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Electronic devices include diodes, transistors and X-ray tubes.
Electronics, the branch of science and technology concerned with the nature, uses, and manufacture of devices in which electrons flow through a gas, vacuum, or semiconductor. All electronic devices are electrical—that is, they use or produce electric energy. Electronic devices are unique in that the flow of electricity in them can be controlled by electrical rather than mechanical means. Examples of electronic devices include diodes, transistors, and X-ray tubes.
Electronics occupies a central place in day-to-day activities in the developed countries of the world. For example, electronic systems are essential to many major industries—such as banking, telecommunications, and transportation—as well as to the government and armed forces.
Electronic devices are fundamental parts of a vast assortment of products, from hearing aids and digital watches to industrial robots and aircraft navigational instruments. Electronic devices are used in the design, manufacture, sale, and repair of a wide variety of products—including electronic devices themselves. Many forms of entertainment are provided by such electronic products as television sets, stereo systems, and videotape recorders.
One of the most noteworthy characteristics of modern electronic devices is that they can be made very small. The branch of electronics concerned with the miniaturization of electronic devices is called microelectronics. Microelectronics has made possible the development of personal electronic computers and many other kinds of electronic products.
In terms of the number of persons employed and in the value of production, the manufacture and sale of electronic devices is one of the major industries of the United States, Japan, South Korea, Great Britain, and many other countries. Some companies specialize in producing electronic parts; others, in the assembly of these parts into finished products; and yet others, in the sale and repair of electronic products.
History of Electronics
The early history of electronics is closely tied to experimentation with the Crookes tube, a type of vacuum tube developed by Sir William Crookes, an English physicist and chemist. While performing experiments with a Crookes tube, Wilhelm Konrad Roentgen, a German physicist, discovered X rays in 1895. In 1897, Ferdinand Braun, another Germany physicist modified the Crookes tube to make the first oscilloscope, an instrument that produces a visual image of an electric signal.
Interest in improving the reception of radio waves led to the invention of the vacuum-tube diode in 1904 by Sir John Fleming, an English electrical engineer, and to the invention of the vacuum-tube triode in 1907 by Lee De Forest, a United States inventor. The invention of the triode was a key event in the history of electronics, since it was the first electronic amplifier.
During World War I there was an increased interest in developing radio and electronics, and by 1920 the development of vacuum tubes and circuits employing them had advanced to the point where their superiority over all other devices used in radio transmitters and receivers was apparent. Regular commercial radio broadcasting in the United States began in 1920, and the demand for household receivers soon made electronics an important industry.
Certain technical limitations in the operation of electron tubes were overcome with the development of the pentode in 1929. The advances being made at this time helped lead to the development of television; the first regular television broadcasting began in 1936, in London.
During World War II, emphasis was placed on the development of electronics for military use. Radar was greatly improved and in 1944 the first large electronic digital computer, ENIAC, was built. The main purpose of the computer was to speed up the calculation of tables of data for aiming artillery. The electronics industry emerged from the war as a major industry. Its growth following the war continued as television manufacturing entered a boom period and military programs demanded more advanced electronic technology.
In 1948 William Shockley, John Bardeen, and Walter H. Brattain of Bell Telephone Laboratories developed the first transistor, a forerunner of the bipolar junction transistor. During the early 1950's the technology was developed to mass-produce transistors. The advantage of semiconductor devices over electron tubes created a demand for techniques to further reduce the amount of space required for electronic components. An important step toward miniaturizing electronic components was the introduction of the integrated circuit in the early 1960's. The techniques necessary to fabricate such circuits were pioneered by Jack Kilby of Texas Instruments in 1959.
During the 1970's and 1980's the size of the components of integrated circuits continued to be reduced and the number of components that could be produced on each chip grew rapidly. With increasing miniaturization, the capabilities of the electronic circuits and the speed at which they could perform their functions greatly increased. Each advance helped reduce the cost of producing electronic products.
Through the 1980's and into the 1990's, the variety of products being built with electronic components increased, and the use of electronic control devices led to greater automation. Microelectronics led to the development of new technologies, such as digital audio recording; to the introduction of new products, such as personal computers; and to the reduction in the size of portable telephones and many other electronic products.
Basic Principles
The operation of electronic devices depends on a fundamental property of matter called electric charge. There are two kinds of electric charge: positive and negative. Atoms contain both positively charged particles (protons) and negatively charged particles (electrons). In electrical devices an electromotive force, or voltage, is created that causes electrically charged particles to move. The movement of charged particles is called an electric current. In electronic devices and in most other electrical devices, the current consists of a flow of electrons; in other electrical devices, such as batteries, the current consists of a flow of ions (atoms lacking or having an excess of electrons).
In most nonelectronic devices, electricity supplies energy for such purposes as heating or lighting, or running electric motors. In most electronic devices, electricity is used to represent and convey some type of information. The information may be simple, such as an indication that the door of an automobile is open, or it may be complex, such as the sounds produced by a group of musical instruments. The changes in voltage or current that are used to represent information are called electrical signals.
An analog electronic device works with continuously varying electrical signals. These signals are typically used to represent quantities that vary continuously. For example, the radio waves used in AM radio have continuously varying amplitudes that represent the varying pitch and loudness of a sound, and television signals have continuously varying frequencies to represent the varying patterns of brightness and darkness of a scene.
A digital electronic device works with sequences of pulse-like signals. These signals are coded to represent numbers, making them especially useful for operations that require numerical calculations, as in a digital computer. In many digital electronic devices, a series of numerical values can be used to represent a continuously varying quantity. For example, the sound recorded on an audio compact disc is recorded digitally.
The representation of the way an electric current or voltage varies with time is called a waveform. Analog signals generally have smoothly varying waveforms called sine waves. Digital signals generally have rectangular waveforms, rising and falling abruptly between two levels.
In general, electronic systems are designed to detect various kinds of signals. They then modify these signals or produce new signals based on the signals they detect. For example, a compact disc player detects the variations in the light reflected from the surface of a rotating disc. Using this information, it produces an electric current that re-creates the sound by means of earphones or loudspeakers.
One of the most important functions performed by electronic systems is the strengthening, or amplification, of weak signals. In a sound system, for example, a compact disc player produces only a weak signal; through amplification, the system can produce the much stronger signals required by earphones or loudspeakers.
Other important functions carried out by electronic systems include rectification, the conversion of an alternating current to a direct current; oscillation, the production of various kinds of regular waveforms from a direct current; and switching operations called logic functions.
Basic Electronic Components
Some basic components of electronic circuits are active components; they can increase the power of an electrical signal because they are powered by a source of electricity separate from the electrical signal. Transistors and oscillators are examples of active components. Other components are passive components; they help give a circuit its electrical characteristics, but do not require a separate source of electric power for their operation. Passive components include resistors, inductors, and capacitors. Resistors are important in controlling the voltage in different parts of a circuit. Capacitors temporarily store electric charge and help oppose voltage changes in a circuit. Inductors store energy in magnetic fields and help oppose changes of current.
Active components in turn are divided into two fundamental groups: (1) semiconductor, or solid-state, devices; and (2) electron tubes. The operation of semiconductor devices depends on the behavior of electrons in a semiconductor, a material whose ability to conduct electricity is between that of a conductor and that of an insulator. Most electronic devices in use today are semiconductor devices. The operation of electron tubes depends on the behavior of electrons moving through a vacuum or gas inside a closed container.
The most common semiconducting material used in electronics is silicon to which small amounts of certain other chemical elements have been added. The addition of these elements, a process called doping, improves the degree to which a semiconductor's ability to conduct electricity can be controlled.
Doping is used to produce either of two types of semiconductors: n-type or p-type semiconductors. An n-type semiconductor contains a large number of electrons that are free to move through it. A p-type semiconductor contains a large number of holes, sites into which an electron can move.
When an electron from a nearby atom moves into a hole, it leaves a hole at its former position; this action is the same as if the hole moved from one point to another. (When a free electron moves into a hole, the electron ceases to be free and the hole ceases to exist.) Both free electrons and holes are charge carriers—that is, under the influence of a voltage, they give rise to a flow of electric charge.
The operation of most semiconductor devices depends on the electrical properties of a pn junction, the boundary between an n-type semiconductor and a p-type semiconductor. The electrical properties of a pn junction are discussed later in this section under the subtitle Semiconductor Diodes.
Various kinds of protective cases are used to house semiconductor devices. These cases are made of metal, plastic, or a ceramic material. Wires or pins that project from the case provide electrical connections to other parts of an electric circuit.
An electron tube is essentially a sealed hollow enclosure in which the movement of electrons can be carefully controlled. The enclosure is typically made of glass and contains various metal parts called electrodes for producing and regulating a beam of electrons.
An electron tube from which all gases have been removed is called a vacuum tube. In most types of vacuum tubes, one of the electrodes must be heated to emit electrons. (This emission is called thermionic emission.) The most important types of vacuum tubes are the cathode-ray tube and the X-ray tube. Such vacuum tubes as the triode, the pentode, and the vacuum-tube diode were once important, but they have been almost entirely replaced by comparable semiconductor devices that are smaller and more durable. In addition, vacuum tubes consume much more electric energy than semiconductor devices because they require electrical heating for thermionic emission.
In some electron tubes, the enclosure is filled with a gas such as mercury vapor or neon. Such gas tubes are important sources of light. They include fluorescent lamps, neon lamps, and electronic flashtubes.
Semiconductor Diodes
Semiconductor, or solid-state, diodes are relatively simple devices. They have two terminals. Their principal function is to allow current to flow in only one direction. A device that performs this function is called a rectifier.
Most semiconductor diodes are junction diodes. A junction diode consists of a small single crystal of silicon that has been doped in such a way that one side is a p-type semiconductor and the other side an n-type semiconductor. The electrical properties of the pn junction where the two types of material meet give the diode its ability to function as a rectifier.
Despite the presence of holes in a p-type semiconductor and the presence of free electrons in an n-type semiconductor, both materials are normally electrically neutral—that is, the number of protons in each material is balanced by an equal number of electrons. Near a pn junction, however, free electrons from the n-type region pass into the neighboring p-type region, where they combine with holes and form negative ions. Holes from the p-type region pass into the n-type region, where they combine with free electrons and form positive ions. These actions leave the p-type material near the junction with a negative charge and the n-type material near the junction with a positive charge. The redistribution of electric charge creates an electrical barrier that opposes any further movement of charge carriers across the junction.
When a voltage is applied across the diode so that the p-type region is at a higher voltage than the n-type region, the electrical barrier across the junction is reduced. The application of such a voltage across a pn junction is called a forward bias. Free electrons from the n-type region and holes in the p-type region are drawn toward the junction, where they unite. The diode therefore conducts an electric current, since this process will continue indefinitely and electrons will continually enter one end of the diode and leave the other.
The application of a reverse voltage across a pn junction is called a reverse bias. With a reverse bias, the electrical barrier at the junction is raised and it blocks the passage of both free electrons from the n-type region and holes from the p-type region. Under these conditions, there is no flow of charge, and the diode does not conduct.
A Schottky diode is formed by placing a metal in contact with an n-type semiconductor. The junction formed in this manner is especially useful in high-frequency circuits such as high-speed logic circuits.
This type of diode provides the circuit with capacitance, an electrical property that allows a device to store electric charge. In a circuit, capacitance works to resist changes in voltage. The value of the capacitance is controlled by the amount of reverse bias applied to the diode. Varactor diodes are useful in tuning circuits.
If a high reverse voltage is applied to a semiconductor diode, the electrical insulating barrier formed by the pn junction will break down and the diode will suddenly conduct a large current. A zener diode is doped in such a way that it will begin conducting at a specific, relatively low reverse voltage. This voltage is called the diode's breakdown voltage. Zener diodes are useful in regulating the voltage in a circuit and for protecting circuits from excessive voltage.
Some diodes are used to detect light or to generate electricity from light. For information about these types of diodes, see the subtitle Photoelectric Cell. Semiconductor lasers and light-emitting diodes (LED's) are diodes used as sources of light.
Types of Transistors
A transistor is a semiconductor device that contains three electrical terminals. One of the terminals serves to control the voltage or current between the other two. Transistors are used primarily to amplify electrical signals or to act as switches in electronic circuits. Transistors can be found in almost every piece of modern electronic equipment.
A transistor forms part of two separate circuits, both of which have a source of electric power. The electrical properties of the transistor are such that a small change in the current or voltage of one of the circuits produces a very large change in the current or voltage of the other. When a transistor is used as an analog device, the change in the second circuit is proportional to the change in the first. When a transistor is used as a digital circuit, the first circuit essentially serves as a switch that turns the second circuit on or off.
A bipolar junction transistor contains three layers of semiconductors. An npn transistor is formed by a thin layer of p-type semiconductor between two layers of n-type semiconductor; a pnp transistor is formed by a thin layer of p-type material between two layers of p-type material. The operation of both types is conceptually similar, so the operation of only one, the npn transistor, will be discussed here.
In an npn transistor, one n-type region is called the emitter and the other the collector; the p-type region is called the base. The terminals are connected to DC power sources in such a way that the base is kept at a higher voltage than the emitter, and the collector is kept at a higher voltage than the base. With this arrangement, a current readily flows from the emitter to the base (the pn junction between them is forward biased), but is blocked from flowing between the base and the collector (the pn junction between them is reversed biased).
As current flows between the emitter and base, free electrons enter the base from the emitter. Because the collector is maintained at a higher voltage than the base, these electrons are then readily drawn from the base into the collector. The base is made to be extremely thin and with a low concentration of holes so that most of the free electrons that enter the base from the emitter will be drawn into the collector. In this way, a small current in the external circuit to which the emitter and base are connected will give rise to a much larger current in the circuit to which the emitter and collector are connected.
The operation of a bipolar junction transistor typically consists of controlling the current in the emitter-collector circuit by varying the amount of current that flows in the emitter-base circuit. Weak input signals applied to the emitter-base circuit are amplified, producing much stronger output signals in the emitter-collector circuit.
In a field-effect transistor, the output current is controlled by an electric field that is varied by changing the voltage of one of the transistor's terminals. A field-effect transistor requires much less input current than a bipolar junction transistor.
There are two major types of field-effect transistors: junction-gate field-effect transistors and metal-oxide semiconductor field-effect transistors (MOSFET's). MOSFET's are the most widely used transistors today.
The operation of a typical MOSFET can be explained with the assistance of the illustration Field-effect Transistor. The three terminals, made of metal, are called the source, gate, and drain. The source and drain are connected to two separate regions of n-type semiconductor. The two n-type regions are separated from each other by a region of p-type semiconductor. Along one side of the p-type region there is a thin layer of silicon dioxide. The layer serves as a support for the gate terminal and it insulates the gate from the p-region.
In this type of MOSFET, a current cannot normally flow between the source and the gate because of the n-type region between them. However, when a positive voltage is applied to the gate, the electric field the gate produces extends through the silicon dioxide layer. The field draws electrons from the p-type region into a thin channel along the oxide layer. The channel forms a conductive path in the p-type region between the source and the drain and allows electric charge to flow between them.
A small change in the voltage applied to the gate will give rise to a large change in the current between the source and the drain. A MOSFET is typically used as a switch: either no voltage is applied to the gate so that the transistor completely blocks any current in the source-drain circuit, or a voltage is applied to the gate that will effectively eliminate any resistance to the current in the source-drain circuit.
Types of Cathode-ray Tubes
A cathode-ray tube (CRT) is a vacuum tube in which one or more beams of electrons are used to form an image on a relatively flat glass screen at one end of the tube. The image is essentially a visual record of a varying electrical signal applied to the tube. CRT's are commonly used as picture tubes for television sets and as computer monitors. They are also used for radar displays, oscilloscope displays, and other applications.
Each beam of electrons in a cathode-ray tube is produced by an electron gun. An electron gun contains a negative electrode called the cathode, which serves as a source of the electrons; and a positive electrode, called an anode, which accelerates the electrons. A negatively charged element called the control grid controls the intensity of the beam.
The electron beam is then focused so that it will come to a point on the screen at one end of the tube. The location at which the beam strikes the screen is controlled by bending the beam vertically and horizontally. In some cathode-ray tubes, such as those used in oscilloscopes, the beam is bent as it passes between pairs of charged metal plates that exert electrostatic forces. In most cathode-ray tubes, the beam is bent by electromagnets mounted around the central part of the tube. Both types are shown in the illustration Cathode-ray Tubes.
The inner face of the screen is covered with a coating of substances called phosphors; the phosphor coating glows at the spot where it is struck by the electrons from the electron gun. A visual image is formed as the beam of electrons passes across the screen; the brightness of each spot struck by the beam is controlled by varying the intensity of the beam. Because the screen is transparent, the image formed is visible from outside the tube.
Microwave tubes are vacuum tubes that produce extremely short radio waves called microwaves. Travelling-wave tubes and klystrons are two other types. Microwave tubes are used in microwave ovens as well as in radar transmitters and other kinds of equipment.
Vacuum tubes that are used to produce a type of penetrating radiation called X rays are called X-ray tubes. Such tubes contain two electrodes; the X rays are produced by bombarding a metal target on one of the electrodes with high-energy electrons from the other electrode. The voltage between the electrodes must be large, ranging from tens of thousands to several million volts, depending upon the intended use of the X rays.
Thyristors are semiconductor devices with three terminals and four or more alternating layers of p-type and n-type semiconductors. Thyristors are typically used as switches in AC high-power circuits—circuits designed to handle large currents or voltages. Because they contain several pn junctions, thyristors can withstand much higher reverse voltages and currents than transistors and semiconductor diodes can.
During operation, a thyristor normally blocks an electric current. However, a voltage applied to one of the thyristor's terminals (the gate) will cause the thyristor to begin carrying a current. Most thyristors will continue to conduct the current until the current reverses direction. The amount of current that a thyristor conducts can be controlled by turning on the thyristor at a specific point in each cycle of the alternating current.
The silicon-controlled rectifier (SCR) is one of the most common kinds of thyristors. One use of the SCR is to provide the DC motors of an electric locomotive with a regulated amount of DC power from overhead AC power lines. An SCR allows current to flow in only direction. The triac is a similar device but can be turned on to allow current to flow through it in either direction.
Photoelectric Cells
Photoelectric cells, or photocells, are devices whose electrical characteristics vary according to the amount of light that strikes them. There are two basic types made from semiconductors: photovoltaic cells and photoconductive cells. A phototube, an older type of photoelectric cell, is a type of electron tube.
A photovoltaic cell is a semiconductor device that converts light into electric energy. The photovoltaic cells in a solar battery typically consist of silicon wafers treated to form an n-type semiconductor covered by a very thin layer of p-type material. When light strikes the wafer, it creates pairs of holes and free electrons. Some of the light penetrates the surface of the wafer and creates holes and free electrons near the pn junction. The electric field that naturally exists across the junction tends to push electrons into the n-type region and holes into the p-type region. If the two regions are connected through an external circuit, electrons will flow from the n-type region through the external circuit to the p-type region.
Photoconductive cells are light-sensitive semiconductor devices that do not have a pn junction. When light strikes the photoconductive cell, it creates free electrons and holes, lowering the semiconductor's electrical resistance. Although the cell does not generate electricity, it can be used with an external power source to produce an electric signal because the resistance of the semiconductor changes in proportion to the amount of light that strikes it. Photoconductive cells are used in certain types of exposure meters in light switches that turn on automatically at nightfall, and in other devices.
Phototubes are vacuum tubes that conduct a current only when light strikes them. Phototubes have been used in a variety of scientific instruments and other devices, but have been largely replaced by photoconductive cells, which perform similar functions.
Triodes are vacuum tubes that, with the proper circuitry, will produce amplification of an electrical signal. They are little used today, but are historically important. A triode contains three electrodes—a cathode, grid, and anode—sealed in an evacuated container. The grid is a cylindrical screen made of metal mesh that surrounds the cathode. The anode is a cylindrical metal plate that encloses both of the other elements.
During the operation of a triode, the cathode is heated to emit electrons. The heat is provided by an encircling wire filament that becomes hot when a current is sent through it. The plate is always operated at a higher voltage than the cathode, so it will attract the electrons emitted by the cathode.
The movement of the electrons between the cathode and plate is regulated by varying the voltage on the grid. When the grid is held at a voltage lower than that of the cathode, it blocks the flow of electrons from the cathode to the plate. As the voltage of the grid approaches that of the cathode, fewer electrons are blocked and the triode current increases. At voltage levels just below the cathode voltage, very small changes in the grid voltage produce very large changes in the triode current. In this way, weak signals fed to the grid can be amplified by the cathode-plate circuit.
Image-recording electronic components are important in video cameras and a number of other electronic applications. These components include various kinds of picture tubes and semiconductor devices called charge-coupled devices (CCD's).
Lasers and masers are electronic devices that produce beams of very uniform electromagnetic waves. A laser produces a beam of light; a maser, a beam of microwave radiation.
In addition to the triode, there are various other kinds of electron tubes that were widely used before the development of semiconductor devices. These devices include the vacuum-tube diode (whose function is similar to that of a semiconductor diode), the pentode (a five-electrode vacuum tube whose function is similar to that of a transistor), and the thyratron (a vacuum tube whose function is similar to that of a thyristor).
Although an electronic product may contain a large number of components, or parts, the number of different kinds of components is relatively small. The same basic components can be combined in specific ways to perform a great diversity of functions. This section describes many of the various fundamental electronic devices and circuit elements that serve as basic electronic components.
Basic Electronic Devices
A device that supplies electricity to an electric circuit is called a power supply. Portable electronic devices are typically powered by batteries, which provide direct current. Household current is alternating current, and the power supply of electronic equipment designed to operate on household current must convert it into direct current. In addition, the power supply provides the current at one or more specific voltages. A power supply performs these functions with a rectifier, filter, and voltage regulator. Most power supplies also contain a transformer to increase or decrease the voltage of the current.
A typical diode can rectify an alternating current—that is, it is able to block part of the current so that it will pass through the diode in only one direction. However, in blocking part of the current, the diode reduces the amount of electric power the current can provide.
A full-wave rectifier is able to rectify an alternating current without blocking any part of it. The voltage between two points in an AC circuit regularly changes from positive to negative and back again. In the full-wave rectifier shown in the illustration on this page at lower left, the positive and negative halves of the current are handled by different pairs of diodes. When the input signal to the rectifier is positive, point x is at a higher voltage than point y. This situation creates a reverse bias in diodes 2 and 3 and a forward bias in diodes 1 and 4. Therefore, current flows only through diodes 1 and 4. When the input to the rectifier is reversed, the input signal becomes negative (drops below the horizontal line) and the voltages at x and y are reversed. This situation creates a reverse bias in diodes 1 and 4 and a forward bias in diodes 2 and 3, so only diodes 2 and 3 conduct.
The output signal produced by the full-wave rectifier is a DC voltage, but it pulsates. To be useful, this signal must be smoothed out to produce a constant voltage at the output. A simple circuit for filtering the signal is one in which a capacitor is in parallel with the output. With this arrangement, the capacitor becomes charged as the voltage of the signal produced by the rectifier increases. As soon as the voltage begins to drop, the capacitor begins to discharge, maintaining the current in the output. This discharge continues until the increasing voltage of the next pulse again equals the voltage across the capacitor.
A voltage regulator is used to help keep the output voltage at a desired level. A simple type of voltage regulator consists of a resistor in series with and a zener diode in parallel with the output of the power supply. The diode is used with a reverse bias above its breakdown voltage so that it will conduct a reverse current. In this mode of operation, a very small change in voltage across the diode causes a very large change in the current through the diode. Should the voltage at the input to the regulator increase, the diode conducts more current and the current through the resistor increases. This action increases the voltage drop across the resistor in such a way that the voltage at the output remains the same.
An amplifier is an electronic device that magnifies an electrical signal by increasing its voltage or current. Amplifiers are essential components of most electronic devices. For example, a radio receiver contains one or more amplifiers to boost the strength of the feeble signal generated by radio waves in an antenna and to produce a signal that can power an earphone or loudspeaker.
In virtually all amplifiers in use today, the signal to be amplified is fed to a transistor. In the transistor, the signal imposes its waveform or some other characteristic onto a stronger electric current, which is produced by the amplifier's power supply.
Amplifiers are grouped into several classes based on the portion of the input signal they reproduce. A class A amplifier conducts for the entire signal; a class B amplifier, for only the positive half of the signal; and a class C amplifier, for only the peaks of the signal. Class A amplifiers are used when a signal needs to be reproduced as accurately as possible, such as in audio equipment. Class C amplifiers are the most efficient of the three; they can produce a large amount of amplification but with distortion. Class B amplifiers are more efficient than Class A amplifiers, but produce less distortion than Class C types.
In many amplifiers part of the output signal is redirected to the input as, feedback. Feedback is used in such a way that it helps in adjusting the input signal. Negative feedback is out of phase with the input and therefore decreases the output signal. It is used to help maintain the output of the amplifier within desired levels. Positive feedback is in phase with the input and therefore increases the output signal. It is used to maximize the output of the amplifier.
Op Amps, are amplifiers that provide a high level of amplification. Operational amplifiers are so named because they can be used to perform mathematical operations using analog signals. Operational amplifiers are linear devices—that is, their output varies directly with their input. A common type, the differential operational amplifier, will produce a signal whose strength is proportional to the difference in the strength of two input signals. Such amplifiers are used in controlling the operation of automation equipment.
An oscillator is an electronic circuit that produces a regularly varying electrical signal from a direct current. Oscillators are at the heart of all radio, television, and radar transmitters. They also play a vital role in the operation of most radio and all television receivers. In a computer, an oscillator circuit called a clock produces regularly spaced pulses that coordinate the operations performed by logic circuits. Such pulses are also used for keeping time in electronic watches.
One common type of oscillator contains a tank circuit, a circuit formed by an inductor and capacitor. Such a circuit produces an oscillating signal whose frequency depends on the capacitance and inductance of the circuit. Within the circuit a DC power supply and a transistor are used to maintain the strength of the oscillating current by means of positive feedback.
Some oscillators contain variable capacitors, which permit the frequency of the oscillations to be modified. A quartz-controlled oscillator contains a quartz crystal to help eliminate any variations in the frequency of the signal.
Types of Circuits
Logic circuits are essentially arrays of switches called logic gates. In general, a switch is a device whose inputs and outputs have a limited number of distinct states, usually "on" or "off." In many logic circuits, these states are referred to as high or low, because the state corresponds to either a high or a low voltage level. Logic circuits are essential for the operation of computers and most other digital equipment.
A major advantage of electronic switches is that their "on" and "off" states can be changed very quickly, in as little as one billionth of a second. Computations in digital electronic computers and other digital devices may involve complex arrays of logic gates and an enormous number of steps. The time required to perform such switching operations must be small or the time required for a calculation would become prohibitively long.
Logic circuits can be built from a variety of electronic components. Circuits built from compatible types of components are referred to as logic families. Two of the most important such families are TTL (transistor-transistor logic), which uses combinations of bipolar transistors to build logic gates; and CMOS (complementary metal-oxide silicon), which uses combinations of metal-oxide field-effect transistors. MOSFET's are particularly suited for use in integrated circuits because they consume little power.
Most logic gates are switched on or off depending on the value of two inputs. For example, an AND gate will produce an output signal only if it receives a signal from all of its inputs. An OR gate will produce an output signal if it receives an input signal from any of its inputs.
For many applications logic circuits are needed that will retain an "on" or "off" state. The simplest such circuit is a two-transistor circuit called a flip-flop. Large arrays of flip-flops and other types of logic circuits are used to form memory circuits— circuits that hold digital information. These arrays are built as integrated circuits. There are two basic kinds: RAM (random-access memory) and ROM (read-only memory).
A microprocessor is a highly complex integrated circuit that contains a large number of logic gates. The gates are organized into various specialized subcircuits for receiving, processing, storing, and relaying data (information) in the form of digital signals. Such logic circuits can be programmed to work with the data in predetermined ways, serving as a computer's central processing unit.
All but the simplest electronic products are made up of a combination of circuits, each designed to perform a specific function. The same basic circuit can be used in a wide variety of products; for example, the power-supply circuit of an audio amplifier is essentially the same as that of a personal computer. Some circuits are designed as self-contained devices, forming units that can be easily interconnected in assembling an electronic product and easily replaced if they become defective.
In conventional circuits, the active and passive components are individual parts that are connected by wires. The connections are greatly simplified in a printed circuit, a circuit in which the components are mounted on a stiff board and connected by strips of metal deposited on the board by a printing process. In integrated circuits, all the various components of the circuit are formed on a single chip of silicon or other semiconductor. Most complex electronic circuits today are built as integrated circuits.
Integrated circuits are very small—their individual components are typically microscopic in size. The entire circuit is generally no more than 1/16 of an inch (1.6 mm) square. Integrated circuits are highly reliable and, compared with the components they replace, inexpensive. Their use reduces the overall size of electronic equipment and helps in designing electronic products as systems made up of separate units, each performing a distinct operation. This construction simplifies manufacturing and maintenance procedures.