Triode

triode: see electron tube.

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triode

A type of vacuum tube that is used in audio and radio amplifiers and oscillator circuits. It is like a diode with the addition of a wire mesh control grid between the cathode and plate (anode) that controls current flow. A filament heats the cathode enabling it to release electrons. When a small voltage is applied to the grid, the current flow between the cathode and plate is changed accordingly. In some triodes, the filament is the cathode. See diode, tetrode and magnetron.

The Triode Uses a Grid

Adding a control grid to the diode allows the current to be varied between the cathode and anode.

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triode [′trī‚ōd]

(electronics)

A three-electrode electron tube containing an anode, a cathode, and a control electrode.

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Triode 

an electron tube with three electrodes, namely, a directly or indirectly heated cathode, a control grid, and an anode, or plate. The triode was invented in 1906 by L. De Forest. At first, triodes were used only as receiver tubes. In 1913 the German scientist A. Meissner showed that the triode could be used as an oscillator tube.

In the simplest triode, a helical grid is placed inside a cylindrical plate. Both the plate and the grid are usually made of a refractory metal, such as Ni, Mo, or Ta. The cathode is located on the common axis of the plate and the grid and is made of pure tungsten or thoriated tungsten carbide or may be oxide coated. The designs and the values of the parameters of currently used triodes vary widely and are determined mainly by the intended use. For example, triodes used in the initial stages of amplifiers and in voltage regulators have a high amplification factor of 30–100. Triodes used in output amplifier stages and in current regulators have a grid-plate characteristic whose linear section is shifted to the left, a low amplification factor of 4–10, and a relatively high plate dissipation of up to 20 watts. Triodes that amplify high-frequency oscillations, particularly in cascades with a common (grounded) grid, have a very steep grid-plate characteristic and a low plate-cathode capacitance.

Receiver triodes are often built as combined units, in which the same bulb contains two or more systems of electrodes; examples include duotriodes, diode-triodes, and triode-pentodes. Nuvi-stors and miniaturized triodes with rigid or flexible leads have come into use. Cermets are used in triodes intended for operation at frequencies above 1 gigahertz; such triodes have a system of planar electrodes and annular leads (to facilitate connection to external resonators or wave guides). Oscillator triodes and modulator triodes (the latter having a lower amplification factor than the former and a grid-plate characteristic whose linear section lies to the left of the plate-current zero axis) have different plate-dissipation capacities and, consequently, different useful output powers. Thus, in triodes with glass envelopes and natural cooling, the useful output power may be as high as several kilowatts; in triodes with an external plate that is a part of the vacuum envelope and with forced-air or liquid cooling, the useful output power reaches 1 megawatt. Triodes intended for pulse operation are characterized by a high pulsed cathode emission capacity and a large useful (pulsed) output of several hundred kilowatts with a negligible mean plate dissipation.

With the development of semiconductor electronics, triodes have been largely supplanted by semiconductor devices. Triodes have retained their importance, however, in a number of devices, for example, high-power oscillators and radio receivers intended for operation at high radiation levels and over broad temperature ranges.

REFERENCE

Kleiner E. Iu. Osnovy teorii elektronnykh lamp. Moscow, 1974.

S. M. MOSHKOVICH