Power Amplifier Classes
Understanding PA Classes
PA class selection is the most fundamental architecture decision in transmitter design. The class determines the operating bias point, which controls how much of the RF cycle the transistor conducts current, directly affecting efficiency and linearity.
PA Class Comparison
| Class | Conduction Angle | Max Efficiency | Linearity |
|---|---|---|---|
| A | 360 deg | 50% | Best |
| AB | 180-360 deg | 50-78% | Good |
| B | 180 deg | 78.5% | Moderate |
| C | < 180 deg | 85-90% | Poor |
| D/E/F | Switching | 90-100% | None (switching) |
Practical Applications
- Class A: LNAs, driver stages, instruments. Maximum linearity.
- Class AB: Most cellular/satellite PAs. Balance of linearity and efficiency.
- Class C: FM transmitters, frequency multipliers. Constant envelope only.
- Class E/F: Switch-mode PAs for high-efficiency with DPD for linearization.
Frequently Asked Questions
What are PA classes?
PA classes define the transistor bias point and conduction angle. Class A: most linear, 50% max efficiency. Class AB: practical compromise. Class B: 78% max. Class C: high efficiency, nonlinear. Class D/E/F: switching modes, 90%+ efficiency.
Why is Class AB most common?
Class AB provides the best practical compromise between linearity (needed for modern modulations) and efficiency (needed for thermal and power management). Most cellular base station PAs use Class AB operated in 6-10 dB backoff with DPD.
Can switching-class PAs be used for linear signals?
Yes, with digital predistortion (DPD) or envelope tracking. Switch-mode PAs (Class E/F) are inherently nonlinear but combined with DPD, they can achieve adequate linearity for OFDM signals while maintaining 50-60% average efficiency.