A triode is an electronic amplification device having three active electrodes. The term most commonly applies to a vacuum tube (or valve in British English) with three elements: the filament or cathode, the grid, and the plate or anode. The triode vacuum tube was the first electronic amplification device, which propelled the electronics age forward, by enabling amplified radio technology and long-distance telephony. Triodes were widely used in consumer electronics until the 1950s, when bipolar junction transistors replaced them; their main remaining use is for high power RF amplifiers in radio transmitters and industrial RF heating devices. The word is derived from the Greek τρίοδος, tríodos, from tri- (three) and hodós (road, way), originally meaning the place where three roads meet.
The first three-element device (mercury-vapor filled with a control grid) was patented on March 4, 1906 by the Austrian Robert von Lieben; independent from that, on October 25, 1906 Lee De Forest patented his two-element Audion. The original Audion did not provide amplification. However it was not until around 1912 that other researchers, while attempting to improve the service life of the Audion, stumbled on the principle of the true vacuum tube. The name triode appeared later, when it became necessary to distinguish it from other generic kinds of vacuum tubes with more or fewer elements (e.g. diodes, tetrodes, pentodes, etc.). The Audion tubes deliberately contained some gas at low pressure. The name triode is only applied to vacuum tubes which have been evacuated of as much gas as possible.
There was a lengthy lawsuit between von Lieben and De Forest.
The directly-heated cathode (or indirectly by means of a filament) produces an electron charge by thermionic emission. This electron stream is attracted to the positively-charged plate (anode), inducing a current. Applying a negative DC voltage ("bias") to the control grid will repel some of the electron stream back towards the cathode, thus isolating the plate from the cathode; full bias will turn the tube off by blocking all current from the cathode. Conversely, increasing the positive DC voltage on the plate will attract more electrons toward it. As grid bias is increased, more of the electron current is repelled, resulting in a smaller current at the plate. An AC signal voltage superimposed on the grid will appear as variations in the plate current; voltage amplification can be obtained by using a suitable value of plate load resistance.
The triode is very similar in operation to the n-channel JFET; it is normally on, and progressively switched off as the grid/gate is pulled increasingly negative of the source/cathode.
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The triode was the first device to provide power gain at audio and radio frequencies, and made radio practical. Triodes are used for amplifiers and oscillators. 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-marketed consumer electronics, having been overtaken by less expensive transistor based solid-state devices. Triodes continue to be used in certain performance sensitive areas such as high-end and professional audio applications, as well as in microphone preamplifiers, electric guitar amplifiers and high power RF amplifiers and transmitters.
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 the operating point of the particular triode.
In the example characteristic shown on the image, if an anode voltage Va of 200 V and a grid voltage bias of -1 volt are selected, a plate (anode) current of 2.25 mA will be present (using the yellow curve on the graph). Changing the grid voltage will change the plate current; by suitable choice of a plate load resistor, amplification is obtained.
In the class A triode amplifier, an anode resistor would be connected between the anode and the positive voltage source. For example, with Ra=10000 Ohms, the voltage drop on it would be
VRa=Ia×Ra=22.5 V if an anode current of Ia=2.25 mA is chosen.
If the input voltage amplitude (at the grid) changes from -1.5 V to -0.5 V (difference of 1 V), the anode current will change from 1.2 to 3.3 mA (see image). This will change the resistor voltage drop from 12 to 33 V (a difference of 21 V).
Since the grid voltage changes from -1.5 V to -0.5 V and the anode resistor voltage drops from 12 to 33 V, an amplification of the signal resulted. The amplification factor is 21 - output voltage amplitude divided by input voltage amplitude.
See also 
-  DRP 179807
- Tapan K. Sarkar (ed.) "History of wireless", John Wiley and Sons, 2006. ISBN 0-471-71814-9, p.335
- Sōgo Okamura (ed), History of Electron Tubes, IOS Press, 1994 ISBN 90-5199-145-2 page 20
-  Patent US841387 from 10/25/1906
- U.S. Patent 879,532
- Les lampes radio — A French page on thermionic valves. Of particular interest is the 17 minute video showing the manual production of triodes.