I.

Introduction

Electronics, field of engineering and applied physics dealing with the design and application of devices, usually electronic circuits, the operation of which depends on the flow of electrons for the generation, transmission, reception, and storage of information. The information can consist, for example, of voice or music (audio signals) in a radio receiver, a picture on a television screen, or numbers and other data in a computer.

Electronic circuits provide different functions to process this information, either in analogue or digital form. Analogue functions include amplification of weak signals to a usable level; generation of radio waves; extraction of information, such as the recovery of an audio signal from a radio wave (demodulation); and control, such as the superimposition of an audio signal on to radio waves (modulation). Digital functions include the coding, storage, and transmission of information-bearing signals in binary form, and the logic operations and numerical processing performed in computers.

II.

Historical Background

The introduction of vacuum tubes at the beginning of the 20th century was the starting point of the rapid growth of electronics. With vacuum tubes the manipulation of signals became possible, which could not be done with the early telegraph and telephone circuit or with the early transmitters that used high-voltage sparks to create radio waves. For example, with vacuum tubes weak radio and audio signals could be amplified, and audio signals, such as music or voice, could be superimposed on radio waves. The development of a large variety of tubes designed for specialized functions made possible the swift progress of radio communication technology before World War II and the development of early computers during and shortly after the war.

The transistor, invented in 1948, has now almost completely replaced the vacuum tube in most of its applications. Incorporating an arrangement of semiconductor materials and electrical contacts, the transistor provides the same functions as the vacuum tube but at reduced cost, weight, and power consumption and with higher reliability. Subsequent advances in semiconductor technology, in part attributable to the intensity of research associated with the space-exploration effort, led to the development of the integrated circuit. Integrated circuits may contain hundreds of thousands of transistors on a small piece of material and allow the construction of complex electronic circuits, such as those in microcomputers, audio and video equipment, and communications satellites.

III.

Electronic Components

Electronic circuits consist of interconnected electronic components. Components are classified into two categories—active or passive. Passive components include resistors, capacitors, and inductors. Components considered active include batteries, generators, vacuum tubes, transistors, and amplifiers.

A.

Vacuum Tubes

A vacuum tube consists of an air-evacuated glass envelope that contains several metal electrodes. A simple, two-element tube (diode) consists of a cathode and an anode that is connected to the positive terminal of a power supply. The cathode—a small metal tube heated by a filament—frees electrons, which migrate to the anode—a metal cylinder around the cathode (also called the plate). If an alternating voltage is applied to the anode, electrons will only flow to the anode during the positive half-cycle; during the negative cycle of the alternating voltage, the anode repels the electrons, and no current passes through the tube. Diodes connected in such a way that only the positive half-cycles of an alternating current (AC) are permitted to pass are called rectifier tubes; these are used in the conversion of alternating current to direct current (DC; see Electricity; Rectification). By inserting a grid, consisting of a spiral of metal wire, between the cathode and the anode and applying a negative voltage to the grid, the flow of electrons can be controlled. When the grid is negative, it repels electrons, and only a fraction of the electrons emitted by the cathode can reach the anode. Such a tube, called a triode, can be used as an amplifier. Small variations in voltage at the grid, such as can be produced by a radio or audio signal, will cause large variations in the flow of electrons from the cathode to the anode and, hence, in the circuitry connected to the anode.

B.

Transistors

Transistors are made from semiconductors. These are materials, such as silicon or germanium, that are “doped” (have minute amounts of foreign elements added) so that either an abundance or a lack of free electrons exists. In the former case, the semiconductor is called n-type, and in the latter case, p-type. By combining n-type and p-type materials, a diode can be produced. When this diode is connected to a battery so that the p-type material is positive and the n-type negative, electrons are repelled from the negative battery terminal and pass unimpeded to the p-region, which lacks electrons. With the battery reversed, the electrons arriving in the p-material can pass only with difficulty to the n-material, which is already filled with free electrons, and the current is almost zero.

The bipolar transistor was invented in 1948 as a replacement for the triode vacuum tube. It consists of three layers of doped material, forming two p-n (bipolar) junctions with configurations of p-n-p or n-p-n. One junction is connected to a battery so as to allow current flow (forward bias), and the other junction has a battery connected in the opposite direction (reverse bias). If the current in the forward-biased junction is varied by the addition of a signal, the current in the reverse-biased junction of the transistor will vary accordingly. The principle can be used to construct amplifiers in which a small signal applied to the forward-biased junction causes a large change in current in the reverse-biased junction.

Another type of transistor is the field-effect transistor (FET). Such a transistor operates on the principle of repulsion or attraction of charges due to a superimposed electric field. Amplification of current is accomplished in a manner similar to the grid control of a vacuum tube. Field-effect transistors operate more efficiently than bipolar types, because a large signal can be controlled by a very small amount of energy.

C.

Integrated Circuits

Most integrated circuits are small pieces, or “chips”, of silicon, perhaps 2 to 4 sq mm in size, in which transistors are fabricated. Photolithography enables the designer to create tens of thousands of transistors on a single chip by proper placement of the many n-type and p-type regions. These are interconnected with very small conducting paths during fabrication to produce complex special-purpose circuits. Such integrated circuits are called monolithic because they are fabricated on a single crystal of silicon. Chips require much less space and power and are cheaper to manufacture than an equivalent circuit built out of individual transistors.

D.

Resistors

If a battery is connected across a conducting material, a certain amount of current will flow through the material. This current is dependent on the voltage of the battery, on the dimensions of the sample, and on the conductivity of the material itself. Resistors with known resistance are used for current control in electronic circuits. The resistors are made from carbon mixtures, metal films, or resistance wire and have two connecting wires attached. Variable resistors, with an adjustable sliding contact arm, are often used to control volume on radios and television sets.

E.

Capacitors

Capacitors consist of two metal plates that are separated by an insulating material. If the positive terminal of a battery is connected to one of the plates, and the negative terminal to the other, equal and opposite charges will accumulate on the two plates. This will continue until the potential difference between the plates equals the battery voltage. If the battery is disconnected, the capacitor retains the charge and the voltage associated with it, until the charge has slowly leaked away through the insulating material. Rapidly changing voltages, such as those caused by an audio or radio signal, produce larger current flows to and from the plates; the capacitor then functions as a conductor for the changing current. This effect can be used, for example, to separate an audio or radio signal from a direct current in order to connect the output of one amplifier stage to the input of the next amplifier stage.

F.

Inductors

Inductors consist of a conducting wire wound into the form of a coil. When a current passes through the coil, a magnetic field is set up around it that tends to oppose rapid changes in current intensity (see Induction). Like a capacitor, an inductor can be used to distinguish between rapidly and slowly changing voltages. When an inductor is used in conjunction with a capacitor, the voltage in the inductor reaches a maximum value at a specific frequency that is dependent on both the capacitance and inductance. This principle, known as tuning, is used in a radio receiver, where a specific frequency is selected by a variable capacitor.

G.

Sensing Devices and Transducers

Measurements of mechanical, thermal, electrical, and chemical quantities are made by devices called sensors and transducers. The sensor is responsive to changes in the quantity to be measured, for example, temperature, position, or chemical concentration. The transducer converts such measurements into electrical signals, which can be fed to instruments for the readout, recording, or control of the measured quantities. Sensors and transducers can operate at locations remote from the observer and in environments unsuitable or impractical for human beings.

Some devices act as both sensor and transducer. A thermocouple has two junctions of wires of different metals; these generate a small electric voltage that depends on the temperature difference between the two junctions. A thermistor is a special resistor, the resistance of which varies with temperature. A variable resistor can convert mechanical movement into an electrical signal. Specially designed capacitors are used to measure distance, and photocells are used to detect light (see Photoelectric Cell). Other devices are used to measure velocity, acceleration, or fluid flow. In most instances, the electric signal is weak and must be amplified by an electronic circuit.

IV.

Power-Supply Circuits

Most electronic equipment requires power for its operation to be supplied in the form of DC voltage and current. These can be provided by batteries or by internal power supplies that convert alternating current as available at the home electric outlet, into regulated DC voltages. The first element in an internal DC power supply is a transformer, which steps up or steps down the input AC voltage to a level suitable for the operation of the equipment. A secondary function of the transformer is to provide electrical ground insulation of the device from the power line to reduce potential shock hazards. The transformer is then followed by a rectifier, normally a diode. In the past, vacuum diodes and a wide variety of different materials such as germanium crystals or cadmium sulphide were employed in the low-power rectifiers used in electronic equipment. Today, silicon rectifiers are used almost exclusively because of their low cost and their high reliability.

Fluctuations and ripples superimposed on the rectified DC voltage (sometimes noticeable as a hum in a malfunctioning audio amplifier) can be filtered out by a capacitor; the larger the capacitor, the smaller the amount of ripple in the voltage. More precise control over voltage levels and ripples can be achieved by a voltage regulator, which also makes the internal voltages independent of fluctuations that may be encountered at an outlet. A simple, often-used voltage regulator is the zener diode. It consists of a solid-state p-n-junction diode, which acts as an insulator up to a predetermined voltage; above that voltage it becomes a conductor that bypasses excess voltages. More sophisticated voltage regulators are usually constructed as integrated circuits.

V.

Amplifier Circuits

Electronic amplifiers are used mainly to increase the voltage, current, or power of a signal. An ideal linear amplifier would provide signal amplification with no distortion, so that the output was proportional to the input. In practice, however, some degree of distortion is always introduced. A non-linear amplifier may produce a considerable change in the waveform of the signal. Linear amplifiers are used for audio and video signals, whereas non-linear amplifiers find use in oscillators, power electronics, modulators, mixers, logic circuits, and other applications where an amplitude cut-off is desired. Although vacuum tubes played a major role in amplifiers in the past, today either discrete transistor circuits or integrated circuits are generally used.

A.

Audio Amplifiers

Audio amplifiers, such as are found in radios, television sets, citizens’ band (CB) radios, and cassette recorders, are generally operated at frequencies below 20 kilohertz (1 kHz = 1,000 cycles/sec). They amplify the electrical signal, which is then converted to sound in a loudspeaker. Operational amplifiers (op-amps), built with integrated circuits and consisting of DC-coupled, multi-stage, linear amplifiers are popular for audio amplifiers. Some devices of this kind have high-power output stages capable of driving loudspeakers directly. Most, however, need to have their output further amplified before the signal is fed into the loudspeakers.

B.

Video Amplifiers

Video amplifiers are used mainly for signals with a frequency spectrum range up to 6 megahertz (1 MHz = 1 million cycles/sec). The signal handled by the amplifier becomes the visual information presented on the television screen, with the signal amplitude regulating the brightness of the spots forming the image on the screen. To achieve its function, a video amplifier must operate over a wide frequency band and amplify all frequencies equally and with low distortion. See Video Recording.

C.

Radio Frequency Amplifiers

These amplifiers boost the signal level of radio or television communication systems. Their frequencies generally range from 100 kHz to 1 gigahertz (1 GHz = 10⁹ cycles/sec) and can extend well into the microwave frequency range.

VI.

Oscillators

Oscillators generally consist of an amplifier and some type of feedback: the output signal is fed back to the input of the amplifier. The frequency-determining elements may be a tuned inductance-capacitance circuit or a vibrating crystal. Crystal-controlled oscillators offer the highest precision and stability. Oscillators are used to produce audio and radio signals for a wide variety of purposes. For example, simple audio-frequency oscillators are used in modern push-button telephones to transmit data to the central telephone exchange when dialling. Audio tones generated by oscillators are also found in alarm clocks, radios, electronic instruments, computers, and warning systems. High-frequency oscillators are used in communications equipment to provide tuning and signal-detection functions. Radio and television stations use precise high-frequency oscillators to produce transmitting frequencies.

VII.

Switching and Timing Circuits

Switching and timing circuits, or logic circuits, form the heart of any device where signals must be selected or combined in a controlled manner. Applications of these circuits include telephone switching, satellite transmissions, and digital computer operations.

Digital logic is a rational process for making simple “true” or “false” decisions based on the rules of Boolean algebra. “True” can be represented by a 1 and “false” by a 0, and in logic circuits the numerals appear as signals of two different voltages. Logic circuits are used to make specific true-false decisions based on the presence of multiple true-false signals at the inputs. The signals may be generated by mechanical switches or by solid-state transducers. Once the input signal has been accepted and conditioned (to remove unwanted electrical signals, or “noise”), it is processed by the digital logic circuits. The various families of digital logic devices, usually integrated circuits, perform a variety of logic functions through logic gates, including “OR”, “AND”, and “NOT”, and combinations of these (such as “NOR”, which includes both OR and NOT). One widely used logic family is TTL (transistor-transistor logic). Another family is CMOS (complementary metal oxide semiconductor logic), which performs similar functions, but consumes less power. Several other, less popular families of logic circuits exist, including the obsolete RTL (resistor-transistor logic) and ECL (emitter-coupled logic); the latter is used for very high-speed systems.

The elemental blocks in a logic device are called digital logic gates. An AND gate has two or more inputs and a single output. The output of an AND gate is true only if all the inputs are true. An OR gate has two or more inputs and a single output. The output of an OR gate is true if any one of the inputs is true and is false if all of the inputs are false. An INVERTER has a single input and a single output terminal and changes a true signal to a false signal, thus performing the NOT function. More complicated logic circuits are built up from elementary gates. They include flip-flops (binary switches), counters, comparators, adders, and more complex combinations.

To perform a desired overall function, large numbers of logic elements may be connected in complex circuits. In some cases microprocessors are used to perform many of the switching and timing functions of the individual logic elements. The processors are specifically programmed with individual instructions to perform a given task or tasks. An advantage of microprocessors is that they make possible the performance of different logic functions, depending on the program instructions that are stored. A disadvantage of microprocessors is that normally they operate in a sequential mode, which may be too slow for some applications. In these cases specifically designed logic circuits are used.

VIII.

Recent Developments

The development of integrated circuits has revolutionized the fields of communications, information handling, and computing. Integrated circuits reduce the size of devices and lower manufacturing and system costs, while at the same time providing high speed and increased reliability. Digital watches, hand-held computers, and electronic games are systems based on microprocessors. Other developments include the digitalization of audio signals, where the frequency and amplitude of an audio signal are coded digitally by appropriate sampling techniques, that is, techniques for measuring the amplitude of the signal at very short intervals. Digitally recorded music, as found on compact discs, shows fidelity far superior to direct-recording methods. New digital recording media, such as digital versatile disk (DVD), enable gigabytes of information to be stored and read conveniently and cheaply, thereby allowing high-quality digital recording of video and other data-intensive information.

Medical electronics has progressed from computerized axial tomography (the use of CAT or CT scanners) to systems that can discriminate more and more of the organs of the human body. Devices that can view blood vessels and the respiratory system have also been developed. The combination of low-cost computing with other new technologies is making powerful new tools more widespread. For example, electronic radiographs (which record X-ray images in electronic form) and computer processing of images allow dentists to identify tooth decay using only one-tenth the quantity of X-rays needed by older methods.

Today’s research to increase the speed and capacity of computers concentrates mainly on the improvement of integrated circuit technology and the development of even faster switching components. Very-large-scale integrated (VLSI) circuits that contain several hundred thousand components on a single chip have been developed. The same technological advances have enabled high-tech systems such as mobile telephones to be produced at relatively low-cost. Very-high-speed computers are being developed in which semiconductors may be replaced by superconducting circuits using Josephson junctions (see Josephson Effect) and operating at temperatures near absolute zero.