Audion tube
Encyclopedia
Amplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
vacuum tube
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...
invented by Lee De Forest
Lee De Forest
Lee De Forest was an American inventor with over 180 patents to his credit. De Forest invented the Audion, a vacuum tube that takes relatively weak electrical signals and amplifies them. De Forest is one of the fathers of the "electronic age", as the Audion helped to usher in the widespread use...
in 1906. It was the forerunner of the triode
Triode
A triode is an electronic amplification device having three active electrodes. The term most commonly applies to a vacuum tube with three elements: the filament or cathode, the grid, and the plate or anode. The triode vacuum tube was the first electronic amplification device...
, in which the current from the filament to the plate
Plate electrode
A plate is a type of electrode that formed part of a vacuum tube. The plate is impressed with a positive charge so that it may capture and flow electrons within a circuit....
was controlled by a third element, the grid. A small amount of power
Electric power
Electric power is the rate at which electric energy is transferred by an electric circuit. The SI unit of power is the watt.-Circuits:Electric power, like mechanical power, is represented by the letter P in electrical equations...
applied to the grid could control a larger current from the filament to the plate, allowing the Audion both to detect
Detector (radio)
A detector is a device that recovers information of interest contained in a modulated wave. The term dates from the early days of radio when all transmissions were in Morse code, and it was only necessary to detect the presence of a radio wave using a device such as a coherer without necessarily...
radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
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
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 wavesRadio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
. De Forest found that gas in a partial vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
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 patent
Patent
A patent is a form of intellectual property. It consists of a set of exclusive rights granted by a sovereign state to an inventor or their assignee for a limited period of time in exchange for the public disclosure of an invention....
s; 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 "loopholeLoophole
A loophole is a weakness that allows a system to be circumvented.Loophole may also refer to:*Arrowslit, a slit in a castle wall*Loophole , a short science fiction story by Arthur C...
" 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 , but the "triode" (three electrode) version was patented in 1908 . De Forest continued to claim that he developed the Audion independently from John Ambrose Fleming
John Ambrose Fleming
Sir John Ambrose Fleming was an English electrical engineer and physicist. He is known for inventing the first thermionic valve or vacuum tube, the diode, then called the kenotron in 1904. He is also famous for the left hand rule...
's earlier research on the thermionic valve (for which he received Great Britain patent 24850 and the American Fleming valve
Fleming valve
The Fleming valve, also called the Fleming oscillation valve, was a vacuum tube diode invented by John Ambrose Fleming and used in the earliest days of radio communication...
patent , 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
Edwin Armstrong
Edwin Howard Armstrong was an American electrical engineer and inventor. Armstrong was the inventor of modern frequency modulation radio....
published an explanation of the Audion, and when the two later faced each other in a dispute over the regeneration
Regenerative circuit
The regenerative circuit or "autodyne" allows an electronic signal to be amplified many times by the same vacuum tube or other active component such as a field effect transistor. It consists of an amplifying vacuum tube or transistor with its output connected to its input through a feedback...
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
Irving Langmuir
Irving Langmuir was an American chemist and physicist. His most noted publication was the famous 1919 article "The Arrangement of Electrons in Atoms and Molecules" in which, building on Gilbert N. Lewis's cubical atom theory and Walther Kossel's chemical bonding theory, he outlined his...
of General Electric
General Electric
General Electric Company , or GE, is an American multinational conglomerate corporation incorporated in Schenectady, New York and headquartered in Fairfield, Connecticut, United States...
, 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
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...
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
Demodulation
Demodulation is the act of extracting the original information-bearing signal from a modulated carrier wave.A demodulator is an electronic circuit that is used to recover the information content from the modulated carrier wave.These terms are traditionally used in connection with radio receivers,...
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 triodeTriode
A triode is an electronic amplification device having three active electrodes. The term most commonly applies to a vacuum tube with three elements: the filament or cathode, the grid, and the plate or anode. The triode vacuum tube was the first electronic amplification device...
that made practical radio broadcasts a reality.
Prior to the introduction of the Audion, radio receivers had used a variety of detector
Detector (radio)
A detector is a device that recovers information of interest contained in a modulated wave. The term dates from the early days of radio when all transmissions were in Morse code, and it was only necessary to detect the presence of a radio wave using a device such as a coherer without necessarily...
s including coherer
Coherer
The coherer was a primitive form of radio signal detector used in the first radio receivers during the wireless telegraphy era at the beginning of the twentieth century. Invented around 1890 by French scientist Édouard Branly, it consisted of a tube or capsule containing two electrodes spaced a...
s, barretters, and crystal detectors The most popular crystal detector consisted of a small piece of galena
Galena
Galena is the natural mineral form of lead sulfide. It is the most important lead ore mineral.Galena is one of the most abundant and widely distributed sulfide minerals. It crystallizes in the cubic crystal system often showing octahedral forms...
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
World War I
World War I , which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918...
.
By the late 1920s such "tube radios" began to become a fixture of most Western world
Western world
The Western world, also known as the West and the Occident , is a term referring to the countries of Western Europe , the countries of the Americas, as well all countries of Northern and Central Europe, Australia and New Zealand...
households, and remained so until the introduction of transistor
Transistor
A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...
radios in the mid 1950s.
In modern electronics
Electronics
Electronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...
, the vacuum tube
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...
has been largely superseded by solid state
Solid state (electronics)
Solid-state electronics are those circuits or devices built entirely from solid materials and in which the electrons, or other charge carriers, are confined entirely within the solid material...
devices such as the transistor
Transistor
A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...
, invented in 1947 and implemented in integrated circuit
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...
s 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
CRT
-Medicine:* Capillary refill time, the rate at with blood refills empty capillaries* Cognitive Retention Therapy, a dementia treatment* Cardiac resynchronization therapy, a treatment for heart failure** CRT-D, an implanted cardiac resynchronization device...
display.