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Single grid amplifying vacuum tube having three active electrodes
the electronic vacuum tube

A triode is an electronic amplifying vacuum tube (or thermionic valve in British English) 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 (Fleming valve), 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 helped make amplified radio technology and long distance telephony possible. some time around 1920, derived from the Greek τρίοδος, tríodos, from tri (three) and hodós (road, way), originally meaning the place where three roads meet.

Before thermionic valves were invented, Philipp Lenard used the principle of grid control while conducting photoelectric experiments in 1902. The first vacuum tube used in radio was the thermionic diode or Fleming valve, invented by John Ambrose Fleming in 1904 as a detector for radio receivers. It was an evacuated glass bulb containing two electrodes, a heated filament (cathode) and a plate (anode).

Triodes came about in 1906 when American engineer Lee de Forest and Austrian physicist Robert von Lieben independently patented tubes that added a third electrode, a control grid, between the filament and plate to control current. Von Lieben's partially evacuated three element tube, patented in March 1906, contained a trace of mercury vapor and was intended to amplify weak telephone signals. Like the von Lieben vacuum tube, De Forest's Audions were incompletely evacuated and contained some gas at low pressure. von Lieben's vacuum tube did not see much development due to his death seven years after its invention, shortly before the outbreak of the First World War. De Forest's Audion did not see much use until its ability to amplify was recognized around 1912 by several researchers, who used it to build the first successful amplifying radio receivers and electronic oscillators. The many uses for amplification motivated its rapid development. By 1913 improved versions with higher vacuum were developed by Harold Arnold at American Telephone and Telegraph Company, which had purchased the rights to the Audion from De Forest, and Irving Langmuir at General Electric, who named his tube the "Pliotron",

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. Then the output voltage and amplification of the triode can be evaluated graphically by drawing a load line on the graph. In the example characteristic shown on the image, suppose we wish to operate it at a quiescent anode voltage Va of 200 V and a grid voltage bias of −1 V. This implies a quiescent plate (anode) current of 2.2 mA (using the yellow curve on the graph). In a class A triode 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, the voltage amplification factor (or mu) is 17 in this case. It is also possible to use triodes as cathode followers in which there is no voltage amplification but a huge reduction in dynamic impedance; in other words, the current is greatly amplified (as it also is in the common cathode configuration described 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).