A triode is an electronic amplification device having three active electrodes. The term most commonly applies to a vacuum tube (or valve in British English) with three elements: the filament or cathode, the grid, and the plate or anode. The triode vacuum tube is the first electronic amplification device.
The original three-element device was patented in 1907 by Lee De Forest who developed it from his original two-element 1906 Audion. The Audion did provide amplification. However it was not until around 1912 that other researchers, while attempting to improve the service life of the audion, stumbled on the principle of the true vacuum tube. The name triode appeared later, when it became necessary to distinguish it from other generic kinds of vacuum tubes with more or fewer elements (eg diodes, tetrodes, pentodes etc.). The Audion tubes deliberately contained some gas at low pressure. The name triode is only applied to vacuum tubes which have been evacuated of as much gas as possible.
There was a parallel independent invention of the triode by Austrian Robert von Lieben.
The principle of its operation is that, as with a thermionic diode, the heated cathode (either directly or indirectly by means of a filament) causes a space charge of electrons that may be attracted to the positively charged plate (anode in UK parlance) and create a current. Applying a negative charge to the control grid will tend to repel some of the (also negatively charged) electrons back towards the cathode: the larger the charge on the grid, the smaller the current to the plate. If an AC signal is superimposed on the DC bias of the grid, an amplified version of the AC signal appears (inverted) in the plate circuit.
The triode is very similar in operation to the n-channel JFET; it is normally on, and progressively switched off as the grid/gate is pulled increasingly negative of the source/cathode.
Although triodes are now largely obsolete in consumer electronics, having been replaced by the transistor, triodes continue to be used in certain high-end and professional audio applications, as well as in microphone preamplifiers and electric guitar amplifiers.
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