Last updated
Triode Audion from 1908. The filament (which was also the cathode) would be at the lower left inside the tube, but the filament has burned out and is no longer present. The filament's connecting and supporting wires are visible. The plate is at the middle top, and the grid is the serpentine electrode below it. The plate and grid connections leave the tube at the right. Triode tube 1906.jpg
Triode Audion from 1908. The filament (which was also the cathode) would be at the lower left inside the tube, but the filament has burned out and is no longer present. The filament's connecting and supporting wires are visible. The plate is at the middle top, and the grid is the serpentine electrode below it. The plate and grid connections leave the tube at the right.

The Audion was an electronic detecting or amplifying vacuum tube [1] invented by American electrical engineer Lee de Forest in 1906. [2] [3] [4] It was the first triode, [1] [5] [6] [7] [8] consisting of an evacuated glass tube containing three electrodes: a heated filament, a grid, and a plate. [4] It is important in the history of technology because it was the first widely used electronic device which could amplify; a small electrical signal applied to the grid could control a larger current flowing from the filament to plate. [4] [5]

Vacuum tube Device that controls electric current between electrodes in an evacuated container

In electronics, a vacuum tube, an electron tube, or valve or, colloquially, a tube, is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied.

Lee de Forest American inventor

Lee de Forest was an American inventor, self-described "Father of Radio", and a pioneer in the development of sound-on-film recording used for motion pictures. He had over 180 patents, but also a tumultuous career—he boasted that he made, then lost, four fortunes. He was also involved in several major patent lawsuits, spent a substantial part of his income on legal bills, and was even tried for mail fraud. His most famous invention, in 1906, was the three-element "Audion" (triode) vacuum tube, the first practical amplification device. Although De Forest had only a limited understanding of how it worked, it was the foundation of the field of electronics, making possible radio broadcasting, long distance telephone lines, and talking motion pictures, among countless other applications.

Triode single-grid amplifying vacuum tube having three active electrodes

A triode is an electronic amplifying vacuum tube consisting of three electrodes inside an evacuated glass envelope: a heated filament or cathode, a grid, and a plate (anode). Developed from Lee De Forest's 1906 Audion, a partial vacuum tube that added a grid electrode to the thermionic diode, the triode was the first practical electronic amplifier and the ancestor of other types of vacuum tubes such as the tetrode and pentode. Its invention founded the electronics age, making possible amplified radio technology and long-distance telephony. Triodes were widely used in consumer electronics devices such as radios and televisions until the 1970s, when transistors replaced them. Today, their main remaining use is in high-power RF amplifiers in radio transmitters and industrial RF heating devices. 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 the sound of tube-based electronics.


The original triode Audion had more residual gas in the tube than later versions and vacuum tubes; the extra residual gas limited the dynamic range and gave the Audion non-linear characteristics and erratic performance. [1] [7] Originally developed as a radio receiver detector [3] by adding a grid electrode to the Fleming valve, it found little use until its amplifying ability was recognized around 1912 by several researchers, [7] [9] who used it to build the first amplifying radio receivers and electronic oscillators. [8] [10] The many practical applications for amplification motivated its rapid development, and the original Audion was superseded within a few years by improved versions with higher vacuum. [7] [9]

Gas-filled tube arrangement of electrodes in a gas within an insulating, temperature-resistant envelope

A gas-filled tube, also known as a discharge tube, is an arrangement of electrodes in a gas within an insulating, temperature-resistant envelope. Gas-filled tubes exploit phenomena related to electric discharge in gases, and operate by ionizing the gas with an applied voltage sufficient to cause electrical conduction by the underlying phenomena of the Townsend discharge. A gas-discharge lamp is an electric light using a gas-filled tube; these include fluorescent lamps, metal-halide lamps, sodium-vapor lamps, and neon lights. Specialized gas-filled tubes such as krytrons, thyratrons, and ignitrons are used as switching devices in electric devices.

Detector (radio)

In radio, a detector is a device or circuit that extracts information from a modulated radio frequency current or voltage. The term dates from the first three decades of radio (1888-1918). Unlike modern radio stations which transmit sound on an uninterrupted carrier wave, early radio stations transmitted information by radiotelegraphy. The transmitter was switched on and off to produce long or short periods of radio waves, spelling out text messages in Morse code. Therefore, early radio receivers had only to distinguish between the presence or absence of a radio signal. The device that performed this function in the receiver circuit was called a detector. A variety of different detector devices, such as the coherer, electrolytic detector, magnetic detector and the crystal detector, were used during the wireless telegraphy era until superseded by vacuum tube technology.

Fleming valve a vacuum tube used as a detector for early radio receivers

The Fleming valve, also called the Fleming oscillation valve, was a thermionic valve or vacuum tube invented in 1904 by Englishman 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 which 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.


One of the earliest Audion radio receivers, constructed by De Forest in 1914. Audion tubes were mounted upside down, with the delicate filament hanging down, to prevent it from sagging and touching the grid. This was a detector (rectifier) and two stage audio amplifier unit; the radio signal came from a separate "tuner" unit. Audion receiver.jpg
One of the earliest Audion radio receivers, constructed by De Forest in 1914. Audion tubes were mounted upside down, with the delicate filament hanging down, to prevent it from sagging and touching the grid. This was a detector (rectifier) and two stage audio amplifier unit; the radio signal came from a separate "tuner" unit.

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 22volt 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.

Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around 20 kHz to around 300 GHz. This is roughly between the upper limit of audio frequencies and the lower limit of infrared frequencies; these are the frequencies at which energy from an oscillating current can radiate off a conductor into space as radio waves. Different sources specify different upper and lower bounds for the frequency range.

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").

Patent Intellectual property conferring a monopoly on a new invention

A patent is a form of intellectual property that gives its owner the legal right to exclude others from making, using, selling, and importing an invention for a limited period of years, in exchange for publishing an enabling public disclosure of the invention. In most countries patent rights fall under civil law and the patent holder needs to sue someone infringing the patent in order to enforce his or her rights. In some industries patents are an essential form of competitive advantage; in others they are irrelevant.

Gridiron (cooking) casserole

A gridiron is a metal grate with parallel bars typically used for grilling meat, fish, vegetables, or combinations of such foods. It may also be two such grids, hinged to fold together, to hold food securely while grilling over an open flame.

The Audion provided power gain; with other detectors, all of the power to operate the headphones had to come from the antenna circuit itself. Consequently, weak transmitters could be heard at greater distances.

Patents and disputes

De Forest and everybody else at the time greatly underestimated the potential of his original device, imagining it to be limited to mostly 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. (Ironically, in the years of patent disputes leading up to World War I, it was only this "loophole" that allowed vacuum triodes to be manufactured at all since none of De Forest's patents specifically mentioned this application).

First internal grid Audion tube.jpg
Audion tube.jpg
(left) The first prototype Audion with the grid (zigzag wires) between the filament and plate. [11] (right) Later design of an audion tube. The grid and plate are in two parts on either side of the central filament. In both these tubes the filament is burned out.

De Forest was granted a patent for his early two-electrode version of the Audion on November 13, 1906 ( U.S. Patent 841,386 ), and 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 Fleming received Great Britain patent 24850 and the American Fleming valve patent U.S. Patent 803,684 ), and De Forest 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".[ citation needed ]

John Ambrose Fleming Electrical engineer and physicist

Sir John Ambrose Fleming FRS was an English electrical engineer and physicist who invented the first thermionic valve or vacuum tube, designed the radio transmitter with which the first transatlantic radio transmission was made, and also established the right-hand rule used in physics. He was the eldest of seven children of James Fleming DD, a Congregational minister, and his wife Mary Ann, at Lancaster, Lancashire, and baptised on 11 February 1850. A devout Christian, he once preached at St Martin-in-the-Fields in London on evidence for the resurrection. In 1932, he and Douglas Dewar and Bernard Acworth helped establish the Evolution Protest Movement. Fleming bequeathed much of his estate to Christian charities, especially those for the poor. He was a noted photographer, painted water colours, and enjoyed climbing the Alps.

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. It is true that after the invention of the true vacuum triode in 1913 (see below), De Forest continued to manufacture various types of radio transmitting and receiving apparatus, (examples of which are illustrated on this page). However, although he routinely described these devices as using "Audions", they actually used high-vacuum triodes, using circuitry very similar to that developed by other experimenters.

In 1914, Columbia University student Edwin Howard Armstrong worked with professor John Harold Morecroft to document the electrical principles of the Audion. Armstrong published his explanation of the Audion in Electrical World in December 1914, complete with circuit diagrams and oscilloscope graphs. [12] In March and April 1915, Armstrong spoke to the Institute of Radio Engineers in New York and Boston, respectively, presenting his paper "Some Recent Developments in the Audion Receiver", which was published in September. [10] A combination of the two papers was reprinted in other journals such as the Annals of the New York Academy of Sciences. [12] When Armstrong and De Forest 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. [7] [13]

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 adsorbed 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. Much of the research that led to the development of true vacuum tubes was carried out by Irving Langmuir in the General Electric (GE) research laboratories.

Kenotron and Pliotron

Audions and early triodes developed from them, 1918.
Bottom row (D): De Forest Audions
Third row (C): Pliotrons, developed at General Electric by Langmuir
Second row (B): triodes developed at Western Electric which bought the rights from De Forest in 1913. These were used in telephone repeaters which made possible the first transcontinental telephone line in 1915.
Top row (A): French triodes. The French government gained the right to manufacture Audions in 1912 when De Forest failed to renew his French patents for lack of $125. Early triode vacuum tubes.jpg
Audions and early triodes developed from them, 1918.
Bottom row (D): De Forest Audions
Third row (C): Pliotrons, developed at General Electric by Langmuir
Second row (B): triodes developed at Western Electric which bought the rights from De Forest in 1913. These were used in telephone repeaters which made possible the first transcontinental telephone line in 1915.
Top row (A): French triodes. The French government gained the right to manufacture Audions in 1912 when De Forest failed to renew his French patents for lack of $125.
General Electric Company Pliotron General electric pliotron pp schenectady 3.jpg
General Electric Company Pliotron

Langmuir had long suspected that certain assumed limitations on the performance of various low-pressure and vacuum electrical devices, might not be fundamental physical limitations at all, but simply due to contamination and impurities in the manufacturing process.

His first success was in demonstrating that, contrary to what Edison and others had long asserted, incandescent lamps could function more efficiently and with longer life if the glass envelope was filled with low-pressure inert gas rather than a complete vacuum. However, this only worked if the gas used was meticulously 'scrubbed" of all traces of oxygen and water vapor. He then applied the same approach to producing a rectifier for the newly developed "Coolidge" X-ray tubes. Again contrary to what had been widely believed to be possible, by virtue of meticulous cleanliness and attention to detail, he was able to produce versions of the Fleming Diode that could rectify hundreds of thousands of volts. His rectifiers were called "Kenotrons" from the Greek keno (empty, contains nothing, as in a vacuum) and tron (device, instrument).

He then turned his attention to the Audion tube, again suspecting that its notoriously unpredictable behaviour might be tamed with more care in the manufacturing process.

However he took a somewhat unorthodox approach. Instead of trying to stabilize the partial vacuum, he wondered if it was possible to make the Audion function with the total vacuum of a Kenotron, since that was somewhat easier to stabilize.

He soon realized that his "vacuum" Audion had markedly different characteristics from the De Forest version, and was really a quite different device, capable of linear amplification and at much higher frequencies. To distinguish his device from the Audion he named it the "Pliotron", from the Greek plio (more or extra, in this sense meaning gain, more signal coming out than went in).

Essentially, he referred to all his vacuum tube designs as Kenotrons, the Pliotron basically being a specialized type of Kenotron. However, because Pliotron and Kenotron were registered trademarks, technical writers tended to use the more generic term "vacuum tube". By the mid-1920s, the term "Kenotron" had come to exclusively refer to vacuum tube rectifiers, while the term "Pliotron" had fallen into disuse. Ironically, in popular usage, the sound-alike brands "Radiotron" and "Ken-Rad" outlasted the original names.

Applications and use

The first Audion AM radio transmitter, built by Lee De Forest and announced April, 1914 First vacuum tube AM radio transmitter.jpg
The first Audion AM radio transmitter, built by Lee De Forest and announced April, 1914
Some of the earliest Audion AM radio transmitters, built by De Forest around 1916. The invention of the Audion oscillator in 1912 made inexpensive sound radio transmission possible, and was responsible for the advent of radio broadcasting around 1920. De Forest Audion AM radio transmitters.jpg
Some of the earliest Audion AM radio transmitters, built by De Forest around 1916. The invention of the Audion oscillator in 1912 made inexpensive sound radio transmission possible, and was responsible for the advent of radio broadcasting around 1920.
Audion advertisement, Electrical Experimenter magazine, 1916 Audion vacuum tube advertisement.png
Audion advertisement, Electrical Experimenter magazine, 1916

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 through 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 one 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 long after 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 such applications as high-powered transmitters, guitar amplifiers and some high fidelity audio equipment.

Related Research Articles

Electronic oscillator electronic circuit that produces a repetitive, oscillating electronic signal, often a sine wave or a square wave

An electronic oscillator is an electronic circuit that produces a periodic, oscillating electronic signal, often a sine wave or a square wave. Oscillators convert direct current (DC) from a power supply to an alternating current (AC) signal. They are widely used in many electronic devices ranging from simplest clock generators to digital instruments and complex computers and peripherals etc. Common examples of signals generated by oscillators include signals broadcast by radio and television transmitters, clock signals that regulate computers and quartz clocks, and the sounds produced by electronic beepers and video games.

A tetrode is a vacuum tube having four active electrodes. The four electrodes in order from the centre are: a thermionic cathode, first and second grids and a plate. There are several varieties of tetrodes, the most common being the screen-grid tube and the beam tetrode. In screen-grid tubes and beam tetrodes, the first grid is the control grid and the second grid is the screen grid. In other tetrodes one of the grids is a control grid, while the other may have a variety of functions.


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 20th century. Its use in radio was based on the 1890 findings of French physicist Edouard Branly and adapted by other physicists and inventors over the next ten years. The device consists of a tube or capsule containing two electrodes spaced a small distance apart with loose metal filings in the space between. When a radio frequency signal is applied to the device, the metal particles would cling together or "cohere", reducing the initial high resistance of the device, thereby allowing a much greater direct current to flow through it. In a receiver, the current would activate a bell, or a Morse paper tape recorder to make a record of the received signal. The metal filings in the coherer remained conductive after the signal (pulse) ended so that the coherer had to be "decohered" by tapping it with a clapper actuated by an electromagnet, each time a signal was received, thereby restoring the coherer to its original state. Coherers remained in widespread use until about 1907, when they were replaced by more sensitive electrolytic and crystal detectors.

Regenerative circuit

A regenerative circuit is an amplifier circuit that employs positive feedback. Some of the output of the amplifying device is applied back to its input so as to add to the input signal, increasing the amplification. One example is the Schmitt trigger, but the most common use of the term is in RF amplifiers, and especially regenerative receivers, to greatly increase the gain of a single amplifier stage.

Radio receiver radio device for receiving radio waves and converting them to a useful signal

In radio communications, a radio receiver, also known as a receiver, wireless or simply radio is an electronic device that receives radio waves and converts the information carried by them to a usable form. It is used with an antenna. The antenna intercepts radio waves and converts them to tiny alternating currents which are applied to the receiver, and the receiver extracts the desired information. The receiver uses electronic filters to separate the desired radio frequency signal from all the other signals picked up by the antenna, an electronic amplifier to increase the power of the signal for further processing, and finally recovers the desired information through demodulation.

Control grid vacuum tube electrode

The control grid is an electrode used in amplifying thermionic valves such as the triode, tetrode and pentode, used to control the flow of electrons from the cathode to the anode (plate) electrode. The control grid usually consists of a cylindrical screen or helix of fine wire surrounding the cathode, and is surrounded in turn by the anode. The control grid was invented by Lee De Forest, who in 1906 added a grid to the Fleming valve to create the first amplifying vacuum tube, the Audion (triode).

Armstrong oscillator

The Armstrong oscillator is an electronic oscillator circuit which uses an inductor and capacitor to generate an oscillation. It is the earliest oscillator circuit, invented by US engineer Edwin Armstrong in 1912 and independently by Austrian engineer Alexander Meissner in 1913, and was used in the first vacuum tube radio transmitters. It is sometimes called a tickler oscillator because its distinguishing feature is that the feedback signal needed to produce oscillations is magnetically coupled into the tank inductor in the input circuit by a "tickler coil" (L2, right) in the output circuit. Assuming the coupling is weak, but sufficient to sustain oscillation, the oscillation frequency f is determined primarily by the tank circuit (L1 and C in the figure on the right) and is approximately given by

Hot-filament ionization gauge

The hot-filament ionization gauge, sometimes called a hot-filament gauge or hot-cathode gauge, is the most widely used low-pressure (vacuum) measuring device for the region from 10−3 to 10−10 Torr. It is a triode, with the filament being the cathode.

Grid-leak detector

A grid leak detector is an electronic circuit that demodulates an amplitude modulated alternating current and amplifies the recovered modulating voltage. The circuit utilizes the non-linear cathode to control grid conduction characteristic and the amplification factor of a vacuum tube. Invented by Lee De Forest around 1912, it was used as the detector (demodulator) in the first vacuum tube radio receivers until the 1930s.

Crystal detector

A crystal detector is an obsolete electronic component in some early 20th century radio receivers that used a piece of crystalline mineral as a detector (demodulator) to rectify the alternating current radio signal to extract the audio modulation which produced the sound in the earphones. It was the first type of semiconductor diode, and one of the first semiconductor electronic devices. The most common type was the so-called cat whisker detector, which consisted of a piece of crystalline mineral, usually galena, with a fine wire touching its surface. The "asymmetric conduction" of electric current across electrical contacts between a crystal and a metal was discovered in 1874 by Karl Ferdinand Braun. Crystals were first used as radio wave detectors in 1894 by Jagadish Chandra Bose in his microwave experiments. who first patented a crystal detector in 1901. The crystal detector was developed into a practical radio component mainly by G. W. Pickard, who began research on detector materials in 1902 and found hundreds of substances that could be used in forming rectifying junctions. The physical principles by which they worked were not understood at the time they were used, but subsequent research into these primitive point contact semiconductor junctions in the 1930s and 1940s led to the development of modern semiconductor electronics.

Invention of radio aspect of history relating to the invention of radio

The invention of radio communication, although generally attributed to Guglielmo Marconi in the 1890s, spanned many decades, from theoretical underpinnings, through proof of the phenomenon's existence, development of technical means, to its final use in signalling.

Electrolytic detector

The electrolytic detector, or liquid barretter, was a type of detector (demodulator) used in early radio receivers. First used by Canadian radio researcher Reginald Fessenden in 1903, it was used until about 1913, after which it was superseded by crystal detectors and vacuum tube detectors such as the Fleming valve and Audion (triode). It was considered very sensitive and reliable compared to other detectors available at the time such as the magnetic detector and the coherer. It was one of the first rectifying detectors, able to receive AM (sound) transmissions. On December 24, 1906, US Naval ships with radio receivers equipped with Fessendon's electrolytic detectors received the first AM radio broadcast from Fessenden's Brant Rock, Massachusetts transmitter, consisting of a program of Christmas music.

Reflex receiver

A reflex radio receiver, occasionally called a reflectional receiver, is a radio receiver design in which the same amplifier is used to amplify the high-frequency radio signal (RF) and low-frequency audio (sound) signal (AF). It was first invented in 1914 by German scientists Wilhelm Schloemilch and Otto von Bronk, and rediscovered and extended to multiple tubes in 1917 by Marius Latour and William H. Priess. The radio signal from the antenna and tuned circuit passes through an amplifier, is demodulated in a detector which extracts the audio signal from the radio carrier, and the resulting audio signal passes again through the same amplifier for audio amplification before being applied to the earphone or loudspeaker. The reason for using the amplifier for "double duty" was to reduce the number of active devices, vacuum tubes or transistors, required in the circuit, to reduce the cost. The economical reflex circuit was used in inexpensive vacuum tube radios in the 1920s, and was revived again in simple portable tube radios in the 1930s.

Autodyne Wikimedia disambiguation page

The autodyne circuit was an improvement to radio signal amplification using the De Forest Audion vacuum tube amplifier. By allowing the tube to oscillate at a frequency slightly different from the desired signal, the sensitivity over other receivers was greatly improved. The autodyne circuit was invented by Edwin Howard Armstrong of Columbia University, New York, NY. He inserted a tuned circuit in the output circuit of the Audion vacuum tube amplifier. By adjusting the tuning of this tuned circuit, Armstrong was able to dramatically increase the gain of the Audion amplifier. Further increase in tuning resulted in the Audion amplifier reaching self-oscillation.

Audion receiver

An audion receiver makes use of a single vacuum tube or transistor to detect and amplify signals. It is so called because it originally used the audion tube as the active element. Unlike a crystal detector or Fleming valve detector, the audion provided amplification of the signal as well as detection. The audion was invented by Lee De Forest.


  1. 1 2 3 Okamura, Sōgo (1994). History of Electron Tubes. IOS Press. pp. 17–22. ISBN   9051991452.
  2. De Forest patented a number of variations of his detector tubes starting in 1906. The patent that most clearly covers the Audion is U.S. Patent 879,532 , Space Telegraphy , filed January 29, 1907, issued February 18, 1908
  3. 1 2 De Forest, Lee (January 1906). "The Audion; A New Receiver for Wireless Telegraphy". Trans. AIEE. American Institute of Electrical and Electronic Engineers. 25: 735–763. doi:10.1109/t-aiee.1906.4764762 . Retrieved January 7, 2013. The link is to a reprint of the paper in the Scientific American Supplement, No. 1665, November 30, 1907, p.348-350, copied on Thomas H. White's United States Early Radio History website
  4. 1 2 3 Godfrey, Donald G. (1998). "Audion". Historical Dictionary of American Radio. Greenwood Publishing Group. p. 28. Retrieved January 7, 2013.
  5. 1 2 Amos, S. W. (2002). "Triode". Newnes Dictionary of Electronics, 4th Ed. Newnes. p. 331. Retrieved January 7, 2013.
  6. Hijiya, James A. (1992). Lee de Forest. Lehigh University Press. p. 77. ISBN   0934223238.
  7. 1 2 3 4 5 Lee, Thomas H. (2004). Planar Microwave Engineering: A Practical Guide to Theory, Measurement, and Circuits. Cambridge University Press. pp. 13–14. ISBN   0521835267.
  8. 1 2 Hempstead, Colin; Worthington, William E. (2005). Encyclopedia of 20th-Century Technology, Vol. 2. Taylor & Francis. p. 643. ISBN   1579584640.
  9. 1 2 Nebeker, Frederik (2009). Dawn of the Electronic Age: Electrical Technologies in the Shaping of the Modern World, 1914 to 1945. John Wiley & Sons. pp. 14–15. ISBN   0470409746.
  10. 1 2 Armstrong, E. H. (September 1915). "Some Recent Developments in the Audion Receiver". Proceedings of the IRE. 3 (9): 215–247. doi:10.1109/jrproc.1915.216677.. Republished as Armstrong, E. H. (April 1997). "Some Recent Developments in the Audion Receiver" (PDF). Proceedings of the IEEE. 85 (4): 685–697. doi:10.1109/jproc.1997.573757.
  11. De Forest, Lee (May 1930). "Evolution of the Vacuum Tube" (PDF). Radio News. Experimenter Publications. 9 (11): 990. Retrieved August 3, 2014.
  12. 1 2 Armstrong, E. H. (December 12, 1914). "Operating Features of the Audion". Electrical World. 64 (24): 1149–1152.
  13. McNicol, Donald Monroe (1946). Radio's Conquest of Space the Experimental Rise in Radio Communication. Taylor & Francis. pp. 178–184.

Further reading

1973 postage stamp honoring De Forest's audion 1973 airmail stamp C86.jpg
1973 postage stamp honoring De Forest's audion