Which class of amplifier provides the least efficiency?

A Class A amplifier is biased so that its active device conducts current throughout the entire 360 degrees of the input signal cycle. Because it never completely turns off, it draws a continuous, heavy steady-state current even when no input signal is present. This continuous conduction means a massive amount of DC power is constantly dissipated as heat rather than being converted into useful RF output. As a result, Class A amplifiers have the lowest efficiency of all standard amplifier classes, typically hovering between 25% and 50%. While Classes AB, B, and C shut off during parts of the cycle to conserve energy, Class A sacrifices this efficiency entirely to ensure perfect replication of the input signal without clipping.