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Triode

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Examples of low power triodes from 1918(left)to miniature tubes of the 1960s(right)
This article is about the electronic vacuum tube. For the Eastern Orthodox structure of hymns, seeTriodion.
ECC83,a dual triode used in 1960-era audio equipment, showing the orange glow of the hot cathode.
3CX1500A7, a modern 1.5 kW power triode used in radio trans­mitters. The cylindrical structure is a heat sink atta­ched to the plate, through which air is blown during operation.

Atriodeis an electronicamplifyingvacuum tube(orthermionic valvein British English) consisting of threeelectrodesinside an evacuated glass envelope: a heatedfilamentorcathode,agrid,and aplate(anode). Developed fromLee De Forest's 1906Audion,a partial vacuum tube that added a grid electrode to thethermionic diode(Fleming valve), the triode was the first practicalelectronic amplifierand the ancestor of other types of vacuum tubes such as thetetrodeandpentode.Its invention helped make amplifiedradio technologyand long-distancetelephonypossible.[1]Triodes were widely used inconsumer electronicsdevices such as radios and televisions until the 1970s, whentransistorsreplaced them. Today, their main remaining use is in high-powerRFamplifiers inradio transmittersand 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[vague]the sound of tube-based electronics.[citation needed]

The name "triode" was coined by British physicistWilliam Eccles[2][3]some time around 1920, derived from theGreekτρίοδος,tríodos,fromtri-(three) andhodós(road, way), originally meaning the place where three roads meet.

History

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Precursor devices

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De Forest Audion tube from 1908, the first triode. The flat plate is visible on the top, with the zigzag wire grid under it. The filament was originally present under the grid but was burnt out.
Lieben-Reisz tube, another primitive triode developed the same time as the Audion by Robert v. Lieben

Before thermionic valves were invented,Philipp Lenardused the principle of grid control while conducting photoelectric experiments in 1902.[4]

The firstvacuum tubeused in radio[5][6]was thethermionic diodeorFleming valve,invented byJohn Ambrose Flemingin 1904 as adetectorforradio receivers.It was an evacuated glass bulb containing two electrodes, a heated filament (cathode) and a plate (anode).

Invention

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Triodes came about in 1906 when American engineerLee de Forest[7]and Austrian physicistRobert von Lieben[8]independently patented tubes that added a third electrode, acontrol grid,between the filament and plate to control current.[9][10]Von Lieben's partially-evacuated three-element tube, patented in March 1906, contained a trace ofmercury vaporand 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 calledAudions,intended to be used as radio detectors.[14][7]The one which became the design of the triode, in which the grid was located between the filament and plate, was patented January 29, 1907.[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 theFirst World War.[19]

De Forest's Audion did not see much use until its ability to amplify was recognized around 1912 by several researchers,[18][1]who used it to build the first successful amplifying radio receivers andelectronic oscillators.[20][21]The many uses for amplification motivated its rapid development. By 1913 improved versions with higher vacuum were developed by Harold Arnold atAmerican Telephone and Telegraph Company,which had purchased the rights to the Audion from De Forest, andIrving LangmuiratGeneral Electric,who named his tube the "Pliotron",[18][1]These were the firstvacuum tubetriodes.[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.). There were lengthy lawsuits between De Forest and von Lieben, and De Forest and theMarconi Company,who representedJohn Ambrose Fleming,the inventor of the diode.[22]

Wider adoption

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The discovery of the triode's amplifying ability in 1912 revolutionized electrical technology, creating the new field ofelectronics,[1]the technology ofactive(amplifying) electrical devices. The triode was immediately applied to many areas of communication. During World War I,AMvoicetwo way radiosets were made possible in 1917 (seeTM (triode)) which were simple enough that the pilot in a single seat aircraft could use it while flying. Triode "continuous wave"radio transmittersreplaced the cumbersome inefficient "damped wave"spark-gap transmitters,allowing the transmission of sound byamplitude modulation(AM). Amplifying trioderadio receivers,which had the power to driveloudspeakers,replaced weakcrystal radios,which had to be listened to withearphones,allowing families to listen together. This resulted in the evolution of radio from a commercial message service to the firstmass communicationmedium, with the beginning ofradio broadcastingaround 1920. Triodes made transcontinental telephone service possible. Vacuum tube trioderepeaters,invented atBell Telephoneafter its purchase of the Audion rights, allowed telephone calls to travel beyond the unamplified limit of about 800 miles. The opening by Bell of the first transcontinental telephone line was celebrated 3 years later, on January 25, 1915. Other inventions made possible by the triode weretelevision,public address systems,electricphonographs,andtalking motion pictures.

The triode served as the technological base from which later vacuum tubes developed, such as thetetrode(Walter Schottky,1916) andpentode(Gilles Holst and Bernardus Dominicus Hubertus Tellegen, 1926), which remedied some of the shortcomings of the triode detailed below.

The triode was very widely used inconsumer electronicssuch as radios, televisions, andaudio systemsuntil it was replaced in the 1960s by thetransistor,invented in 1947, which brought the "vacuum tube era" introduced by the triode to a close. Today triodes are used mostly in high-power applications for which solid statesemiconductor devicesare 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. They also remain in use as vacuum fluorescent displays (VFDs), which come in a variety of implementations but all are essentially triode devices.

Construction

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Structure of a modern low-power triode vacuum tube. The glass and outer electrodes are shown partly cut away to reveal the construction.
Schematic symbolused incircuit diagramsfor a triode, showing symbols for electrodes.

All triodes have a hotcathodeelectrode heated by afilament,which releases electrons, and a flat metalplate electrode(anode) to which the electrons are attracted, with agridconsisting of a screen of wires between them to control the current. These are sealed inside a glass container from which the air has been removed to a high vacuum, about 10−9atm. 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. The operating lifetime of a triode is about 2000 hours for small tubes and 10,000 hours for power tubes.

Low power triodes

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Low power triodes have a concentric construction(see drawing right),with the grid and anode as circular or oval cylinders surrounding the cathode. Thecathodeis a narrow metal tube down the center. Inside the cathode is afilamentcalled the "heater" consisting of a narrow strip of high resistancetungstenwire, which heats the cathode red-hot (800 - 1000 °C). This type is called an "indirectly heated cathode".The cathode is coated with a mixture ofalkaline earthoxides such as calcium andthorium oxidewhich reduces itswork functionso it produces more electrons. The grid is constructed of a helix or screen of thin wires surrounding the cathode. The anode is a cylinder or rectangular box of sheet metal surrounding the grid. It is blackened to radiate heat and is often equipped with heat-radiating fins. The electrons travel in a radial direction, from cathode through the grid to the anode. The elements are held in position bymicaor ceramicinsulatorsand are supported by stiff wires attached to the base, where the electrodes are brought out to connecting pins. A "getter",a small amount of shinybariummetal evaporated onto the inside of the glass, helps maintain the vacuum by absorbing gas released in the tube over time.

High-power triodes

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High-power triodes generally use afilamentwhich serves as the cathode (a directly heated cathode) because the emission coating onindirectly heated cathodesis destroyed by the higher ion bombardment in power tubes. Athoriated tungstenfilament is most often used, in whichthoriumadded to the tungsten diffuses to the surface and forms a monolayer which increases electron emission. As the monolayer is removed by ion bombardment it is continually renewed by more thorium diffusing to the surface. These generally run at higher temperatures than indirectly heated cathodes. The envelope of the tube is often made of more durable ceramic rather than glass, and all the materials have higher melting points to withstand higher heat levels produced. Tubes with anode power dissipation over several hundred watts are usually actively cooled; the anode, made of heavy copper, projects through the wall of the tube and is attached to a large external finned metalheat sinkwhich is cooled by forced air or water.

Lighthouse tubes

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Soviet lighthouse tube6С5Д(6S5D)

A type of low power triode for use atultrahigh frequencies(UHF), the "lighthouse" tube, has a planar construction to reduce interelectrodecapacitanceand leadinductance,which gives it the appearance of a "lighthouse". The disk-shaped cathode, grid and plate form planes up the center of the tube - a little like a sandwich with spaces between the layers. The cathode at the bottom is attached to the tube's pins, but the grid and plate are brought out to low inductance terminals on the upper level of the tube: the grid to a metal ring halfway up, and the plate to a metal button at the top. These are one example of "disk seal" design. 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.

As well, high-frequency performance is limited by transit time: the time required for electrons to travel from cathode to anode. Transit time effects are complicated, but one simple effect is input conductance, also known as grid loading. At extreme high frequencies, electrons arriving at the grid may become out of phase with those departing towards the anode. This imbalance of charge causes the grid to exhibit a reactance that is much less than its low-frequency "open circuit" characteristic.

Transit time effects are reduced by reduced spacings in the tube. Tubes such as the 416B (a Lighthouse design) and the 7768 (an all-ceramic miniaturised design) are specified for operation to 4 GHz. They feature greatly reduced grid-cathode spacings of the order of 0.1 mm.

These greatly reduced grid spacings also give a much higher amplification factor than conventional axial designs. The 7768 has an amplification factor of 225, compared with 100 for the 6AV6 used in domestic radios and about the maximum possible for an axial design.

Anode-grid capacitance is not especially low in these designs. The 6AV6 anode-grid capacitance is 2 picofarads (pF), the 7768 has a value of 1.7 pF. The close electrode spacing used in microwave tubesincreasescapacitances, but this increase is offset by their overall reduced dimensions compared to lower-frequency tubes.

Operation

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Triode with separate cathode and filament.
Triode in which filament serves as cathode.
Filament omitted from diagram.
Schematic circuit symbols for triodes. (F) filament, (C) cathode, (G) grid, (P) plate

In the triode,electronsare released into the tube from the metalcathodeby heating it, a process calledthermionic emission.The cathode is heated red hot by a separate current flowing through a thin metalfilament.In some tubes the filament itself is the cathode, while in most tubes there is a separate filament which heats the cathode but is electrically isolated from it. The interior of the tube is wellevacuatedso that electrons can travel between the cathode and the anode without losing energy in collisions with gas molecules. A positive DC voltage, which can be as low as 20V or up to thousands of volts in some transmitting tubes, is present on the anode. The negative electrons are attracted to the positively chargedanode(or "plate" ), and flow through the spaces between the grid wires to it, creating a flow of electrons through the tube from cathode to anode.

The magnitude of this current can be controlled by a voltage applied on the grid (relative to the cathode). The grid acts like a gate for the electrons. A more negative voltage on the grid will repel more of the electrons, so fewer get through to the anode, reducing the anode current. A less negative voltage on the grid will allow more electrons from the cathode to reach the anode, increasing the anode current. Therefore, an input AC signal on the grid of a few volts (or less), even at a very high impedance (since essentially no current flows through the grid) can control a much more powerful anode current, resulting inamplification.When used in its linear region, variation in the grid voltage will cause an approximately proportional variation in the anode current; this ratio is called thetransconductance.If a suitable load resistance is inserted in the anode circuit, although the transconductance is somewhat lowered, the varying anode current will cause a varying voltage across that resistance which can be much larger than the input voltage variations, resulting involtage gain.

The triode is a normally "on" device; and current flows to the anode with zero voltage on the grid. The anode current is progressively reduced as the grid is made more negative relative to the cathode. Usually a constant DC voltage ( "bias" ) is applied to the grid along with the varying signal voltage superimposed on it. That bias is required so that the positive peaks of the signal never drive the grid positive with respect to the cathode which would result in grid current and non-linear behaviour. A sufficiently negative voltage on the grid (usually around 3-5 volts in small tubes such as the 6AV6, but as much as –130 volts in early audio power devices such as the '45), will prevent any electrons from getting through to the anode, turning off the anode current. This is called the "cutoff voltage". Since beyond cutoff the anode current ceases to respond to the grid voltage, the voltage on the grid must remain above the cutoff voltage for faithful (linear) amplification as well as not exceeding the cathode voltage.

The triode is somewhat similar in operation to the n-channelJFET;it is normally on, and exhibits progressively lower and lower plate/drain current as the grid/gate is pulled increasingly negative relative to the source/cathode. Cutoff voltage corresponds to the JFET's pinch-off voltage (Vp) or VGS(off); i.e., the voltage point at which output current essentially reaches zero. This similarity is limited, however. The triode's anode current is highly dependent on anode voltage as well as grid voltage, thus limiting thevoltage gain.Because, in contrast, the JFET's drain current is virtually unaffected by drain voltage, it appears as a constant-current device, similar in action to a tetrode or pentode tube (high dynamic output impedance). Both the JFET and tetrode/pentode valves are thereby capable of much higher voltage gains than the triode which seldom exceeds 100. However thepower gain,or the output power obtained from a certain AC input voltage is often of greater interest. When these devices are used ascathode followers(orsource followers), they all have a voltage "gain" of just under 1, but with a largecurrent gain.

Applications

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Although S.G. Brown's Type G Telephone Relay (using a magnetic "earphone" mechanism driving a carbon microphone element) was able to give power amplification and had been in use as early as 1914, it was a purely mechanical device with limited frequency range and fidelity. It was suited only to a limited range of audio frequencies - essentially voice frequencies.[23]

The triode was the first non-mechanical device to provide power gain at audio and radio frequencies, and maderadiopractical. Triodes are used foramplifiersandoscillators.Many types are used only at low to moderate frequency and power levels. Large water-cooled triodes may be used as the final amplifier in radio transmitters, with ratings of thousands of watts. Specialized types of triode ( "lighthouse" tubes, with low capacitance between elements) provide useful gain at microwave frequencies.

Vacuum tubes are obsolete in mass-marketedconsumer electronics,having been overtaken by less expensive transistor-basedsolid-statedevices. However, more recently, vacuum tubes have been making somewhat of a comeback. Triodes continue to be used in certain high-powerRFamplifiers andtransmitters.While proponents of vacuum tubes claim their superiority in areas such ashigh-endandprofessional audioapplications, the solid-state MOSFET has similar performance characteristics.[24]

Characteristics

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ECC83 triode operating characteristic

In triode datasheets, characteristics linking the anode current (Ia) to anode voltage (Va) and grid voltage (Vg) are usually given. From here, a circuit designer can choose theoperating pointof the particular triode. Then the output voltage and amplification of the triode can be evaluated graphically by drawing aload lineon the graph.

In the example characteristic shown on the image, suppose we wish to operate it at a quiescent anode voltage Vaof 200 V and a gridvoltage biasof −1 V. This implies a quiescent plate (anode) current of 2.2 mA (using the yellow curve on the graph). In aclass-Atriode amplifier, one might place an anode resistor (connected between the anode and the positive power supply). If we choose Ra= 10000 Ω, the voltage drop on it would be V+− Va= Ia× Ra= 22 V for the chosen anode current of Ia= 2.2 mA. Thus we require a power supply voltage V+= 222 V in order to obtain Va= 200 V on the anode.

Now suppose we impress on the −1 V bias voltage a signal of 1 V peak-peak, so that the grid voltage varies between −0.5 V and −1.5 V. When Vg= −0.5 V, the anode current will increase to 3.1 mA, lowering the anode voltage to Va= V+− 10 kΩ × 3.1 mA = 191 V (orange curve). When Vg= −1.5 V, the anode current will decrease to 1.4 mA, raising the anode voltage to Va= V+− 10 kΩ × 1.4 mA = 208 V (green curve). Therefore a 1 V peak-peak signal on the input (grid) causes an output voltage change of about 17 V.

Thus voltage amplification of the signal is obtained. The ratio of these two changes, thevoltage amplification factor(ormu) is 17 in this case. It is also possible to use triodes ascathode followersin which there is no voltage amplification but a huge reduction in dynamicimpedance;in other words, thecurrentisgreatly amplified(as it also is in thecommon-cathode configurationdescribed above). Amplifying either the voltage or current results in power amplification, the general purpose of an amplifying tube (after all, either the current or voltage alone could be increased by decreasing the other just using a transformer, a passive device).

See also

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References

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  1. ^abcdNebeker, 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.ISBN978-0470409749.
  2. ^Turner, L. B. (1921).Wireless Telegraphy and Telephony.Cambridge University Press. p. 78.ISBN110762956X.
  3. ^Ginoux, Jean-Marc; Rosetto, Bruno, "The Singing Arc: The oldest memrister?" inAdamatzky, Andrew; Chen, Guanrong (2013).Chaos, CNN, Memristors and Beyond.World Scientific. p. 500.ISBN978-9814434812.
  4. ^Burns, Russell W. (2004).Communications: An International History of the Formative Years.London: Institute of Electrical Engineers. p. 339.ISBN0863413277.
  5. ^Aitken, Hugh G.J. (2014).The Continuous Wave: Technology and American Radio, 1900-1932.Princeton University Press. p. 195.ISBN978-1400854608.
  6. ^Fisher, David E.; Fisher, Marshall (1996).Tube: The Invention of Television.Counterpoint. p. 54.ISBN1887178171.
  7. ^abcTyne, Gerald F. J. (September 1943)."The Saga of the Vacuum Tube, Part 6"(PDF).Radio News.30(3). Chicago, IL: Ziff-Davis: 26–28, 91.RetrievedNovember 30,2016.
  8. ^abTyne, Gerald F. J. (November 1943)."The Saga of the Vacuum Tube, Part 8"(PDF).Radio News.30(5). Chicago, IL: Ziff-Davis: 26–28.RetrievedNovember 30,2016.
  9. ^abAnton A. Huurdeman, The Worldwide History of Telecommunications, John Wiley & Sons - 2003, page 226
  10. ^John Bray, The Communications Miracle: The Telecommunication Pioneers from Morse to the Information Superhighway, Springe - 2013, pages 64-65
  11. ^[1]DRP 179807
  12. ^Tapan K. Sarkar(ed.) "History of wireless", John Wiley and Sons, 2006.ISBN0-471-71814-9,p.335
  13. ^Sōgo Okamura (ed),History of Electron Tubes,IOS Press, 1994ISBN90-5199-145-2page 20
  14. ^De Forest, Lee (January 1906)."The Audion; A New Receiver for Wireless Telegraphy".Trans. AIEE.25.American Institute of Electrical and Electronics Engineers: 735–763.doi:10.1109/t-aiee.1906.4764762.RetrievedMarch 30,2021.The link is to a reprint of the paper in theScientific American Supplement,Nos. 1665 and 1666, November 30, 1907 and December 7, 1907, p.348-350 and 354-356
  15. ^U.S. patent 879,532,Space Telegraphy,filed January 29, 1907, issued February 18, 1908
  16. ^Hijiya, James A. (1997).Lee de Forest and the Fatherhood of Radio.Lehigh University Press. p. 77.ISBN0934223238.
  17. ^abOkamura, Sōgo (1994).History of Electron Tubes.IOS Press. pp. 17–22.ISBN9051991452.
  18. ^abcLee, Thomas H. (2004).Planar Microwave Engineering: A Practical Guide to Theory, Measurement, and Circuits.Cambridge University Press. pp. 13–14.ISBN0521835267.
  19. ^John Bray, The Communications Miracle: The Telecommunication Pioneers from Morse to the Information Superhighway, Springe - 2013, page 64
  20. ^Hempstead, Colin; William E. Worthington (2005).Encyclopedia of 20th-Century Technology, Vol. 2.Taylor & Francis. p. 643.ISBN1579584640.
  21. ^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.S2CID2116636..Republished asArmstrong, 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.
  22. ^James A. Hijiya,Lee de Forest and the Fatherhood of Radio Political, and Economic DevelopmentLehigh University Press, 1992.ISBN0934223238,pages 93-94
  23. ^Tyne, Gerald F.J., Saga of the Vacuum Tube, 1977, Howard W. Sams, pp 201~202
  24. ^"Tubes Versus Solid-State Audio Amps—The Last Word (Or" House Of Fire, "Part 2)".www.electronicdesign.com.Retrieved2022-04-20.
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