BASIC ELECTRONIC DEVICES

Electronics is concerned with the theory design, and application of devices to control streams of electrons moving through solids, liquids, gases, and spaces, which are free of air (vacuums). Electronics is a very wide field. It is a specialized branch of the study of electricity.

 

Distinction between Electrical Device and Electronic Device

There is a clear distinction between an electrical device and an electronic device. An electrical device performs any of the two functions of either converting electricity to another form of energy or modifying the properties of electric currents. An electronic device on the other hand performs the basic function of emitting electrons and directing them in particular ways in the internal circuits of appliances. Electrical devices are things such as switches, transformers, generators, transmission lines, motors, pumps, circuit breakers, resistors, capacitors and so forth. Electronic devices are diodes, triodes, transistors, gas discharge tubes, thyristors and so forth. These electronic devices have made it possible to make appliances such as televisions, video cassette recorders, DVD players, video and audio CD players, video games, hi-fi sets, computers, radios, audio cassette players, mobile phones, remote controllers, guitars, organs, pianos, microwave ovens, calculators, broadcast equipment and others.

 

Basic Emission Theory

Electron emission is the release of electrons from the surface of a conductor into the space immediately surrounding it. The operations of a number of electronic devices for example transistors depend largely on electron emission.

Consider a conductor such as a metallic strip or wire. Whenever a voltage is applied across the ends of this conductor a flow of electrons otherwise called electric current will begin in it. Ordinarily this electric current will be confined within the conductor itself. It is however possible through the use of appropriate electron emission processes to cause the electrons flowing along the conductor to escape from its surface into the surrounding space. These processes of electron emission are called thermionic, photoelectric, secondary, and field emissions respectively. The first three have been widely employed, while the last one has not been of much practical use.

 

1) Thermionic emission

Thermionic emission is the release of electrons from a conductor into the surrounding space as a result of heat energy applied to the conductor. In practice, this conductor called emitter is made from molybdenum or nickel (or their alloy) and is coated with a layer of strontium, barium, and calcium oxides. This emitter is heated to very high temperatures. Electrons begin to leave the surfaces of the emitter at 957?C. The higher the temperature, the more the electrons emitted.

 

2) Photoelectricity

A direct relationship exists between light energy and electrical energy. This relationship is broadly referred to as photoelectricity. Photoelectricity is described by three phenomena namely photoelectric emission, photovoltaic effect, and photoconductive effect.

 

a) Photoelectric Emission

This is an effect whereby electrons are given off from the surface of a material as a result of light energy falling on the material.

Photoelectric emission is mostly evident in metals. It is the release of electrons from a surface by shining light energy on it. Although photoelectric emission theoretically can occur from the surface of any conductor only very few conductors have been so used. This is because most conductors do not emit electrons efficiently.

Photoelectric emission occurs when ultraviolet light having very short wavelength is shone on the conductor. Photons of light energy striking the surface of the conductor cause the electrons near the surface to escape. This effect finds application in the phototube; a kind of photo-emissive cell consisting of a special vacuum or gas-filled glass tube in which are placed a photosensitive cathode and an anode acting as an electron collector. When a phototube connected to a circuit, receives direct light energy, electrons are given off at its cathode and they move towards the anode and are absorbed at its surface. This electric current is released for use in the circuit to which the phototube is connected.

 

b) Photovoltaic Effect

This is a situation whereby an electromotive force develops as a result of light energy falling on a region separating two dissimilar materials. The photovoltaic effect is used in photovoltaic cells also called solar cells or solar batteries. A typical solar cell is made up of semiconductor materials. When a solar cell connected to a circuit receives incident light energy, electrons are emitted on its surface and they flow into the circuit. Solar cells produce low currents and are currently not more than 30 per cent efficient at best. They are used in calculators, cameras, watches and others.

 

c) Photoconductive Effect

In this case, an improvement in the electrical conductance of a material is achieved as a result of light energy falling on the material. All the above photoelectric effects are made use of in various photoelectric devices, otherwise called electric eyes. Photoconductive effect is applied in photoconductive cells. In order to be used, a photoconductive cell needs to be connected to a source of electromotive force, such as a battery since it cannot create an electric current like a solar cell or phototube does. When a photoconductive cell is connected to a circuit and light energy falls on it, its electrical resistance reduces and thus it allows more current to flow through it. In all photoelectric devices, the more intense the light energy falling on them, the stronger the electric current they produce or allow to pass through them. These devices are employed in alarms, relays, switches, door openers, camera exposure meters, calculators, spacecrafts, and several other appliances.

 

3) Secondary Emission

In secondary emission, the energy possessed by a primary electron current is used to free electrons from the surface of another material. This is employed in a device called a photo multiplier tube. This tube contains a series of conductors called dynodes arranged in a specific configuration.

In operation, a primary electron is directed at the surface of a dynode. This frees a number of other electrons, which are reflected to another dynode from whose surface more electrons are freed. This process continues with successive dynodes. The result is that after say, the seventh dynode a secondary electron current far larger than the initial primary electron current is obtained.

 

4) Field Emission

Field emission makes use of electric field to free electrons. In this case, a very strong electric field directed at the surface of a conductor imparts enough energy to the electrons close to the surface as to enable them escape into the surrounding space.

 

Principal Electronic Functions

After electrons are emitted, they are controlled by electronic devices for the performance of certain functions. Principal among these functions are amplification, oscillation, and switching. Through the use of various combinations of these functions, it is possible to make a wide range of electronic appliances.

 

A) Amplification

Amplification is the strengthening of an electric signal. The equipment, which performs this function, is called an amplifier. The output signal has same properties as the input signal. However, the output signal is much bigger. An electric signal essentially is an electric current. Normally these signals transmitted from broadcasting stations are weak by the time they reach the receiving antennas of radio and television sets.

Amplifiers inside these sets strengthen these weak signals and thus make them strong enough to be converted into sounds and pictures. An amplified signal may be millions or even billions of times stronger than the original signal. Stereo hi-fi sets, video cassette recorders, CD players, projectors, telephone and communication equipment, intercoms, mobile phones, walkie-talkies and others all make use of amplifiers. Another example is the case of a public address system. Amplifiers in the system, strengthen the signals entering the microphone. As a result, sound coming out of the loudspeaker is magnified several times.

 

B) Oscillation

Oscillation is the changing of a direct current into an electrical signal of desired frequency. The equipment, which does this, is called an oscillator. Electrical signals generated at broadcasting stations have comparatively low frequencies. In order for these signals to travel far, they have to leave the stations at very high frequencies. The higher the frequency, the further the signals will go. Because of this, broadcasting stations make use of oscillators to produce high frequency electric signals needed to convey sound and picture information over great distances. Oscillators inside television and radio sets also help to select the desired channel or station. Oscillators are also used for producing microwaves, which are employed in communications equipment, certain ovens and others.

 

C) Switching

In the field of electronics, switching means directing a signal to one or more destinations. Typically, several destinations are possible. This is accomplished by digital switches. Switching has made it possible to make computers, operate them on the internet and also operate millions of telephones in the same system. A typical digital switch operates by allowing or disallowing a signal to pass through it as the case may be. Another form of switching is rectification. Rectification is the changing of alternating current into direct current. The device that does this is a rectifier, a rectifier allows current to move in one direction only.

The need for rectification lies in the fact that electronic appliances require d.c. to operate their internal circuits, although a.c. is transmitted from the electric power station. Electrical equipment such as light bulbs, electric irons, fans, and heaters can operate on a.c. However, electronic equipment such as televisions, radios, stereo sets, and videocassette recorders, video and audio CD players have rectifiers which first convert the incoming a.c to d.c before being routed through their internal circuits. Battery-powered flashlights, calculators, watches, cameras and so forth do not need rectifiers because batteries supply d.c.

 

Basic Electronic Devices and their Uses

An electronic device is an object, which emits and controls the flow of electrons in a desirable manner. Electronic devices constitute the principal components of appliances such as televisions, radios, stereo sets, video sets, video and audio CD players, computers etc. Any of these appliances usually contains from a few hundreds to millions of electronic devices. In fact, computers and workstations of large capacities contain billions of electronic devices.

Electronic devices range from various earlier-invented vacuum tubes to the more recent transistors. Their sizes also vary from the relatively bulky light emitting diodes (LED’s) to the tiny transistor. When these electronic devices are electrically connected, they constitute an integrated circuit. Integrated circuits are borne by microchips. A microchip as big as the tip of a child’s thumb can contain as many as a thousand transistors.

 

A) Vacuum Tubes

A vacuum tube is a sealed and almost air-less glass or metallic enclosure in which is placed specially designed conductors, used for the purpose of controlling electronic signals. These electronic signals consist of streams of electrons moving between the conductors, called electrodes in the vacuum tube.

A typical vacuum tube has two basic electrodes namely cathode and anode. Because electrons are released at the surface of the cathode and are absorbed at the surface of the anode, both are respectively called emitter, and collector. In some vacuum tubes, a third electrode called grid is also situated between the anode and the cathode. The function of the grid is to control the amount of electrons flowing through the vacuum tube. A heater is also encased in the tube.

When in use, voltage applied across the terminals of the vacuum tube develops a negative charge at the emitter and a positive charge at the collector. As the emitter is heated, it gives off electrons, which are repelled towards the collector at whose surface they are absorbed. The grid will allow or prevent electrons to pass depending on the strength of the negative charge on the grid.

There are several kinds of vacuum tubes of which diodes and triodes are among the most common.

i) Diode

A diode has only one pair of emitter and collector. It does not have a grid. It is mainly used as a rectifier in electronic circuits. Whenever an a.c is applied to a diode, the device will only allow electric current to pass through it at instances when the charge on the emitter is negative. Because of this, the current, which passes out of the diode, comes out as direct current.

Diodes are grouped according to their functions in circuits, or according to the materials from which their electrodes are made. In the first group belong rectifier diodes, light emitting diodes (LED’s), photo diodes etc. In the second group belong silicon diodes and germanium diodes.

 

ii) Triode

A triode has three electrodes namely emitter, collector, and grid. In electronic circuits, triodes are principally used either as amplifiers or oscillators.

When used as an amplifier, an electric current is connected to the emitter while a much weaker electric signal is applied to the grid. The properties of the week signal from the grid are then passed to the stream of electrons flowing from the emitter to the collector. The result is that a much stronger but similar electric signal comes out from the terminal of the collector.

When used as an oscillator, part of the current flowing between the emitter and collector is diverted to the grid. This results in a current of higher frequency.

 

B) Transistors

Transistors are electronic devices, which are quite recent. They are much smaller than the earlier-invented vacuum tubes. Whereas a vacuum tube controls the flow of electrons in an almost airless space, a transistor controls the flow of electrons in semiconductor materials. A semiconductor material is one, which is not electrically conductive or insulative enough as to be grouped with conductors or insulators. Silicon, germanium, selenium, and gallium arsenide are examples of semiconductors.

The principal function of transistors in electronic circuits is amplification. Whenever an a.c is fed into a transistor, the output of this device will be an a.c having the same properties but much bigger than the input.

Most transistors are made from crystals of silicon. Thus, the finished product is sometimes called silicon chip. In making a transistor, minute amounts of certain impurities are added to the silicon crystals. Depending on the type of transistor, these impurities either enhance the flow of electric current by freeing more electrons, or retard the flow of electric current by hindering the liberation of electrons, within the transistor. In the latter case, a deficiency of electrons called holes is induced in the silicon.

If the silicon contains mostly holes, it is designated as p-type while if it contains more free electrons it is designated as n-type. In p-type semiconductor, electric current is defined as the movement of holes, while in n-type semiconductor, it is defined as flow of electrons. A transistor consists of strips of n-type and p-type semiconductor materials. Lead wires are connected to these materials and these constitute terminals through which electricity enters or leaves the transistor. The whole arrangement is encased for protection against damage.

There are two broad types of transistors namely the bipolar junction transistors (BJT) and the field effect transistors (FET). Although there are differences in construction and mode of operation of both types, they however perform the same basic function of amplifying electronic signals.

 

1) Bipolar Junction Transistors (BJT)

A BJT consists of a strip of either ptype or n-type semiconductor material placed between and in contact with two thicker strips of the opposite type. Thus, two forms of BJT are possible. These are the n-p-n and p-nap transistors. In both, the middle strip is the base while one of the outer strips is the emitter and the other the collector. In an n-p-n transistor, electrons flow from the emitter through the base to the collector while in a p-n-p transistor, it is the holes which move through the base.

 

2) Field Effect Transistors (FET)

Whereas a BJT has three strips of semiconductor materials, an FET has two strips. One of the strips is called channel while the other is called gate. In a circuit electric current flows through the channel but it is controlled by the voltage connected to the gate. The channel and the gate are made of opposite semiconductor materials. If one is n-type, the other is p-type and vice versa.

 

iii) Gas Discharge Tubes

Essentially, a gas discharge tube consists of a sealed glass containing some quantity of gas and at least two electrodes, called the emitter and collector. There are several types of gas discharge tubes of which some are fluorescent lamps, neon lamps, mercury vapour lamps, high pressure sodium lamps, low pressure sodium lamps, and metal halide lamps.

In operation, a sufficiently large voltage applied to the terminals of a gas discharge tube causes electrons to leave the surfaces of the emitter and travel to the collector at whose surfaces they are absorbed. The emitter and collector are located at opposite ends, inside the tube. Some discharge tubes also contain a third electrode called grid. A grid is a means by which the flow of electrons inside the tube can be controlled. A voltage applied to the grid interferes with the movement of the electrons. As electrons travel through the tubes, they collide with the atoms of the gas inside the tube, and electromagnetic radiation is given off.

The tube of a fluorescent lamp contains a mixture of argon gas and mercury vapour under low pressure. Collision of electrons with the atoms of these gases results in ultraviolet radiation being given off. Ultraviolet radiation is not visible. Because of this, the interior of the tube of a fluorescent lamp is coated with substances called phosphors. As ultraviolet radiation strikes these phosphors, visible light is given off from the tube.

A neon lamp operates like a fluorescent lamp, although its tube is filled with neon gas. The electromagnetic radiations given off by some neon lamps can pass through opaque (non-transparent) materials for example, paper. For this reason, neon lamps are used in banks, and supermarkets to detect fake paper currencies. The colour of light emitted by a gas discharge tube depends on the type of gas inside it and also in some cases, the substance used for coating the interior of the tube.

Gas discharge tubes and vacuum tubes belong to a very large group of electronic devices called electron tubes. One equally important electron tube is the cathode ray tube. This tube uses a stream of electrons to trace a picture or pattern on a glass screen. Important applications of the cathode-ray tube are the picture tube of a television set, computer monitor, and visual display screens in several other appliances.