A simple, horrendously inefficient amplifier that makes use of the linear gains of transistors to drive an output. The input signal] uses a transistor to control the bias current fed to another transistor that is sitting quiecently in the middle of its linear range. The problem is that when the transistor is quiescent, there is still 1/2 the maximum amount of power being dissipated through the circuit. It is not uncommon for class A amplifiers to drain more power than the devices that they drive.