What are power amplifier classes A, AB, B, C, D, E and F?
PA classes describe how the transistor is biased and driven. Classes A, AB, B and C are defined by conduction angle: 360° for A (max efficiency 50%), between 180° and 360° for AB, 180° for B (max π/4 ≈ 78.5%) and below 180° for C (approaching 100% as the angle approaches zero, with vanishing output). Classes D, E and F use switching or harmonic waveform shaping to reach a theoretical 100% efficiency.[1][2]
How do the classes compare?
| Class | Principle | Ideal max efficiency | Linearity |
|---|---|---|---|
| A | Always conducting | 50% | Best |
| AB | 180°–360° | 50–78.5% | Good; most linear PAs |
| B | 180° | 78.5% | Crossover distortion in push-pull |
| C | <180° | →100% | Poor; constant-envelope only |
| D | Switching pair, square voltage | 100% | Switching |
| E | Single switch, load network shapes ZVS | 100% | Switching |
| F / inverse F | Harmonic terminations square up voltage/current | 100% (infinite harmonics) | Saturated |
How does class E work?
Class E, introduced by Nathan and Alan Sokal in 1975, uses a single switch and a load network designed so that the switch voltage and its slope are zero at turn-on (zero-voltage and zero-derivative switching), eliminating capacitive switching loss. Its switch peak voltage is about 3.56 VDD.[3]
How does class F work?
Class F terminates the transistor output in a short at even harmonics and an open at odd harmonics so the drain voltage approaches a square wave and current a half-sine, reducing overlap. Practical designs control the second and third harmonics.[1]
Which class is used where?
Linear modulations (OFDM, QAM) use class AB, often in Doherty configurations with DPD. Constant-envelope systems (FM, GFSK, GSM) can use saturated C, E or F. Envelope tracking and polar transmitters combine switching efficiency with linear modulation.
Frequently asked questions
Why is class A limited to 50%?
DC power is constant while the maximum RF swing delivers only half of it to the load.
Is class AB a compromise?
Yes, between class A linearity and class B efficiency.
Are switching PAs linear?
Not in amplitude; they need polar or outphasing architectures for amplitude modulation.
See also
References
Online sources accessed October 21, 2026; printed works cited by edition. See all sources.
- “RF Power Amplifiers for Wireless Communications, 2nd ed.”. S. C. Cripps, Artech House. 2006.
- “RF Microelectronics, 2nd ed.”. B. Razavi, Prentice Hall. 2012.
- “Class E: A New Class of High-Efficiency Tuned Single-Ended Switching Power Amplifiers”. N. O. Sokal and A. D. Sokal, IEEE Journal of Solid-State Circuits, vol. 10, no. 3, pp. 168-176. June 1975.