How do you design a high-efficiency RF power amplifier and what are the different efficiency classes?

High-efficiency PA design involves selecting appropriate class based on application. Class AB provides moderate efficiency (50-60%) with good linearity for variable-envelope signals. Class C offers higher efficiency (70-80%) for constant-envelope FM/PM signals but poor linearity. Switching classes (D, E, F) achieve theoretical 100% by operating transistors as switches. Class E uses optimized load network for zero-voltage switching at turn-on. Class F uses harmonic tuning with open/short at harmonics. For modern wireless (OFDM), efficiency enhancement techniques include: Envelope tracking (dynamic supply modulation), Doherty combining (efficiency back-off improvement), and Digital pre-distortion (linearize efficient but nonlinear PAs).