electron tube

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electron tube,

device consisting of a sealed enclosure in which electrons flow between electrodes separated either by a vacuum (in a vacuum tube) or by an ionized gas at low pressure (in a gas tube). The two principal electrodes of an electron tube are the cathode and the anode or plate. The simplest vacuum tube, the diodediode
, two-terminal electronic device that permits current flow predominantly in only one direction. Most diodes are semiconductor devices; diode electron tubes are now used only for a few specialized applications.
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, has only those two electrodes. When the cathode is heated, it emits a cloud of electrons, which are attracted by the positive electric polarity of the anode and constitute the current through the tube. If the cathode is charged positively with respect to the anode, the electrons are drawn back to the cathode. However, the anode is not capable of emitting electrons, so no current can exist; thus the diode acts as a rectifier, i.e., it allows current to flow in only one direction. In the vacuum triode a third electrode, the grid, usually made of a fine wire mesh or similar material, is placed between the cathode and anode. Small voltage fluctuations, or signals, applied to the grid can result in large fluctuations in the current between the cathode and the anode. Thus the triode can act as a signal amplifier, producing output signals some 20 times greater than input. For even greater amplification, additional grids can be added. Tetrodes, with 2 grids, produce output signals about 600 times greater than input, and pentodes, with 3 grids, 1,500 times. X-ray tubes maintain a high voltage between a cathode and an anode. This enables electrons from the cathode to strike the anode at velocities high enough to produce X rays. A cathode-ray tubecathode-ray tube
(CRT), special-purpose electron tube in which electrons are accelerated by high-voltage anodes, formed into a beam by focusing electrodes, and projected toward a phosphorescent screen that forms one face of the tube.
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 can produce electron beams that strike a screen to produce pictures, as in some oscilloscopes and older television displays. Gas tubes behave similarly to vacuum tubes but are designed to handle larger currents or to produce luminous discharges. In some gas tubes the cathode is not designed as an electron emitter; conduction occurs when a voltage sufficient to ionize the gas exists between the anode and the cathode. In these cases the ions and electrons formed from the gas molecules constitute the current. Electron tubes have been replaced by solid-state devices, such as transistorstransistor,
three-terminal, solid-state electronic device used for amplification and switching. It is the solid-state analog to the triode electron tube; the transistor has replaced the electron tube for virtually all common applications.
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, for most applications. However they are still used in high-power transmitters, specialty audio equipment, and some oscilloscopes. A klystron is a special kind of vacuum tube that is a powerful microwavemicrowave,
electromagnetic wave having a frequency range from 1,000 megahertz (MHz) to 300,000 MHz, corresponding to a wavelength range from 300 mm (about 12 in.) to 1 mm (about 0.04 in.). Like light waves, microwaves travel essentially in straight lines.
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 amplifier; it is used to generate signals for radar and television stations.

See also magnetronmagnetron
, vacuum tube oscillator (see electron tube) that generates high-power electromagnetic signals in the microwave frequency range. Its operation is based on the combined action of a magnetic field applied externally and the electric field between its electrodes.
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; photoelectric cellphotoelectric cell
or photocell,
device whose electrical characteristics (e.g., current, voltage, or resistance) vary when light is incident upon it. The most common type consists of two electrodes separated by a light-sensitive semiconductor material.
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Electron tube

A device in which electrons can travel through a sealed chamber containing at least two electrodes and gas at a very low pressure. The gas pressure usually ranges from about 10-6 to 10-9 atm (10-1 to 10-4 pascal). At the low extreme of this pressure range, electron tubes are sometimes referred to as vacuum tubes, and at the high extreme as gas tubes.

At least one of the electrodes must emit electrons, and at least one must collect electrons. The emitting electrode, the cathode, may emit electrons through one or more of four mechanisms: thermionic or primary emission, secondary emission photoelectric emission, or field emission. Electrons must acquire more energy than they have in the conduction band of a metal in order to escape from the surface of a metal. They acquire this energy, respectively, in the four mechanisms listed above, from heat, electron or ion impact, a photon impact, or an external electric field. Photoelectric emission is used in light-sensing devices, often in combination with secondary electron multiplication to amplify the current. Secondary emission, sometimes in combination with thermionic emission, plays an important role in magnetrons and in crossed-field amplifiers. Field emission is used in some experimental amplifiers, flat-panel display devices, and x-ray tubes, but by far the most common type of emitting electrode used in electron tubes is the thermionic cathode. See Field emission, Photoemission, Secondary emission, Thermionic emission, X-ray tube

A diode is a two-electrode tube, with a cathode and a collecting electrode. A. Fleming (1904) developed the first thermionic diode using an oxide cathode. Because the collecting electrode is usually operated at a positive potential with respect to the cathode in order to collect much of the available electron current from the cathode, it is called an anode. Even so, because of the thermal energy of thermionic electrons, the anode can collect some electrons when it has a slightly negative potential.

L. DeForest (1906) added a third electrode to a diode in order to control the current flow from cathode to anode. This third electrode, the grid, took the form of a fairly open array or mesh made of wires with a diameter small compared to their spacing. In this geometry, much of the electric field from the anode terminates on the grid, and the field from the grid that terminates on the cathode exerts a primary influence on the space-charge current that flows to or through the grid. When the grid is at a negative potential with respect to the cathode, current flows due to the anode field that leaks through the grid, but the grid can collect no current. When the grid and anode are both positive, much more current flows and divides between the grid and anode.

Unfortunately, in triode amplifiers at high frequencies, the capacitance between the anode and grid electrodes, in combination with typical grid circuit reactances, can cause positive feedback, regeneration, or oscillation unless circuits that provide compensating negative feedback are used. For this reason W. Schottky (1919) invented the tetrode, which has a second or screen grid between the first or control grid and the anode. This grid was operated at a constant positive potential and effectively shielded the control grid from the anode. At large signal levels, it also created a problem by collecting secondary electrons emitted from the anode as a result of primary electron impacts when the instantaneous voltage on the anode was less than the screen grid voltage. This problem was dealt with in two ways. G. Jobst and D. H. Tellegen (1926) introduced the pentode, which has a third very open suppressor grid between the screen grid and the anode. It was connected to the cathode. This created an electric field which returned secondary electrons to the anode. A more elegant solution to the secondary electron problem was provided in the beam-power tetrode. In these tetrodes the anode was placed far enough from the screen grid that the charge of the electrons traveling between the screen grid and anode actually depressed the potential in the space between the screen and anode enough to return secondary electrons to the anode.

The tubes discussed so far act as valves that control the flow of a current to a load. The potential energy of the current is derived from a direct-current power source. There is another class of electron tubes, most of which are referred to as microwave tubes, in which electrons are accelerated to a velocity at which they have a kinetic energy that is equivalent to the full voltage of the power supply that was used to accelerate them. If these electrons are bunched periodically in time, they can be made to give up their energy to the electric field in a gap or gaps in a very high frequency or microwave circuit. Microwave tubes include the inductive output tube, the klystron, traveling-wave tubes, crossed-field devices, and cyclotron-resonance devices.

In the inductive output tube, invented by A. V. Haeff (1939), an electron beam is amplitude-modulated with a grid and then accelerated through a hole in the first accelerating electrode to form the high-velocity beam of electrons that passes through a gap in the center conductor of the coaxial external cavity resonator and into the collector. Inductive output tubes are used in many television transmitters operating between 470 and 900 MHz.

The klystron, invented by R. Varian and S. Varian (1939), has a similar output cavity and collector, but has a beam which is first accelerated in a diode electron gun and then velocity-modulated in another reentrant cavity gap. Fast electrons overtake slowed electrons and yield an intensity-modulated beam by the time the electrons reach the output cavity. Additional cavities may be interposed between the input and output cavities to provide very high gain (often as high as 60 dB).

In traveling-wave tubes (see illustration) invented by R. Kompfner (1946), a high-velocity electron beam is velocity-modulated by, and gives up its energy to, periodically loaded or helical waveguides which slow the electromagnetic wave to a velocity nearly equal to that of the electron beam. Again very high gain is possible.

Basic elements of a typical traveling-wave tubeenlarge picture
Basic elements of a typical traveling-wave tube

Input-output tubes, klystrons, and traveling-wave tubes are used in television broadcasting, satellite communications systems, radar, scientific accelerators, medical accelerators used for cancer therapy, and military countermeasures equipment.

In magnetrons and crossed-field amplifiers, electrons circulate about a cylindrical cathode in a radial direct-current electric field and an axial magnetic field. Concentric with, and outside, the cathode is a periodically loaded transmission line that propagates a wave having components that travel in synchronism with the rotating electron cloud. The electrons follow orbits that allow them to take energy from the radial direct-current electric field and transfer it to the circumferential radio-frequency electric field of the wave on the circuit. Magnetrons are used in huge quantities in household microwave ovens. They and crossed-field amplifiers are also used in ground-based, shipboard, and airborne radars.

Cyclotron-resonance devices including gyrotrons, gyroklystrons, and gyro-traveling-wave tubes again employ electrons that have been accelerated to the full energy provided by the electrical power supply. The beam is formed in a magnetic field so that it has a great deal of momentum perpendicular to the magnetic field, and the electrons follow helical paths. A radio-frequency electric field perpendicular to the axis of the electron trajectories will modulate the energy of the electrons and hence the relativistic mass and the cyclotron frequency. This azimuthal velocity modulation causes the electrons to draw into rodlike bunches that can give up their energy to a circuit supporting either the same alternating electric field that bunched them (in a gyrotron), or to an alternating electric field in another circuit (in a gyroklystron). Cyclotron-resonance devices can be built using very long circuits producing very weak electric fields, and as a result, having very low losses at very high frequencies. Efficient gyrotrons have been built at frequencies as high as several hundred gigahertz and have produced continuous power of hundreds of kilowatts.

McGraw-Hill Concise Encyclopedia of Physics. © 2002 by The McGraw-Hill Companies, Inc.

electron tube

[i′lek‚trän ‚tüb]
An electron device in which conduction of electricity is provided by electrons moving through a vacuum or gaseous medium within a gastight envelope. Also known as radio tube; tube; valve (British usage).
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

electron tube

(Or tube, vacuum tube, UK: valve, electron valve, thermionic valve, firebottle, glassfet) An electronic component consisting of a space exhausted of gas to such an extent that electrons may move about freely, and two or more electrodes with external connections. Nearly all tubes are of the thermionic type where one electrode, called the cathode, is heated, and electrons are emitted from its surface with a small energy (typically a Volt or less). A second electrode, called the anode (plate) will attract the electrons when it is positive with respect to the cathode, allowing current in one direction but not the other.

In types which are used for amplification of signals, additional electrodes, called grids, beam-forming electrodes, focussing electrodes and so on according to their purpose, are introduced between cathode and plate and modify the flow of electrons by electrostatic attraction or (usually) repulsion. A voltage change on a grid can control a substantially greater change in that between cathode and anode.

Unlike semiconductors, except perhaps for FETs, the movement of electrons is simply a function of electrostatic field within the active region of the tube, and as a consequence of the very low mass of the electron, the currents can be changed quickly. Moreover, there is no limit to the current density in the space, and the electrodes which do dissapate power are usually metal and can be cooled with forced air, water, or other refrigerants. Today these features cause tubes to be the active device of choice when the signals to be amplified are a power levels of more than about 500 watts.

The first electronic digital computers used hundreds of vacuum tubes as their active components which, given the reliability of these devices, meant the computers needed frequent repairs to keep them operating. The chief causes of unreliability are the heater used to heat the cathode and the connector into which the tube was plugged.

Vacuum tube manufacturers in the US are nearly a thing of the past, with the exception of the special purpose types used in broadcast and image sensing and displays. Eimac, GE, RCA, and the like would probably refer to specific types such as "Beam Power Tetrode" and the like, and rarely use the generic terms.

The cathode ray tube is a special purpose type based on these principles which is used for the visual display in television and computers. X-ray tubes are diodes (two element tubes) used at high voltage; a tungsten anode emits the energetic photons when the energetic electrons hit it. Magnetrons use magnetic fields to constrain the electrons; they provide very simple, high power, ultra-high frequency signals for radar, microwave ovens, and the like. Klystrons amplify signals at high power and microwave frequencies.
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electron tube

An earlier term for a vacuum tube. See vacuum tube.

An RCA Victor Tube
Radio Corporation of America (RCA) was formed in 1919 and sold to GE in 1986. The RCA Victor brand evolved from the 1929 acquisition of the Victor Talking Machine Company and has been used for decades to brand vacuum tubes and phonograph records.
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Prior to the introduction of the pentode, electron tubes had lower power than their applications needed.

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