vacuum tube(redirected from Field emitter microtube)
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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 diode, 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 tube 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 Electron tubes have been replaced by solid-state devices, such as transistors, for most applications.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 microwave amplifier; it is used to generate signals for radar and television stations.
(in Russian, elektronnaia lampa), an electron-tube device whose operation is based on a change in a stream of electrons emitted from a cathode and traveling in a vacuum by an electric field formed by means of electrodes. Depending on their output power, vacuum tubes are classified as receiving tubes or as oscillator tubes. The output power of receiving tubes is not higher than 10 watts (W); that of oscillator tubes is higher than 10 W.
The first vacuum tubes—vacuum diodes and triodes—were developed in the early 20th century on the basis of the production technology for incandescent lamps. They very much resembled incandescent lamps in appearance, having a glass bulb with a tungsten filament serving as the cathode in the center. (The word lampa [“lamp”] in the Russian term elektronnaia lampa underscores the similarity between early vacuum tubes and incandescent lamps; the adjective elektronnaia [“electron”] indicates the fundamental differences between the two. Although the appearance of vacuum tubes had changed considerably as early as the 1930’s, so that vacuum tubes no longer resembled incandescent lamps, the word lampa has been retained to this day in the Russian term.)
In the first half of the 20th century, vacuum tubes had a decisive influence on the evolution of radio engineering. Such tubes served as the basis for the development of radio communication, radio broadcasting, television, radar, and—for first-generation computers—computer technology. Between 1921 and 1941, the annual world output of vacuum tubes increased from a million to hundreds of millions. However, advances in semiconductor electronics made the further development of radio equipment based on receiving tubes no longer worthwhile. In the 1960’s and 1970’s, the development of such equipment was discontinued. As a result, the annual world production of receiving tubes decreased by roughly a factor of three between 1960 and 1975.
Nevertheless, advances in semiconductor electronics did not affect the development of oscillator tubes, since the output power of radio-frequency semiconductor devices does not exceed 10–100 watts. The oscillator tubes produced today include triodes and tetrodes. They are characterized by a power rating of 50 W to 3 megawatts (MW) in the continuous mode and of up to 10 MW in the pulsed mode. In the development of new types of oscillator tubes, primary attention is given to such considerations as reducing the grid current and raising the power amplification factor to 25–30 decibels (dB). Another consideration is the linearity of the grid-plate transfer characteristic, that is, the linearity of the dependence of the plate current on the control-grid voltage; in present-day tubes, third-order distortions have been reduced to –45 dB. A fourth consideration is increasing the efficiency of oscillator tubes. For example, in triodes with magnetic electron focusing, the efficiency may be as high as 90 percent. Such triodes are used for high-frequency heating.
REFERENCESVlasov, V. F. Elektronnye i ionnye pribory, 3rd ed. Moscow, 1960.
Yingst, T., et al. “Lampy bol’shoi moshchnosti s setochnym upravleniem—1972 g.” Trudy Instituta inzhenerov po elektrotekhnike i radioelektronike, 1973, vol. 61, no. 3, pp. 121–52. (Translated from English.)
Kleiner, E. Iu. Osnovy teorii elektronnykh lamp. Moscow, 1974.
V. F. KOVALENKO
vacuum tube[′vak·yəm ‚tüb]
vacuum tubeAn electronic device that controls the flow of electrons in a vacuum. It is used as a switch, amplifier or display screen (CRT). Used as on/off switches, vacuum tubes allowed the first computers to perform digital computations. Although tubes made a comeback in high-end stereo components, they have long since been abandoned for TVs and computer monitors. See vacuum tube types, audiophile, tube amplifier and Vintage Radio Museum.
|Early Vacuum Tube|
|Early vacuum tubes were used to amplify signals for radio and other audio devices. This one was made in 1915. Tubes were not used as switches in calculating machines until 1939. (Image courtesy of AT&T.)|
|Tubes in the 21st Century|
|Many audiophiles claim vacuum tubes amplify music better than transistors. These high-end Model One amplifiers (collectively weighing 212 pounds) were designed by legendary audio engineer Mark Levinson.|
|Vacuum tubes have come in myriad shapes and sizes over the years, and the Vintage Radio and Communications Museum has one of the finest collections. (Images courtesy of Vintage Radio and Communications Museum of Connecticut, www.vrcmct.org)|
|Amazing - A Tube Could Be This Small|
|This image is from an article in the October 1947 issue of Mechanix Illustrated that highlighted the huge reduction in vacuum tube size. The article's author could not have imagined that in the future several trillion transistors could fit inside the small vacuum tube. (Image courtesy of Mechanix Illustrated.)|