What is the difference between Class A, Class AB, Class B, and Class C amplifier operation?
PA Class Comparison
The conduction angle determines the tradeoff between linearity and efficiency. Class A bias keeps the transistor conducting at all times, faithfully reproducing the input waveform but wasting power because current flows even when there is no signal. At full drive, the DC power is equally split between the output signal and the waste heat, giving 50% efficiency.
| Parameter | LNA | Driver | Power Amplifier |
|---|---|---|---|
| Noise Figure | 0.3-2.0 dB | 3-8 dB | 5-15 dB (not specified) |
| Gain | 10-25 dB | 10-20 dB | 8-15 dB |
| P1dB | -10 to +10 dBm | +15 to +25 dBm | +30 to +50 dBm |
| OIP3 | +5 to +25 dBm | +25 to +40 dBm | +40 to +55 dBm |
| DC Power | 10-100 mW | 0.5-5 W | 5-500 W |
Bias and Operating Point
Class AB reduces the bias so that the transistor turns off for a small portion of the negative cycle. The resulting waveform clipping is minimal for small and moderate signals (good linearity), and the efficiency improves because less DC current flows during the no-signal portions. Most practical linear PAs operate in Class AB with conduction angles of 200-300°.
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
- Interface compatibility: verify impedance, connector type, and mechanical form factor match the system architecture
Stability Considerations
Class B biases the transistor at the cutoff point so it conducts for exactly half the RF cycle. This produces significant harmonic distortion (the half-sinusoid output contains all harmonics) but achieves 78.5% theoretical efficiency. A push-pull Class B amplifier uses two transistors biased at cutoff, each amplifying one half of the waveform, to reconstruct the full waveform and cancel even harmonics.
Frequently Asked Questions
Why is Class A used at all?
Class A provides the highest linearity and is used in test equipment, precision measurement, high-fidelity applications, and as driver stages where output power is modest and linearity is critical. At low power levels, the efficiency penalty is acceptable because the absolute wasted power is small.
What is the practical efficiency of Class AB?
At the backed-off power levels needed for OFDM/QAM signals (6-8 dB below P1dB): practical PAE is 10-25% for Class AB without efficiency enhancement. With Doherty or ET: 30-50%. The theoretical Class AB maximum (50-78%) is achieved only at full compression, which is too nonlinear for most modulation formats.
What about Class D, E, and F?
These are switching-mode amplifier classes where the transistor acts as a switch (on/off) rather than a current source. They achieve 80-95% efficiency but produce square-wave or shaped outputs that require filtering. Used for constant-envelope modulation (FM), outphasing transmitters, and polar transmitters where amplitude modulation is applied separately.