Amplifier by their physical placement in the signal chain; a preamplifier may precede other signal processing stages, for example. The first practical electrical device which could amplify was the triode vacuum tube, invented in 1906 by Lee De Forest, which led to the first amplifiers around 1912. Today most amplifiers use transistors. The first practical device that could amplify was the triode vacuum tube, invented in 1906 by Lee De Forest, which led to the first amplifiers around 1912. Vacuum tubes were used in almost all amplifiers until the 1960s–1970s when the transistor, invented in 1947, replaced them. Today, most amplifiers
Valve transmitters circuit. This circuit design has been used at 1296 MHz using disk seal triode valves such as the 2C39A. The grid is kept at ground, the drive is applied to the cathode through a capacitor. The heater supply must be isolated with great care from the cathode as unlike the other designs the cathode is not connected to RF ground. The cathode may be at the same DC potential as the grid if a valve such as the 811A (zero bias triode) is used, otherwise the cathode must be positive with respect to the grid to provide proper bias. This