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Contents:
1. Dynamical theory of
Electromagnetic waves
2. Transmission and reception
3. Transmitter:
4. Receiver:
5. Antenna:
6. Modulation:
7. Band theory of solids:
8. Semiconductors
9. Doping:
10. Biasing:
11. Rectifier (semi conductor Diode):
12. Solar cell:
13. Transistor:
14. SHORT QUESTIOB AND ANSWERS
ELECTROMAGNETIC WAVES
Introduction:
Electromagnetic radiation can be described in terms of a stream of photons, which are massless
particles each traveling in a wave-like pattern and moving at the speed of light. Each photon contains a
certain amount (or bundle) of energy, and all electromagnetic radiation consists of these photons. The only
difference between the various types of electromagnetic radiation is the amount of energy found in the
photons. Radio waves have photons with low energies, microwaves have a little more energy than radio
waves, infrared has still more, then visible, ultraviolet, X-rays, and the most energetic of all gamma-rays.
Actually, the electromagnetic spectrum can be expressed in terms of energy, wavelength, or frequency.
A wave is a vibration that is propagated in space. The
alternating electrical current produces an electric and
magnetic field that are also alternating: the succession of
cycles going "back and forth" constitutes the wave. The wave
of the electric field and the wave of the magnetic field are
propagated perpendicularly to the direction of propagation and to
each other. At extremely low frequencies, the electric field and the
magnetic field are specified separately. At higher frequencies,
electric and magnetic fields are not separable, and are named
"electromagnetic waves" or "electromagnetic fields". Any
electromagnetic wave is characterized by two parameters: its
frequency and its wavelength.
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An electromagnetic wave consists of two
primary components—an electric field and a
magnetic field. The electric field results from the force of voltage,
and the magnetic field results from the flow of current. Although electromagnetic fields
that are radiated are commonly considered to be waves, under certain circumstances their
behavior makes them appear to have some of the properties of particles. In general, however, it is easier to
picture electromagnetic radiation in space as horizontal and vertical lines of force oriented at right angles
to each other. These lines of force are made up of an electric field (E) and a magnetic field (H), which
together make up the electromagnetic field in space. The electric and magnetic fields radiated from an
antenna form the electromagnetic field. This field is responsible for the transmission and reception of
electromagnetic energy through free space. An antenna, however, is also part of the electrical circuit of a
transmitter or a receiver and is equivalent to a circuit containing inductance, capacitance, and resistance.
Therefore, the antenna can be expected to display definite voltage and current relationships with respect
to a given input. A current through the antenna produces a magnetic field, and a charge on the antenna
produces an electric field. These two fields combine to form the induction field. All of the antenna
parameters are expressed in terms of a transmission antenna, but are identically applicable to a receiving
antenna. For a transmitting antenna, this is the antenna itself. For a receiving antenna, this is at the (radio)
receiver rather than at the antenna. Tuning is done by adjusting the length of an electrically long linear
antenna to alter the electrical resonance of the antenna.
Antenna tuning is done by adjusting an inductance or capacitance combined with the active antenna (but
distinct and separate from the active antenna). The inductance or capacitance provides the reactance which
combines with the inherent reactance of the active antenna to establish a resonance in a circuit including the
active antenna. The established resonance being at a frequency other than the natural electrical resonant
frequency of the active antenna. Adjustment of the inductance or capacitance changes this resonance.
The sub-system , that takes the information signal and
processes it preceding to transmission. The transmitter
modulates the information onto a carrier signal, amplifies
the signal and broadcasts it over the channel.
Channel: The medium which transports the modulated signal to the receiver.
Air acts as the channel for broadcasts like radio. May also be a wiring system
like cable TV or the Internet.
The sub-system that takes in the transmitted signal
from the channel and processes, it to retrieve the information signal. The receiver must
be able to discriminate the signal from other signals which may using the same channel (called tuning),
amplify the signal for processing and demodulate (remove the carrier) to retrieve the information. It also
then processes the information for reception (for example, broadcast on a loudspeaker).
Description
An antenna (or aerial) is a transducer that transmits or receives
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electromagnetic waves. In other words, antennas convert electromagnetic radiation into
electrical current, or vice versa. Antennas generally deal in the transmission and reception
of radio waves, and are a necessary part of all radio equipment. Antennas are used in
systems such as radio and television broadcasting, point-to-point radio communication,
wireless LAN, cell phones, radar, and spacecraft communication. Antennas are most
commonly employed in air or outer space, but can also be operated under water or even
through soil and rock at certain frequencies for short distances.
Physically, an antenna is an arrangement of one or more conductors, usually called elements in this
context. In transmission, an alternating current is created in the elements by applying a voltage at the
antenna terminals, causing the elements to radiate an
electromagnetic field. In reception, the inverse occurs: an
electromagnetic field from another source induces an
alternating current in the elements and a corresponding
voltage at the antenna's terminals. Some receiving
antennas (such as parabolic and horn types) incorporate
shaped reflective surfaces to collect the radio waves striking them and direct or focus them onto the actual
conductive elements.
Modulation means to vary or change. Modulation is the process where a Radio
Frequency or Light Wave's amplitude, frequency, or phase is changed in order to transmit intelligence.
The characteristics of the carrier wave are instantaneously varied by another "modulating" waveform.
In wireless we first take a signal, say a telephone conversation, and then impress it on a constant radio
wave called a carrier. Once done the voice signal varies or modulates this radio wave. The two go together
over the air. A voice frequency in the audible or audio range, what we can hear, thus modulates or varies a
constant frequency in the radio range, which we can't hear. That's an important point. Modulation makes
voice band and radio band frequencies work together. Different modulation techniques, such as A.M.,
F.M., and so on, represent different ways to shape or form electromagnetic radio waves.
There are many reasons to modulate a signal in a particular way.
a) Amplitude Modulation:
Amplitude Modulation occurs when a voice signal's varying voltage is applied to a carrier frequency.
The carrier frequency's amplitude changes in accordance with the modulated voice signal, while the
carrier's frequency does not change. Amplitude modulation
means a carrier wave is modulated in proportion to the strength
of a signal. The carrier rises and falls instantaneously with each
high and low of the conversation. As shown in figure. Low
frequency commercial broadcast stations in the "A.M band" use
amplitude modulation.
b) Frequency Modulation:
Frequency Modulation occurs when a carrier's Center
frequency is changed based upon the input signal's amplitude. Unlike Amplitude Modulation, the carrier
signal's amplitude is Unchanged. This makes FM modulation more immune to noise than AM and
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improves the overall signal-to-noise ratio of the communications system. Power output is
also constant, differing from the varying AM power output.
The amount of analog bandwidth necessary to transmit a FM signal is greater than the
amount necessary for AM, a limiting constraint for some systems.
The word frequency in FM relates, instead, to the rate at which this method varies a
carrier wave, not to any particular radio
frequency it is used on. This will become more
clear as we go on.
c)Phase Modulation :
Phase Modulation is similar to Frequency Modulation in which the natural flow of the alternating
current waveform is delayed temporarily. Instead of the frequency of the carrier wave changing, the
PHASE of the carrier changes.
As you might imagine, this type of modulation is easily adaptable to data modulation applications.
d) Pulse Modulation :
We migrate from analog modulation (continuous in both time and value) to
digital modulation (discrete in both time and value) through pulse modulation
(discrete in time but could be continuous in value).
Quantum mechanics states that the energy of an electron in an atom cannot
be any arbitrary value. There are fixed energy
levels which the electrons can occupy, and values
in between these levels are impossible. The
energy levels are grouped into two bands: the valence band and the conduction band. Electrons in the
conduction band may move freely throughout the substance in the presence of an electrical field.
In insulators and semiconductors, the atoms in the substance influence each other so that between the
valence band and the conduction band there exists a forbidden band of energy
levels, which the electrons cannot occupy. In order for a current to flow, a relatively
large amount of energy must be furnished to an electron for it to leap across this
forbidden gap and into the conduction band. Thus, large voltages yield relatively
small currents
An important parameter in the band theory is the Fermi level, the top of the
available electron energy levels at low temperatures. The position of the Fermi level
with the relation to the conduction band is a crucial factor in determining electrical
properties.
Insulator Energy Bands:
Most solid substances are insulators, and in terms of the band theory of solids this implies that there is a
large forbidden gap between the energies of the valence electrons and the energy at which the electrons
can move freely through the material (the conduction band).
Semiconductor Energy Bands:
For intrinsic semiconductors like silicon and germanium, the Fermi level is essentially halfway between
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the valence and conduction bands. Although no conduction occurs at 0 K, at higher
temperatures a finite number of electrons can reach the conduction band and provide
some current. In doped semiconductors, extra energy levels are added.
The increase in conductivity with temperature can be modeled in terms of the Fermi
function, which allows one to calculate the population of the conduction band.
Conductor Energy Bands:
In terms of the band theory of solids, metals are unique as good conductors of
electricity. This can be seen to be a result of their valence electrons being essentially free. In the band
theory, this is depicted as an overlap of the valence band and the conduction band so that at least a
fraction of the valence electrons can move through the material
A semiconductor, such as silicon, has properties somewhere between those of
a conductor and an insulator. The ability of a semiconductor to conduct
electricity can be changed dramatically by adding small numbers of a different element to the
semiconductor crystal. This process is called doping. Early experiments showed that an electric current
through a semiconductor was carried by the flow of positive charges as well as negative charges
(electrons).
The property of semiconductors that makes them most useful for constructing
electronic devices is that their conductivity may easily be modified by introducing
impurities into their crystal lattice. The process of adding controlled impurities to a semiconductor is
known as doping. The amount of impurity, or dopant, added to an intrinsic (pure) semiconductor varies
its level of conductivity. Doped semiconductors are often referred to as extrinsic. Current conduction in a
semiconductor occurs via mobile or "free" electrons and holes, collectively known as charge carriers.
Doping a semiconductor such as silicon with a small amount of impurity atoms, such as phosphorus or
boron, greatly increases the number of free electrons or holes within the semiconductor. When a doped
semiconductor contains excess holes it is called "p-type", and when it contains excess free electrons it is
known as "n-type", where p (positive for holes) or n (negative for electrons) is the sign of the charge of the
majority mobile charge carriers.
The PN Junction:
It is possible to turn a crystal of pure silicon into a moderately good electrical conductor by adding an
impurity such as arsenic or phosphorus (for an N-type semiconductor) or aluminum or gallium (for a Ptype semiconductor). Useful applications start to happen only when a single semiconductor crystal
contains both P-type and N-type regions. Here we will examine the properties of a single silicon crystal
which is half N-type and half P-type.
The recombination of free electrons and holes in the vicinity of the junction leaves a narrow region on
either side of the junction that contains no mobile charge. This narrow region which has been depleted of
mibile charge is called the depletion layer. It extends into both the p-type and n-type regions. The
diffusion of holes from the p-type side of the depletion layer leaves behind some uncovered fixed negative
charges (the acceptor ions). Similarly, fixed positive charges (donor ions) are uncovered on the n-type side
of the depletion layer. There is then a separation of charges: negative fixed charges on the p-type side of
the depletion layer and positive fixed charges on the n-type side. This separation of charges causes an
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electric field to extend across the depletion layer. A potential difference must therefore
exist across the depletion layer. The variation of potential with distance .
The p-n junction is formed within a single crystal rather than simply joining two pieces
together. Electrical contacts on either side of the crystal enable connection to an external
circuit. The resulting device is called a junction diode.
The potential difference across a p-n junction
can be applied in two different ways: forward biasing
and reverse biasing.
i) Forward bias:
In forward bias, the p-type is connected with the positive terminal and the n-type is connected with
the negative terminal. With a battery connected this way, the
holes in the P-type region and the electrons in the N-type
region are pushed towards the junction. This reduces the
width of the depletion zone. The positive charge applied to the P-type
material
repels the holes, while the negative charge applied to the N-type
material
repels the electrons. As electrons and holes are pushed towards the
junction, the
distance between them decreases. This lowers the barrier in potential. With increasing forward-bias
voltage, the depletion zone becomes thin enough that the zone's electric field can't counteract charge
carrier motion across the p–n junction, reducing electrical resistance. The electrons which cross the p–n
junction into the P-type material (or holes which cross into the N-type material) will diffuse in the nearneutral region. Therefore, the amount of minority diffusion in the near-neutral zones determines the
amount of current that may flow through the diode.
ii) Reverse biased:
If a diode is reverse biased, the voltage at the cathode is higher than that at the anode. Therefore, no
current will flow until the diode breaks down. Connecting the P-type regi on to
the negative terminal of the battery and the N-type region to the positive
terminal, corresponds to reverse bias. Because the p-type material is now
connected to the negative terminal of the power supply, the 'holes' in the P-type
material are pulled away from the junction, causing the width of the depletion
zone to increase. Similarly, because the N-type region is connected to the
positive terminal, the electrons will also be pulled away from the
junction. Therefore the depletion region widens, and does so
increasingly with increasing reverse-bias voltage. This increases the
voltage barrier causing a high resistance to the flow of charge carriers
thus allowing minimal electric current to cross the p–n junction. The
strength of the depletion zone electric field increases as the reverse-bias voltage increases. Once the electric
field intensity increases beyond a critical level, the p–n junction depletion zone breaks-down and current
begins to flow,
A rectifier is an
electrical device that
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converts alternating current (AC) to direct current (DC), a process known as rectification.
Rectifiers have many uses including as components of power supplies and as detectors of
radio signals. Rectifiers may be made of solid state diodes, vacuum tube diodes, mercury
arc valves, and other components.
Half wave Rectifier:
When a diode is connected to a source of alternating
voltage, it will be alternately forward-biased, and then reversebiased, during each cycle of the AC sine-wave. When a single diode is
used in a rectifier circuit, current will flow through the circuit only
during one-half of the input voltage cycle. For this reason, this
rectifier circuit is called a half-wave rectifier. The output of a half-wave rectifier circuit is pulsating DC.
Full-wave Rectifier:
The circuit consists of two Half-wave rectifiers connected to a single load resistance with each diode
taking it in turn to supply current to the load. When point A is positive with respect to point B, diode D1
conducts in the forward direction as indicated by the arrows. When point B is positive (in the negative half
of the cycle) with respect to point A, diode D2 conducts in the forward direction and the current flowing
through resistor R is in the same direction for both circuits. As the output voltage across the resistor R is
the sum of the two waveforms, this type of circuit is also known as a "bi-phase" circuit.
LEDs emit light when an electric current passes through them.
The light emitting diode (LED) is commonly used as an indicator.
It can show when the power is on, act as a warning indicator. It needs to be fed from a DC supply, with the
anode positive and the cathode negative. The light output of individual light-emitting diodes is small
compared to incandescent and compact fluorescent lamps, multiple diodes are often used together.
LEDs have a broad range of uses. Many are used for a variety of signs and signals, such as traffic lights,
motorcycle lights, light bars on emergency vehicles, elevator push-button lights, status lights on electronic
equipment, exit signs, and other message displays. Infrared LEDs are used in remote controls for TVs and
VCRs. Some LEDs are used for phototherapy for acne.
Solar cells are designed to convert
into electrical energy. They do this without the
of either chemical reactions or moving parts.
A solar cell (also called photovoltaic cell) is a solid state device that
converts the energy of sunlight directly into electricity by the
photovoltaic effect. Assemblies of cells are used to make solar
modules, also known as solar panels. The energy generated from
modules, referred to as solar power, is an example of solar energy.
The solar cell works in three steps:
1.Photons in sunlight hit the solar panel and are absorbed by semiconducting
materials, such as silicon.
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2.Electrons (negatively charged) are knocked loose from their atoms, allowing them to
flow through the material to produce electricity. Due to the special composition of solar
cells, the electrons are only allowed to move in a single direction.
3.An array of solar cells converts solar energy into a usable amount of direct current (DC)
electricity.
Structure:
Modern solar cells are based on semiconductor. They are basically just P-N junction
photodiodes with a very large light-sensitive area. The photovoltaic effect, which causes the cell to convert
light directly into electrical energy, occurs in the three energy-conversion layers. The first of these three
layers necessary for energy conversion in a solar cell is the top junction layer (made of N-type
semiconductor ). The next layer in the structure is the core of the device; this is the absorber layer (the P-N
junction). The last of the energy-conversion layers is the back junction layer (made of
P-type semiconductor).
A transistor is a semiconductor device used to amplify and switch electronic
signals. It is made of a solid piece of semiconductor material, with at least three terminals for connection to
an external circuit. The leads are labeled base (B), collector (C) and emitter (E). The BASE - which is the
lead responsible for activating the transistor. The COLLECTOR - which is the positive lead? The EMITTER
- which is the negative lead these terms refer to the internal operation of a transistor but they are not much
help in understanding how a transistor is used. A voltage or current applied to one pair of the transistor's
terminals changes the current flowing through another pair of terminals. Because the controlled (output)
power can be much more than the controlling (input) power, the transistor provides amplification of a
signal.
Types of transistor:
There are two types of standard transistors, NPN and PNP, with different circuit.
Working of PNP transistor:
The other type of BJT is the PNP, consisting of a layer of N-doped semiconductor between two layers
of P-doped material. A small current leaving the base is amplified in the collector output. That is, a PNP
transistor is "on" when its base is pulled low relative to the emitter.The arrows in the NPN and PNP
transistor symbols are on the emitter legs and point in the direction of the conventional current flow when
the device is in forward active mode.A mnemonic device for the NPN / PNP distinction, based on the
arrows in their symbols and the letters in their names, is not pointing in for NPN and pointing in for PNP.
Working of NPN transistor:
NPN is one of the two types of bipolar transistors, consisting of a layer of P-doped semiconductor
(the "base") between two N-doped layers. A small current entering the base is amplified in the collector
output. That is, an NPN transistor is "on" when its base is pulled high relative to the emitter Most of the
NPN current is carried by electrons, moving from emitter to collector as minority carriers in the P-type
base region. Most bipolar transistors used today are NPN, because electron mobility is higher than hole
mobility in semiconductors, allowing greater currents and faster operation.
Amplification is the process of increasing the strength of a
SIGNAL. A signal is just a
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general term used to refer to any particular current, voltage, or power in a circuit. An
amplifier is the device that provides amplification (the increase in current, voltage, or
power of a signal) without appreciably altering the original signal. Transistors are
frequently used as amplifiers. Some transistor circuits are CURRENT amplifiers, with a
small load resistance; other circuits are designed for VOLTAGE amplification and have a
high load resistance; others amplify POWER.
Q: No: 1. What is the electromagnetic spectrum?
Answer:
The range of wavelengths for electromagnetic waves--from the very long to the very short--is called the
Electromagnetic Spectrum:
Radio and TV waves are the longest usable waves, having a wavelength of 1 mile (1.5 kilometer) or more.
Microwaves are used in telecommunication as well as for cooking food. Infrared waves are barely visible.
They are the deep red rays you get from a heat lamp.
Visible light
waves are the radiation you can see with your eyes. Their wavelengths are in the range of 1/1000
centimeter.
Ultraviolet rays are what give you sunburn and are used in "black lights" that make object glow.
X-rays go through the body and are used for medical purposes.
Gamma rays are dangerous rays coming from nuclear reactors and atomic bombs. They have the shortest
wavelength in the electromagnetic spectrum of about 1/10,000,000 centimeter.
Q: No: 2. What are the characteristics of electromagnetic waves?
Answer:
Electromagnetic waves are transverse waves, similar to water waves in the ocean or the waves seen on a
guitar string. This is as opposed to the compression waves of sound. As you learned in Wave Motion, all
waves have amplitude, wavelength, velocity and frequency.
Amplitude
The amplitude of electromagnetic waves relates to its intensity or brightness (as in the case of visible light).
With visible light, the brightness is usually measured in lumens. With other wavelengths the intensity of
the radiation, which is power per unit area or watts per square meter is used. The square of the amplitude
of a wave is the intensity.
Wavelength
The wavelengths of electromagnetic waves go from extremely long to extremely short and everything in
between. The wavelengths determine how matter responds to the electromagnetic wave, and those
characteristics determine the name we give that particular group of wavelengths.
Velocity
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The velocity of electromagnetic waves in a vacuum is approximately 186,000 miles per
second or 300,000 kilometers per second, the same as the speed of light. When these waves
pass through matter, they slow down slightly, according to their wavelength.
Frequency
The frequency of any waveform equals the velocity divided by the wavelength. The
units of measurement are in cycles per second or Hertz.
Q: No: 3. How are these electromagnetic waves created and detected?
Answer:
When electrons move, they create a magnetic field. When electrons move back and forth or oscillate,
their electric and magnetic fields change together, forming an electromagnetic wave. This oscillation can
come from atoms being heated and thus moving about rapidly or from alternating current (AC) electricity.
The opposite effect occurs when an electromagnetic wave hits matter. In such a case, it could cause atoms to
vibrate, creating heat, or it can cause electrons to oscillate, depending on the wavelength of the radiation.
There are a number of different types of detectors of electromagnetic radiation. We know the common
ones for detecting visible light: the eye, camera film, and the detectors on some calculators. Your skin can
also detect both visible light and infrared heat rays.
Electronic devices are necessary to detect most of the longer waves, such as radio waves. Special
film can detect shorter wavelengths such as X-rays
Q: No: 4. Is air a good conductor or a bad conductor and why?
Answer:
Air is not a conductor of electricity if it conducts electricity we cannot live. And if there is any ions present
then only that can conducts electricity but air is a mixture of gases. So it is not a conductor.
Lightning is proof that air is a conductor, however it is not a 'good' conductor it is about average. Air
conductivity changes with moisture (humidity)and temperature level. The higher the moisture in the air,
the higher its conductivity. Yes it is the water in the air that conducts generally, but the question was not
"is oxygen / nitrogen / methane / etc" a good conductor. Our "Air" or "Atmosphere" is made up of a great
number of components.
Q: No: 5. Why transistor is called Amplifier?
Answer:
Because, an electrical signal can be amplified by using a device which allows a small current or voltage to
Control the flow of a much larger current from a dc power source. Transistors are the basic device
Providing control of this kind. So that transistor works as amplifier.
Q: No: 6. How is water a good conductor of electricity? My ohmmeter indicates a lot of resistance in a
bucket of water?
Answer:
Water itself is not a conductor of electricity. In order for a substance to carry charge, two conditions must
first be met:

There must be charged particles within the substance (either ions or electrons) and

These particles must be free to move It is true that in water (H2O) the discrete molecules are free
to move, however, they carry no charge.
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The only reason water can sometimes conduct electricity is because of the minerals
(metallic solids) already present in the water.
Q: No: 7. Is it true that air is a poor heat conductor?
Answer:
It depends what you are comparing it to, if for instance you compare it to a vaccuum then
it is an infinitely better conductor. Compared to copper however it is very poor, becuase
obviously air is a mixture of various gasses and gasses are mostly empty space; because
transferal of heat is through one particle passing on energy to another through vibrations it stands to
reason that the less particles you have in a set volume the less efficiently it will transfer heat.
Q:8 What you mean by modulation ?
Answer:
Modulation is the process of varying one or more properties of a high-frequency periodic waveform,
called the carrier signal, with a modulating signal which typically contains information to be transmitted.
Listed below are the approximate wavelength, frequency, and energy limits
of the various regions of the electromagnetic spectrum.
Wavelength
(m)
Frequency
(Hz)
Energy
(J)
Radio
> 1 x 10-1
< 3 x 109
< 2 x 10-24
Microwave
1x10-3-1 x 10-1
3x109-3x 1011
2x10-242x10-22
Infrared
7x10-7-1 x 10-3
3x1011-4x 1014
2 x10-223x10-19
Optical
4x10-7- 7x10-7
4x1014-.5x 1014
3x10-19 5x10-19
UV
1x10-8-4 x 10-7
7.5x1014-3x 1016
5 x10-19 2x10-17
X-ray
1x10-11-1x10-8
3x1016-3x 1019 2 x 10-17-2
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x10-14
Gammaray
< 1 x 10-11
> 3 x 1019
> 2 x 10-14
An electrical signal can be amplified by using a device which allows a small current or voltage to
control the flow of a much larger current from a dc power source. Transistors are the basic device
providing control of this kind. There are two general types of transistors, bipolar and field-effect.
Very roughly, the difference between these two types is that for bipolar devices an input current
controls the large current flow through the device, while for field-effect transistors an input voltage
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provides the control. In this experiment we will build a two-stage amplifier using two bipolar
transistors
Before going into the basic transistor amplifier, there are two terms you should
be familiar with: AMPLIFICATION and AMPLIFIER. Amplification is the process of
increasing the strength of a SIGNAL. A signal is just a general term used to
refer to any particular current, voltage, or power in a circuit. An amplifier is the device
that provides amplification (the increase in current, voltage, or power of a signal) without
appreciably altering the original signal.
Transistors are frequently used as amplifiers. Some transistor circuits are CURRENT amplifiers,
with a small load resistance; other circuits are designed for VOLTAGE amplification and have a
high load resistance; others amplify POWER.
Now take a look at the NPN version of the basic transistor amplifier in figure 2-12 and let's
see just how it works.
So far in this discussion, a separate battery has been used to provide the necessary forwardbias voltage. Although a separate battery has been used in the past for convenience, it is not
practical to use a battery for emitter-base bias. For instance, it would take a battery
slightly over .2 volts to properly forward bias a germanium transistor, while a similar silicon
transistor would require a voltage slightly over .6 volts. However, common batteries do not
have such voltage values. Also, since bias voltages are quite critical and must be held within
a few tenths of one volt, it is easier to work with bias currents flowing through resistors of
high ohmic values than with batteries.
By inserting one or more resistors in a circuit, different methods of biasing may be achieved
and the emitter-base battery eliminated. In addition to eliminating the battery, some of these
biasing methods compensate for slight variations in transistor characteristics and changes in
transistor conduction resulting from temperature irregularities. Notice in figure 2-12 that the
emitter-base battery has been eliminated and the bias resistor RB has been inserted between the
collector and the base. Resistor RB provides the necessary forward bias for the emitter-base
junction. Current flows in the emitter-base bias circuit from ground to the emitter, out the
base lead, and through RB to VCC. Since the current in the base circuit is very small (a few
hundred microamperes) and the forward resistance of the transistor is low, only a few tenths of
a volt of positive bias will be felt on the base of the transistor. However, this is enough
voltage on the base, along with ground on the emitter and the large positive voltage on the
collector, to properly bias the transistor.
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