Editing Vacuum tube (section)

===Triodes===
{{Main|Triode}}
[[File:Triode tube 1906.jpg|thumb|The first triode, the [[Lee de Forest|de Forest]] [[Audion]], invented in 1906]]
[[File:Triody var.jpg|right|thumb|Triodes as they evolved over some 45 years of tube manufacture, from the RE16 in 1918 to a 1960s era miniature tube]]
[[File:Triode.PNG|thumb|upright|Triode symbol. From top to bottom: plate (anode), control grid, cathode, heater (filament)]]

In the 19th century, telegraph and telephone engineers had recognized the need to extend the distance that signals could be transmitted. In 1906, [[Robert von Lieben]] filed for a patent for a [[cathode-ray tube]] which used an external magnetic deflection coil and was intended for use as an amplifier in telephony equipment.<ref>{{cite web | title = Robert von Lieben&nbsp;– Patent Nr&nbsp;179807 Dated November 19, 1906 | publisher = Kaiserliches Patentamt | date = 19 November 1906 | url = http://www.hts-homepage.de/Lieben/DRP179807.pdf | access-date = 30 March 2008 | url-status = live | archive-url = https://web.archive.org/web/20080528125703/http://www.hts-homepage.de/Lieben/DRP179807.pdf | archive-date = 28 May 2008 }}</ref> This von Lieben magnetic deflection tube was not a successful amplifier, however, because of the power used by the deflection coil.<ref>F. B. Llewellyn. (Mar. 1957). [https://worldradiohistory.com/Archive-Radio-News/50s/Radio-News-1957-03-R.pdf "Birth of the Electron Tube Amplifier".] New York: Ziff-Davis. ''Radio & Television News''. p. 44</ref> Von Lieben would later make refinements to [[triode]] vacuum tubes.

[[Lee de Forest]] is credited with inventing the triode tube in 1907 while experimenting to improve his original (diode) [[Audion]].<ref>Fleming, J. A. (1919). [https://archive.org/details/thermionicvalvei00flemrich/page/114/mode/2up?view=theater'' The Thermionic Valve and its Developments in Radiotelegraphy and Telephony'']. London: The Wireless Press Ltd. p. 115. Retrieved Oct 2021</ref> By placing an additional electrode between the filament ([[cathode]]) and [[plate electrode|plate]] (anode), he discovered the ability of the resulting device to amplify signals. As the voltage applied to the [[control grid]] (or simply "grid") was lowered below the cathode voltage, the current flowing from the filament to the plate decreased. The negative electrostatic field created by the grid in the vicinity of the cathode inhibited the passage of emitted electrons and reduced the current to the plate. With the grid negative relative to the cathode, no direct current could pass from the cathode to the grid. Consequently, a change of voltage applied to the grid, requiring no power input as no current flowed, could make a change in the plate current and could lead to a much larger voltage change at the plate; the result was voltage and power [[Amplifier|amplification]]. In 1908, de Forest was granted a patent ({{US patent|879532}}) for such a three-electrode version of his original Audion for use as an electronic amplifier in radio communications. This eventually became known as the triode.

[[File:General electric pliotron pp schenectady 3.jpg|thumb|upright|[[General Electric Company]] Pliotron, at the [[Science History Institute]]]]

De Forest's original device was made with conventional vacuum technology. The vacuum was not a "hard vacuum" but rather left a very small amount of residual gas. The physics behind the device's operation was not fully understood. The residual gas would cause a blue glow  due to visible ionization when the plate voltage exceeded about 60 volts. In 1912, de Forest and [[John Stone Stone]] brought the Audion for demonstration to AT&T's engineering department, where Harold D. Arnold of  realized that the blue glow was caused by ionized gas. He recommended that AT&T purchase the patent. Arnold developed high-vacuum tubes which operated at high plate voltages without a blue glow; they were tested in the summer of 1913 on AT&T's long-distance network.<ref>{{cite web |url=http://www.corp.att.com/attlabs/reputation/timeline/15tel.html |title=AT&T Labs Research {{pipe}} AT&T |access-date=2013-08-21 |url-status=live |archive-url=https://web.archive.org/web/20131005081147/http://www.corp.att.com/attlabs/reputation/timeline/15tel.html |archive-date=5 October 2013 }}</ref>

Finnish inventor [[Eric Tigerstedt]] significantly improved on the original triode design in 1914, while working on his [[sound-on-film]] process in Berlin, Germany. Tigerstedt's innovation was to make the electrodes concentric cylinders with the cathode at the centre, thus greatly increasing the collection of emitted electrons at the anode.<ref>{{cite book |first1=Antti V. |last1=Räisänen |first2=Arto |last2=Lehto |title=Radio Engineering for Wireless Communication and Sensor Applications |url=https://archive.org/details/radioengineering00rais_350 |url-access=limited |page=[https://archive.org/details/radioengineering00rais_350/page/n25 7] |publisher=Artech House |date=2003 |isbn=978-1580536691}}</ref>

[[Irving Langmuir]] at the [[General Electric]] research laboratory ([[Schenectady, New York]]) had improved [[Wolfgang Gaede]]'s [[diffusion pump|high-vacuum diffusion pump]] and used it to settle the question of thermionic emission and conduction in a vacuum. Consequently, General Electric started producing hard vacuum triodes (which were branded Pliotrons) in 1915.<ref>{{Cite web|url=http://edisontechcenter.org/GEresearchLab.html |title=General Electric Research Lab History |date=2015 |author=Edison Tech Center |website=edisontechcenter.org |access-date=2018-11-12}}</ref> Langmuir patented the hard vacuum triode, but de Forest and AT&T successfully asserted priority and invalidated the patent.

Pliotrons were closely followed by the French type '[[TM (triode)|TM]]' and later the British type 'R', which were in widespread use by the allied military by 1916. Historically, vacuum levels in production vacuum tubes typically ranged from 10 [[Pascal (pressure)|μPa]] down to 10&nbsp;nPa  ({{convert|10|uPa|Torr|sigfig=1|lk=out|disp=out}} down to {{convert|10|nPa|Torr|sigfig=1|disp=out}}).<ref>J.Jenkins and W.H.Jarvis, "Basic Principles of Electronics, Vol. 1 Thermionics", Pergamon Press (1966), Ch. 1.10 p. 9</ref>

The triode and its derivatives (tetrodes and pentodes) are [[transconductance]] devices, in which the controlling signal applied to the grid is a ''voltage'', and the resulting amplified signal appearing at the anode is a ''current''.<ref>Departments of the Army and the Air Force (1952). [https://archive.org/details/BasicTheoryAndApplicationOfElectronTubes/page/n53/mode/2up?view=theater ''Basic Theory and Application of Electron Tubes'']. Washington D. C.: USGPO. p. 42. Retrieved Oct 2021</ref> By comparison the later [[bipolar junction transistor]] uses a small current to control a larger current.

For vacuum tubes, transconductance or mutual conductance ({{math|''g''<sub>m</sub>}}) is defined as the change in the plate(anode)/cathode current divided by the corresponding change in the grid to cathode voltage, with a constant plate(anode) to cathode voltage. Typical values of {{math|''g''<sub>m</sub>}} for a small-signal vacuum tube are 1 to 10 millisiemens. It is one of the three main parameters of a vacuum tube, the other two being its gain μ and plate resistance {{math|''R''<sub>p</sub>}} or {{math|''R''<sub>a</sub>}}; these parameters are related by the Van der Bijl equation: <math>g_m = {\mu \over R_p}</math>

The plate current of the triode was not accurately proportional to the grid voltage, i.e. the  operating characteristic was non-linear, causing early tube audio amplifiers to exhibit harmonic distortion at low volumes. Plotting plate current as a function of applied grid voltage, it was seen that there was a range of grid voltages for which the transfer curve was approximately linear.

To use this range, a negative bias voltage had to be applied to the grid to position the [[Direct current|DC]] operating point in the linear region. This was called the idle condition, and the plate current at this point the "idle current". The controlling voltage was superimposed onto the bias voltage, resulting in a nearly linear variation of plate current in response to positive and negative variation of the input voltage around that point.

This concept is called ''[[grid bias]]''. Many early radio sets had a third battery called the "C battery" (unrelated to the present-day [[C battery|C cell]], a format). The C battery's positive terminal was connected to the cathode of the tubes ("ground" in most circuits) and the negative terminal supplied bias voltage to the grids of the tubes.

Later circuits, after tubes were made with heaters isolated from their cathodes, used [[cathode bias]]ing, avoiding the need for a separate negative power supply. For cathode biasing, a relatively low-value resistor is connected between the cathode and ground. Current flow through the resistor makes the cathode positive with respect to the grid, which is at ground potential for DC.

However C batteries continued to be included in some equipment even when the "A" and "B" batteries had been replaced by power from the AC mains. That was possible because there was essentially no current draw on these batteries; they could thus last for many years (often longer than all the tubes) without requiring replacement.

When triodes were first used with [[Tuned circuit|tuned rather than resistive]] loads in radio-frequency transmitters and receivers, it was found that tuned amplification stages had a tendency to oscillate unless their gain was very limited, due to the parasitic capacitance, termed [[Miller effect|Miller capacitance]], between the plate (the amplifier's output) and the control grid (the amplifier's input).

Eventually the technique of ''neutralization'' was developed whereby the RF transformer connected to the plate (anode) included an additional winding in the opposite phase, connected to the grid through a small capacitor. When properly adjusted this cancelled the Miller capacitance. This technique was successfully employed in the [[Neutrodyne]] radio during the 1920s. Neutralization was dependent upon the frequency; it required careful adjustment and did not work over a wide range of frequencies.