An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the power of a signal.
It does this by taking energy from a power supply and controlling the output to match the input signal shape but with
a larger amplitude. In this sense, an amplifier modulates the output of the power supply to make the output signal
stronger than the input signal.
An amplifier is effectively the opposite of an attenuator: while an amplifier provides gain, an attenuator provides loss.
An amplifier can either be a separate piece of equipment or an electrical circuit within another device.
The ability to amplify is fundamental to modern electronics, and amplifiers are widely used in almost all
electronic equipment. The types of amplifiers can be categorized in different ways. One is by the frequency
of the electronic signal being amplified; audio amplifiers amplify signals in the audio (sound) range of
less than 20 kHz, RF amplifiers amplify frequencies in the radio frequency range between 20 kHz and 300 GHz.
Another is which quantity, voltage or current is being amplified; amplifiers can be divided into voltage
amplifiers, current amplifiers, transconductance amplifiers, and transresistance amplifiers. A further
distinction is whether the output is a linear or nonlinear representation of the input.
Amplifiers can also be categorized by their physical placement in the signal chain.
The first practical electronic device that could amplify was the Audion (triode) vacuum tube, invented in 1906 by
Lee De Forest, which led to the first amplifiers. The terms "amplifier" and "amplification" (from the Latin amplificare,
'to enlarge or expand') were first used for this new capability around 1915 when triodes became widespread.
For the next 50 years, vacuum tubes were the only devices that could amplify.
All amplifiers used them until the 1960s, when transistors appeared. Most amplifiers today use transistors,
though tube amplifiers are still produced.