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 RF amplifier Single Ended Triode (DH-SET) audio amplifiers use radio transmitting valves originally designed to operate as RF amplifiers in the high frequency range. The most efficient valve-based RF amplifiers operate Class C. If used with no tuned circuit in the output, this would distort the input signal, producing harmonics. However, class C amplifiers normally use a high Q output network which removes the harmonics, leaving an undistorted sine wave identical to the input waveform. Class C is suitable only for amplifying signals with a constant amplitude, such as FM, FSK, and some CW (Morse code) signals. Where the amplitude of the