How do I select a power amplifier to meet both output power and linearity requirements?
Power Amplifier Selection Criteria
The PA selection process starts with the system requirements: required output power at the antenna, cable/filter losses between PA and antenna, required linearity (ACPR, EVM, or IM3 specification), operating frequency and bandwidth, and efficiency (especially for battery-powered or high-power systems). These requirements constrain the PA technology, operating class, and specific device.
| Parameter | Class A | Class AB | Class F/Doherty |
|---|---|---|---|
| Max Efficiency | 50% | 50-78% | 70-90% |
| Linearity | Excellent | Good | Moderate (needs DPD) |
| P1dB Backoff | 0-3 dB | 3-6 dB | 6-10 dB |
| Complexity | Low | Low | High |
| Common Use | Test, small signal | General PA | Base station, broadcast |
Frequently Asked Questions
How much P1dB margin do I need?
Minimum 2-3 dB above the peak signal power for linear operation without DPD. With DPD, 0-1 dB above peak may be acceptable. For reliability and temperature margin, add another 1-2 dB. Total typical margin: 3-5 dB above peak signal power.
Should I pick the highest IP3 amplifier?
Not necessarily. Higher IP3 often comes with higher DC power consumption and cost. Choose the minimum IP3 that meets your linearity specification with appropriate margin. Use the cascade IP3 formula to determine the required IP3 for each stage.
How do I account for temperature?
PA performance degrades at high temperature: gain drops 0.01-0.03 dB/°C, P1dB drops 0.5-1 dB over 50°C, and efficiency decreases. Specify the PA for worst-case temperature. GaN devices degrade less than GaAs or LDMOS over temperature.