Editing Triode (section)

== Operation ==
{{multiple image
| align = right
| direction = horizontal
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| image1   = Triode with filament and cathode labeled.svg
| caption1 = Triode with separate cathode and filament.
| width1   = 123
| image2   = Triode with filament labeled.svg
| caption2 = Triode in which filament serves as cathode.
| width2   = 120
| image3   = Triode with cathode labeled.svg
| caption3 = Filament omitted from diagram.
| width3   = 120
| footer   = Schematic circuit symbols for triodes. (''F'') filament, (''C'') cathode, (''G'') grid, (''P'') plate 
}}

In the triode, [[electron]]s are released into the tube from the metal [[cathode]] by heating it, a process called [[thermionic emission]].  The cathode is heated red hot by a separate current flowing through a thin metal [[electrical filament|filament]]. 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 well [[vacuum|evacuated]] so 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 charged [[anode]] (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 in [[amplifier|amplification]]. 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 the [[transconductance]].  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 in [[voltage 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-channel [[JFET]]; 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 (V<sub>p</sub>) 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 the [[voltage 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 the [[power gain]], or the output power obtained from a certain AC input voltage is often of greater interest. When these devices are used as [[cathode follower]]s (or [[source follower]]s), they all have a voltage "gain" of just under 1, but with a large [[current gain]].