Classifications
One domain of classification of vacuum tubes uses the number of active electrodes, neglecting the filament or heater in devices with indirectly-heated cathodes (where the heater is electrically separate from the cathode). A device with two active elements is a diode, usually used for rectification. Devices with three elements are triodes used for amplification and switching. Additional electrodes create tetrodes, pentodes, and so forth, which have multiple additional functions made possible by the additional controllable electrodes.
Other classifications are:
- by frequency range (audio, radio, VHF, UHF, microwave),
- by power rating (small-signal, audio power, high-power radio transmitting),
- by design (e.g., sharp- versus remote-cutoff in some pentodes)
- by application (receiving tubes, transmitting tubes, amplifying or switching, rectification, mixing),
- special qualities (long life, very low microphonic and low noise audio amplification, and so on).
Multiple classifications may apply to a device; for example similar dual triodes can be used for audio preamplification and as flip-flops in computers, although linearity is important in the former case and long life in the latter.
Tubes have different functions, such as cathode ray tubes which create a beam of electrons for display purposes (such as the television picture tube) in addition to more specialized functions such as electron microscopy and electron beam lithography. X-ray tubes are also vacuum tubes. Phototubes and photomultipliers rely on electron flow through a vacuum, though in those cases electron emission from the cathode depends on energy from photons rather than thermionic emission. Since these sorts of "vacuum tubes" have functions other than electronic amplification and rectification they are described in their own articles.
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