Audion tube

"Audion" redirects here. For other uses, see Audion (disambiguation).

"Triode" Audion from 1908. (The 1906 Audion was a 2-element device with the signal applied to a wire wrapped around the glass envelope.)

The Audion is an electronic amplifying vacuum tube invented by Lee De Forest in 1906. It was the forerunner of the triode, in which the current from the filament to the plate was controlled by a third element, the grid. A small amount of power applied to the grid could control a larger current from the filament to the plate, allowing the Audion both to detect radio signals (that is, make them audible) and to provide amplification. However, De Forest's Audion is quite distinct from the true vacuum triode in that it is not capable of linear amplification.

History

A 1916 article titled "The Vacuum Detector and How It Works" from The Electrical Experimenter magazine

It had been known since the middle of the 19th century that gas flames were electrically conductive, and early wireless experimenters had noticed that this conductivity was affected by the presence of radio waves. De Forest found that gas in a partial vacuum heated by a conventional lamp filament behaved much the same way, and that if a wire were wrapped around the glass housing, the device could serve as a detector of radio signals. In his original design, a small metal plate was sealed into the lamp housing, and this was connected to the positive terminal of a 22 volt battery via a pair of headphones, the negative terminal being connected to one side of the lamp filament. When wireless signals were applied to the wire wrapped around the outside of the glass, they caused disturbances in the current which produced sounds in the headphones.

This was a significant development as existing commercial wireless systems were heavily protected by patents; a new type of detector would allow De Forest to market his own system. He eventually discovered that connecting the antenna circuit to a third electrode placed directly in the current path greatly improved the sensitivity; in his earliest versions, this was simply a piece of wire bent into the shape of a grid-iron (hence "grid").

Compared to all competing devices at the time, the Audion was unique in that it did not draw significant power from antenna/tuned circuit, which allowed the tuning circuitry to operate with maximum selectivity. With virtually all other systems, all of the power to operate the headphones had to come from the antenna circuit itself, which tended to "damp" the tuned circuits, limiting their ability to separate stations (distinguish discrete frequencies).

Patents and disputes

Arguments still continue about whether De Forest really invented the triode vacuum tube. What is apparent is that he (and everybody else at the time) greatly underestimated the potential of his original device, imagining it to have mostly limited military applications. It is significant that he apparently never saw its potential as a telephone repeater amplifier, even though crude electromechanical "note magnifiers" had been the bane of the telephone industry for at least two decades. (In fact, for several years it was only this "loophole" that allowed vacuum triodes to be manufactured at all, since none of the original patents specifically mentioned this application.)

De Forest was granted a patent for his early two-electrode version of the Audion on November 13, 1906 (U.S. Patent 841,386), but the "triode" (three electrode) version was patented in 1908 (U.S. Patent 879,532). De Forest continued to claim that he developed the Audion independently from John Ambrose Fleming's earlier research on the thermionic valve (for which he received Great Britain patent 24850 and the American Fleming valve patent (U.S. Patent 803,684), and became embroiled in many radio-related patent disputes. De Forest was famous for saying that he "didn't know why it worked, it just did". He always referred to the vacuum triodes developed by other researchers as "Oscillaudions", although there is no evidence that he had any significant input to their development.

In 1914 Edwin Armstrong published an explanation of the Audion,^[1] and when the two later faced each other in a dispute over the regeneration patent, Armstrong was able to demonstrate conclusively that De Forest still had no idea how it worked.

The problem was that (possibly to distance his invention from the Fleming valve) De Forest's original patents specified that low-pressure gas inside the Audion was essential to its operation (Audion being a contraction of "Audio-Ion"), and in fact early Audions had severe reliability problems due to this gas being absorbed by the metal electrodes. The Audions sometimes worked extremely well; at other times they would barely work at all.

As well as De Forest himself, numerous researchers had tried to find ways to improve the reliability of the device by stabilizing the partial vacuum. One of these, Dr Irving Langmuir of General Electric, took a somewhat unorthodox approach: instead of trying to prevent the absorption of the gas, he deliberately started out with a higher vacuum and looked for ways of making the Audion work under those conditions. He succeeded, but quickly realized that, though superficially similar to the Audion, his "vacuum" tube was really a completely different device, capable of linear amplification and at much higher frequencies.

One of the major weakness of De Forest's claims is that true vacuum triodes simply will not work if there is any trace of gas left in the envelope. In fact, before vacuum tubes could become commercially viable, quite elaborate techniques had to be developed to both initially evacuate the tubes and soak up any gas molecules that subsequently found their way in. This flies directly in the face of his original patent specification, which specifically states that gas is essential to the operation of the Audion.

Another weakness is that none of his Audion schematics denoted the provision for any sort of "grid bias", an essential feature of any true vacuum triode operation.

Unlike the Audion, the vacuum triode could not demodulate radio signals directly (although Langmuir and other researchers soon found alternative ways to do this), but it was capable of linear (i.e. undistorted) amplification, which turned out to be a vastly more useful feature. It is ironic that many "faulty" Audions, which had lost their ability to demodulate radio signals due to gas absorption, had actually turned into crude linear amplifiers (which was why they lost their demodulating ability), but nobody realized this at the time.

Applications and use

De Forest continued to manufacture and supply Audions to the US Navy up until the early 1920s, for maintenance of existing equipment, but elsewhere they were regarded as well and truly obsolete by then. It was the vacuum triode that made practical radio broadcasts a reality.

Prior to the introduction of the Audion, radio receivers had used a variety of detectors including coherers, barretters, and crystal detectors The most popular crystal detector consisted of a small piece of galena crystal probed by a fine wire commonly referred to as a "cat's-whisker detector". They were very unreliable, requiring frequent adjustment of the cat's whisker and offered no amplification. Such systems usually required the user to listen to the signal though headphones, sometimes at very low volume, as the only energy available to operate the headphones was that picked up by the antenna. For long distance communication huge antennas were normally required, and enormous amounts of electrical power had to be fed into the transmitter.

The Audion was a considerable improvement on this, but the original devices could not provide any subsequent amplification to what was produced in the signal detection process. The later vacuum triodes allowed the signal to be amplified to any desired level, typically by feeding the amplified output of from triode into the grid of the next, eventually providing more than enough power to drive a full-sized speaker. Apart from this, they were able to amplify the incoming radio signals prior to the detection process, making it work much more efficiently.

Vacuum tubes could also be used to make superior radio transmitters. The combination of much more efficient transmitters and much more sensitive receivers revolutionized radio communication during World War I.

By the late 1920s such "tube radios" began to become a fixture of most Western world households, and remained so until the introduction of transistor radios in the mid 1950s.

In modern electronics, the vacuum tube has been largely superseded by solid state devices such as the transistor, invented in 1947 and implemented in integrated circuits in 1959, although vacuum tubes remain to this day in high-powered transmitters, and till the 21st Century, in every television set worldwide as its CRT display.

See also

• Triode

References

1. ^ E. H. Armstorng (1917-08-02). "Operating Features of the Audion". Annals of the New York Academy of Sciences 27 (1): 215–243. doi:10.1111/j.1749-6632.1916.tb55188.x. http://www3.interscience.wiley.com/journal/119811389/abstract?CRETRY=1&SRETRY=0. 

• Radio Corp. V. Radio Engineering Laboratories, 293 U.S. 1 (United States Supreme Court 1934).

External links

• 1906 photograph of the original Audion tube, from New York Public Library