An amplifier is a device which changes a small movement into a larger movement. However, in general the most common and useful amplifiers actually use a small amount of energy to control a larger amount of energy. Furthermore it is of significant advantage if the amplified output is in a linear relationship with that of the input. This relationship is known as the gain of the amplifier.
The most common type of amplifier is the electronic amplifier, commonly used in radio and television transmitters and receivers, hi-fi units, microcomputers and other electronic digital equipment, and guitar and other instrument amplifiers.
Another type of amplifier is the fluidic amplifier, based on the fluidic triode.