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Vacuum tube and valve types

Vacuum tube

In electronics, a vacuum tube (American English) or (thermionic) valve (British English) is a device generally used to amplify a signal. Once used in most electronic devices, vacuum tubes are now used only in specialized applications. For most purposes, the vacuum tube has been replaced by the much smaller and less expensive transistor, either as a discrete device or in an integrated circuit. At the start of the 21st century there has been renewed interest in the vacuum tube, this time in the form of the Field-emitter microtube.

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Triode

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 and still remains in use for amplification purposes, especially in AF application where the "tried and true" are revered at a premium.

Triodes are now all but obsolete due to transistors but do still find application where reliability and power consumption are not concerns, but low component count and high power capacity are.

Tetrode

A tetrode is a two-grid vacuum tube. It has four electrodes instead of three, as in the case of a triode.

A tetrode is a group of wire bundles used in electrophysiological studies in the neurosciences to record extracellular field potentials from nervous tissue, e.g. the brain. They consist of bundles of 4 thin (e.g. 30 µm diameter) wires glued together. The idea is that the wires are spaced close enough to each other to 'see' overlapping populations of neurons, but wide enough so that the exact waveform of the signal recorded would be different on each of the wires. These differences would then be used to distinguish the contributions of different neurons based on the shape of their spikes (the extracellular correlates of their action potential) by spike sorting.