class D audio amplifier is basically a non-linear switching
amplifier or PWM amplifier. Class-D amplifiers theoretically can
reach 100% efficiency, as there is no period during a cycle were the
voltage and current waveforms overlap as current is drawn only
through the transistor that is ON.
2.3.6 Class F Amplifier
Class-F
amplifiers boost both efficiency and output by using harmonic
resonators in the output network to shape the output waveform into a
square wave. Class-F amplifiers are capable of high efficiencies of
more than 90% if infinite harmonic tuning is used.
2.3.7Class G Amplifier
Class G offers enhancements to
the basic class AB amplifier design. Class G uses multiple power
supply rails of various voltages and automatically switches between
these supply rails as the input signal changes. This constant
switching reduces the average power consumption, and therefore power
loss caused by wasted heat.
2.3.8 Class I Amplifier
The
class I amplifier has two sets of complementary output switching
devices arranged in a parallel push-pull configuration with both sets
of switching devices sampling the same input waveform. One device
switches the positive half of the waveform, while the other switches
the negative half similar to a class B amplifier. With no input
signal applied, or when a signal reaches the zero crossing point, the
switching devices are both turned ON and OFF simultaneously with a
50% PWM duty cycle canceling out any high frequency signals.
To produce the positive half of
the output signal, the output of the positive switching device is
increased in duty cycle while the negative switching device is
decreased by the same and vice versa. The two switching signal
currents are said to be interleaved at the output, giving the class
I amplifier the named of: “interleaved PWM amplifier” operating
at switching frequencies in excess of 250kHz.
2.3.9Class S Amplifier
A class S power amplifier is a
non-linear switching mode amplifier similar in operation to the
class D amplifier. The class S amplifier converts analogue input
signals into digital square wave pulses by a delta-sigma modulator,
and amplifies them to increases the output power before finally
being demodulated by a band pass filter. As the digital signal of
this switching amplifier is always either fully “ON” or “OFF”
(theoretically zero power dissipation), efficiencies reaching 100%
are possible.
2.4 Class T Amplifier
The class T amplifier is
another type of digital switching amplifier design. Class T
amplifiers are starting to become more popular these days as an
audio amplifier design due to the existence of digital signal
processing (DSP) chips and multi-channel surround sound amplifiers
as it converts analogue signals into digital pulse width modulated
(PWM) signals for amplification increasing the amplifiers
efficiency. Class T amplifier designs combine both the low
distortion signal levels of class AB amplifier and the power
efficiency of a class D amplifier.
We
have seen here a number of classification of amplifiers ranging from
linear power amplifiers to non-linear switching amplifiers, and have
seen how an amplifier class differs along the amplifiers load line.
The class AB, B and C amplifiers can be defined in terms of the
conduction angle (θ), as follows:Amplifier Class by Conduction
Angle.
Amplifier
Class
|
Description
|
Conduction
Angle
|
Class-A
|
Full
cycle 360o of Conduction
|
Θ
=2Ï€
|
Class-B
|
Half
cycle 180o of Conduction
|
Θ
= π
|
Class-AB
|
Slightly
more than 180o of conduction
|
Î
< θ < 2π
|
Class-C
|
Slightly
less than 180o of conduction
|
Θ
< π
|
Class-D
to T
|
ON-OFF
non-linear switching
|
Θ
= 0
|
Table
(2.1) classification of amplifiers by conducting angle
1. AUDIO FREQUENCY AMPLIFIERS (A.F. AMPLIFIERS)
Audio
Frequency Amplifiers is
the amplifier that
amplify the audio frequencies, Generally audio frequencies are in
the range of 20 Hz to 20 kHz. Some of the HI-FI audio amplifiers may
amplify up to 100 kHz.
These
are used to supply audio frequency power to operate the loud
speakers. Most of the modern audio amplifiers are based on solid
state dives such as transistors, in early stages they are made of
vacuum tubes.
2. Intermediate Frequency Amplifiers (I.F. Amplifiers):
The
Intermediate frequencies are amplified by this amplifier. These
amplifiers are used in TV, radio and radar. They provide the maximum
voltage amplification of a radio, TV or radar signal, before the
video or audio information carried by the signal is demodulated.
Their
frequency of operation is lower than that of the received radio
signal, but higher than the audio or video signals eventually
produced by the system. The type of equipment decides the frequency
at which I.F. amplifiers operate.
3. Radio Frequency Amplifiers (R.F. Amplifiers):
This
amplifier increases the power of low-frequency radio signal. These
are used to drive antenna of a transmitter. Radio Frequency
amplifiers are tuned amplifiers whose the frequency of operation is
controlled by the tuned circuit. This circuit can be adjusted
depending on the amplifier purpose. Input resistance is generally
low, as is gain.
A
special feature of RF amplifiers is low noise performance. So they
are used in the earliest stages of a receiver. The background noise
generally produced by any electronic device is kept to a low value as
the amplifier handles very low amplitude signals from the antenna.
Hence low noise FET transistors used in these stages.
4. Ultrasonic Amplifiers:
Ultrasonic
amplifiers amplify the ultrasonic waves. These are in the frequency
range of around 20 kHz up to about 100 kHz. They are used for
specific purposes such as ultrasonic cleaning, ultrasound scanning,
remote control systems etc. Each type will operate at narrow band of
frequencies within the ultrasonic range.
5. Wideband Amplifiers:
Wide
band amplifiers will amplify a band of frequencies. They amplify from
DC to several tens of MHz They are used in equipment such as
oscilloscopes etc. These are used where there is a need to accurately
measure signals over a wide range of frequencies. Because of their
wide bandwidth, gain is low.
6. Direct Coupled Amplifiers (DC Amplifiers):
Direct
coupled or DC amplifiers are used to amplify very low frequency
signals. The output of one stage is coupled with the input of the
next stage in these amplifiers. This amplifier amplifies the DC
frequency that is zero frequency. They are mostly used in many
electrical control systems and measuring instruments.
7. Video Amplifiers:
Video
amplifiers are used to improve the video signal and display it with
high resolution. The video signal carries all the information about
picture in TV and radar systems. They are a special type of wide band
amplifier. They are used specifically for signals that are to be
applied to video equipment.
The
bandwidth of video amplifiers depends on use. In TV receivers they
extend from 0Hz to 6MHz and is still wider in radar. These amplifiers
are used to amplify the signals received from DVDs, computer
monitors. They can also be used to amplify the video quality in small
TVs that are installed in the vehicles.
8. Buffer Amplifiers :
Buffer
amplifiers are commonly used for electrical impedance transformation
from one circuit to another. They have amplifier gain of 1.They are
used for isolating the circuits from each other. They have high
impedance at the input and low impedance at the output .
Therefore
can be used as impedance matching device. This implies that signals
are not attenuated between circuits, which happens when a circuit
with high output impedance feeds a signal directly to another circuit
having a low input impedance.
9. Operational Amplifiers :
Operational
amplifiers are high gain electronic voltage amplifiers. The
operational amplifiers are used to perform mathematical operations on
voltages . They are used in the form of ICs initially they were
developed with the vacuum tubes. The operational amplifier has two
input terminal mainly.
They
are inverting and non-inverting. They can be used as inverting
amplifiers, non-inverting amplifiers, summing amplifiers, difference
amplifier etc.
10. Transistor Amplifiers:
Transistor
is an electronic device. This is also used as an amplifier.
Transistor amplifiers amplify the voltage or current of the input
signal.
There
are two types of transistor devices.
1)
BJT (Bipolar junction transistors).
2)FET
(Field Effect Transistors).
Transistor
amplifiers are analyzed in different configurations. They are Common
Base, Common Emitter and Common Collector using BJT. By using FET,
transistor amplifiers are analyzed in the following configurations,
Common Gate Common Source and Common Drain.
In
Bipolar Junction Transistors, small current at the terminal of base
can control the current at emitter and collector, while in Field
Effect Transistors (FET) small voltage at the gate can control the
voltage at source and drain.
2.5 APPLICATION OF FEEDBACK AMPLIFIER
Amplifiers
has many uses nowaday because most of the electronic devices are
connected with an electronic amplifiers, some of its application are
as follows :-
-
amplifiers are used as a device that increase sound from low frequencies to the high frequencies. e.g electronic amp in a set of loud speaker.
-
Amplifiers are used in hi-fi system. e.g. class AB amplifier.
-
Amplifiers are used as a sound device in a mobile devices and personal computers.
-
Amplifiers are used cars audio sub-woofer amplifiers.
-
Amplifiers can be used as tools that hooked up to take the readings from human body at the hospitals, patient is going to be connected to sensor that run by instrumental amplifiers. These circuit is used in nearly every medical devices. Most of the biomedical sensors are with very high impedance and generate tiny signals such as blood pressure sensor, ultra sound transducers, polarized and non polarize electrons and radiation thermometric transducers.The required is with very high impedance presented by an instrumentation amplifier because the characteristic of potentials can subjected to loading effect, which can cause distortion of the signals. An easily recognizable medical application of amplifiers is like in electrocardiography medical which can maintain the changes in the hearts dipole electric field.In conclusion the electronic amplifiers have uses in nearly every field of electronics, they fulfill a specific role in circuit needing the advantages of high input impedance with good gain while providing common mode noise rejection and fully differential inputs. With such widespread use, this is a device that every engineer should have in his tool belt.
CHAPTER THREE
SUMMARY AND CONCLUSION
3.1 SUMMARY
Then we have seen that
the quiescent DC operating point (Q-point) of an amplifier determines
the amplifier classification. By setting the position of the Q-point
at half way on the load line of the amplifiers characteristics curve,
the amplifier will operate as a class A amplifier. By moving the
Q-point lower down the load line changes the amplifier into a class
AB, B or C amplifier. Then the class of operation of the amplifier
with regards to its DC operating point can be given as:
Fig (3.1)
Amplifier Classes
and Efficiency
As well as audio
amplifiers there are a number of high efficiency Amplifier
Classes relating to switching amplifier designs that use
different switching techniques to reduce power loss and increase
efficiency. Some amplifier class designs listed below use RLC
resonators or multiple power-supply voltages to reduce power loss, or
are digital DSP (digital signal processing) type amplifiers which use
pulse width modulation (PWM) switching techniques.
3.2 CONCLUSION
The
study of feedback amplifier is very wide field which is never been
emphasizes unless is to take some part of it. The feedback amplifiers
was discuss in this literature like, its types, classification and
some of its applications.
REFERENCE
-
Feedback Amplifiers ; theory and design (kluwer academy) p.64, ISBN 0-7923-7643-9.
-
ELEC 312 lecture note chapter 4 p. 17-20, by Rabin Raut, phd.
-
Feedback Amps p.16-19, by professor W. marshal leach jr. (Geoge institute of technology, school of electrical and computer engineering).
-
Feedback Amplifiers chapter 9, by De bashis (Associate professor west Bengal university of technology).
-
Laboratory manual electronic circuit & Analysis by C.S Swapna (assistant professor and R.Sanvers pillai). At MLR Institute of technology & management), Dundigal, Quhbullapur (M), Hyderabad.
-
Electronic Amplifier circuit Theory and Design. By Professor Joseph mypettil and associate professor Malcolm myers, (Stanford university).
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