triode

An electronic valve having three electrodes; a positive anode, an electron-emitting cathode, and a negatively biased control grid. A triode controls the flow of electrons from the cathode to the anode. It can be used as an amplifier or oscillator.

A triode is a type of 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 was the first electrical amplification device. The principle of its operation is that, like in a vacuum tube based diode, the heated filament causes a flow of electrons that hit the plate and create an electric charge to it. The control grid is then charged negatively to repel some of the (also negatively charged) electrons back towards the filament: the larger the charge on the grid, the smaller the charge created on the plate. The name triode appeared later, probably when it became necessary to distinguish it from other generic kinds of vacuum tubes with more elements (Tetrodes, Pentodes). The original Audion tubes were not vacuum tubes however, as they deliberately contained some gas at low pressure. The name triode is only applied to vacuum tubes.

Triodes are largely obsolete, having been replaced by the transistor, but do still find application where power consumption and overall size are not concerns, but low component count and high power capacity are.