Why non linear amplifiers are more battery efficient than linear ones?
When you push a non-linear amplifier transistor to extreme non-linearity, it becomes a fast switch, whose on-off cycle period is 1/f (where f is the RF carrier frequency). A switch dissipates no power when OFF (where current is zero), and it dissipates no power when ON (where voltage across it is zero). This is also why PWM controllers are also highly efficient. A transistor or FET wastes power when it conducts current while simultaneously having a voltage across its terminals. For a Class-A amplifier, this is the normal condition, 100% of the time.