A triode is an electronic amplifying vacuum tube 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, 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. Triodes were widely used in consumer electronics devices such as radios and televisions until the 1970s, when transistors replaced them. Today, their main remaining use is in high-power RF amplifiers in radio transmitters and 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 the sound of tube-based electronics.
Examples of low power triodes from 1918 (left) to miniature tubes of the 1960s (right)
ECC83, a dual triode used in 1960-era audio equipment
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.
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.
An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal. It is a two-port electronic circuit that uses electric power from a power supply to increase the amplitude of a signal applied to its input terminals, producing a proportionally greater amplitude signal at its output. The amount of amplification provided by an amplifier is measured by its gain: the ratio of output voltage, current, or power to input. An amplifier is defined as a circuit that has a power gain greater than one.
A McIntosh stereo audio amplifier with output power of 50 watts per channel used in home component audio systems in the 1970s.
De Forest's prototype audio amplifier of 1914. The Audion (triode) vacuum tube had a voltage gain of about 5, providing a total gain of approximately 125 for this three-stage amplifier.
Power amplifier by Skyworks Solutions in a Smartphone.
An LM741 general purpose op-amp