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 audio amplifier technical specification buffer (giving gain) to then feed a concertina phase splitter is a classic push–pull front end, typically followed by a driver (triode) and (triode or pentode) output stage (in ultra linear in many cases) to form the classic push–pull amplifier circuit. The push–pull output circuit shown is a simplified variation of the Williamson topology, which comprises four stages: The cascode (a contraction of the phrase "cascade to cathode") is a two-stage amplifier composed of a transconductance amplifier followed by a current buffer. In valve circuits, the cascode is often constructed from two triodes connected in series, with one operating as