Any device suited to increase an input signal.
Amplifiers are generally incorporated within a feedback device. M.D. RUBIN even describes the feedback as "a unilateral power amplifier, with a fraction of the output of the amplifier substracted from or added to the input of the amplifier" (1968, p.9)
H. ODUM describes a "constant gain amplifier", within an energy network in which: "… the amount of energy supplied from the upper flow is that necessary to increase the force expressed by the system by a constant factor, called the gain" (1983, p.39)