An electronic amplifier is a device for increasing the power of a signal. An idealized amplifier can be said to be "a piece of wire with gain", as the output is an exact replica of the input, but larger. It does this by taking power from a power supply and controlling the output to match the input signal shape but with a larger amplitude, in this sense an amplifier may be considered as modulating the output of the power supply.
Real world amplifiers are not ideal and this control is thus imperfect. One consequence is that the power supply itself may influence the output, and must itself be considered when designing the amplifier. The amplifier circuit has an "open loop" performance, that can be described by various parameters. The majority of modern amplifiers apply some negative feedback to form a control loop surrounding the gain stage itself.