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
Single-ended triode SET) is used when directly heated triodes are used. There are also single-ended tetrode, beam tetrode/beam power tube/kinkless tetrode, and pentode amplifiers with the same functionality and similar circuitry; e.g. this Mullard design. A typical triode audio power amplifier will have a driver that provides voltage gain, coupled to a triode (like 2A3 and 300B) or a pentode or kinkless tetrode such as EL34 or KT88 connected as a triode, connected to the loudspeaker through an audio transformer in a common cathode arrangement. The triode is biased to Class A operation by applying a suitable negative bias voltage to its