Triode
A triode is an electronic amplifying vacuum tube (or thermionic valve in British English) consisting of three electrodes inside an evacuated glass envelope: a heated filament or cathode, a grid, and a plate (anode).In recent years there has been a resurgence in demand for low power triodes due to renewed interest in tube-type audio systems by audiophiles who prefer[vague] the sound of tube-based electronics.[9][10] Von Lieben's partially-evacuated three-element tube, patented in March 1906, contained a trace of mercury vapor and was intended to amplify weak telephone signals.[11][12][13][8] Starting in October 1906[9] De Forest patented a number of three-element tube designs by adding an electrode to the diode, which he called Audions, intended to be used as radio detectors.[15][7][16] Like the von Lieben vacuum tube, De Forest's Audions were incompletely evacuated and contained some gas at low pressure.[17][18] von Lieben's vacuum tube did not see much development due to his death seven years after its invention, shortly before the outbreak of the First World War.[17] The name "triode" appeared later, when it became necessary to distinguish it from other kinds of vacuum tubes with more or fewer elements (diodes, tetrodes, pentodes, etc.).During World War I, AM voice two way radio sets were made possible in 1917 (see TM (triode)) which were simple enough that the pilot in a single seat aircraft could use it while flying.Today triodes are used mostly in high-power applications for which solid state semiconductor devices are unsuitable, such as radio transmitters and industrial heating equipment.However, more recently the triode and other vacuum tube devices have been experiencing a resurgence and comeback in high fidelity audio and musical equipment.Since the filament eventually burns out, the tube has a limited lifetime and is made as a replaceable unit; the electrodes are attached to terminal pins which plug into a socket.Low power triodes have a concentric construction (see drawing right), with the grid and anode as circular or oval cylinders surrounding the cathode.The elements are held in position by mica or ceramic insulators and are supported by stiff wires attached to the base, where the electrodes are brought out to connecting pins.A "getter", a small amount of shiny barium metal evaporated onto the inside of the glass, helps maintain the vacuum by absorbing gas released in the tube over time.Smaller examples dispense with the octal pin base shown in the illustration and rely on contact rings for all connections, including heater and D.C. cathode.The interior of the tube is well evacuated so that electrons can travel between the cathode and the anode without losing energy in collisions with gas molecules.While proponents of vacuum tubes claim their superiority in areas such as high-end and professional audio applications, the solid-state MOSFET has similar performance characteristics.It is also possible to use triodes as cathode followers in which there is no voltage amplification but a huge reduction in dynamic impedance; in other words, the current is greatly amplified (as it also is in the common-cathode configuration described above).
Triodionamplifyingvacuum tubeelectrodesfilamentcathodeLee De ForestAudionthermionic diodeFleming valveelectronic amplifiertetrodepentoderadio technologytelephonyconsumer electronicstransistorsradio transmittersWilliam EcclesPhilipp LenardJohn Ambrose Flemingdetectorradio receiversRobert von Liebencontrol gridmercury vaporAudionsFirst World Warelectronic oscillatorsAmerican Telephone and Telegraph CompanyIrving LangmuirGeneral ElectrictetrodespentodesMarconi Companyelectronicsactivetwo way radioTM (triode)continuous wavedamped wavespark-gap transmittersamplitude modulationloudspeakerscrystal radiosearphonesmass communicationradio broadcastingrepeatersBell Telephonetelevisionpublic address systemsphonographstalking motion picturesWalter Schottkyaudio systemstransistorsemiconductor devicesSchematic symbolcircuit diagramsplate electrodetungstenindirectly heated cathodealkaline earththorium oxidework functioninsulatorsgetterbariumindirectly heated cathodesthoriated tungstenthoriumheat sinkultrahigh frequenciescapacitanceinductanceelectronsthermionic emissionevacuatedamplificationtransconductancevoltage gainpower gaincathode followerssource followerscurrent gainamplifiersoscillatorssolid-statetransmittershigh-endprofessional audiooperating pointload linevoltage biasclass-AimpedanceFaithful amplificationHanso IdzerdaList of vacuum tubesTapan K. SarkarArmstrong, E.H.Electronic componentsSemiconductordevicesMOS transistorsBiCMOSBioFETChemical field-effect transistorComplementary MOSDepletion-load NMOSFin field-effect transistorFloating-gate MOSFETInsulated-gate bipolar transistorMOS field-effect transistorMulti-gate field-effect transistorPower MOSFETThin-film transistorOther transistorsBipolar junction transistorDarlington transistorDiffused junction transistorField-effect transistorJunction Gate FET (JFET)Organic FET (OFET)Light-emitting transistorOrganic LET (OLET)