Fleming valve

The Fleming valve, also called the Fleming oscillation valve, was a thermionic valve or vacuum tube invented in 1904 by English physicist John Ambrose Fleming as a detector for early radio receivers used in electromagnetic wireless telegraphy. It was the first practical vacuum tube and the first thermionic diode, a vacuum tube whose purpose is to conduct current in one direction and block current flowing in the opposite direction. The thermionic diode was later widely used as a rectifier — a device that converts alternating current (AC) into direct current (DC) — in the power supplies of a wide range of electronic devices, until beginning to be replaced by the selenium rectifier in the early 1930s and almost completely replaced by the semiconductor diode in the 1960s. The Fleming valve was the forerunner of all vacuum tubes, which dominated electronics for 50 years. The IEEE has described it as "one of the most important developments in the history of electronics", and it is on the List of IEEE Milestones for electrical engineering. The valve consists of an evacuated glass bulb containing two electrodes: a cathode in the form of a "filament", a loop of carbon or fine tungsten wire, similar to that used in the light bulbs of the time, and an anode (plate) consisting of a sheet metal plate. Although in early versions, the anode was a flat metal plate placed next to the cathode, in later versions, it became a metal cylinder surrounding the cathode. In some versions, a grounded copper screen surrounded the bulb to shield it against the influence of external electric fields. In operation, a separate current flows through the cathode "filament", heating it so that some of the electrons in the metal gain sufficient energy to escape their parent atoms into the vacuum of the tube, a process called thermionic emission. The AC to be rectified is applied between the filament and the plate. When the plate has a positive voltage with respect to the filament, the electrons are attracted to it and an electric current flows from filament to plate.

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Fleming valve

Crystal detector

A crystal detector is an obsolete electronic component used in some early 20th century radio receivers that consists of a piece of crystalline mineral which rectifies the alternating current radio signal. It was employed as a detector (demodulator) to extract the audio modulation signal from the modulated carrier, to produce the sound in the earphones. It was the first type of semiconductor diode, and one of the first semiconductor electronic devices.

Grille de contrôle

vignette|Photo macro d'une pentode EF91 avec sa grille réceptrice. En électronique, la grille de contrôle est l'électrode de commande d'un tube à vide ou d'un transistor à effet de champ (FET). La grille reçoit le signal à amplifier. Une modification de la différence de potentiel entre la grille et la cathode (ou la source pour un FET) provoque une variation du courant anodique (ou de drain). Généralement la grille est portée à un potentiel négatif par rapport à la cathode (ou source) grâce à un circuit dit circuit de polarisation.

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