The Power Amplifier stage of traditional RF amplifiers must be biased in the linear region to avoid creating distortion and harmonics. This constrains the efficiency to be around 55-60%, which means that for every watt of RF output, an additional .8 watts of power is wasted. This wasted power contributes to power supply costs, cooling requirements, and ultimately, cost, weight, and size.
By using a class D power amplifier, much higher efficiency can be achieved, between 80 to 90%. A class D amplifier is not linear, so instead of attempting to linearly amplify to SSB signal, it is actually created in the RF Power Amplifier stage. The class D power amplifier is driven by a phase-modulated signal, which is amplitude modulated by the drain supply in the amplifier.