How do I calculate the thermal dissipation of an RF amplifier from its efficiency and output power?
Amplifier Thermal Analysis
Every RF power amplifier converts DC power to RF output power with some efficiency, and all unconverted power becomes heat. This heat must be removed through the thermal path to prevent the semiconductor junction from exceeding its rated temperature, which would cause permanent damage or accelerated aging.
| 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
What junction temperature is safe?
GaN: typically rated to 225°C, derate to 175°C for long life. GaAs: rated to 175°C, derate to 150°C. LDMOS: rated to 200°C, derate to 160°C. Every 10°C reduction doubles the estimated lifetime (Arrhenius relationship).
Natural or forced air cooling?
Natural convection heatsinks provide Rth of 1-5°C/W depending on size. Forced air reduces this to 0.3-1°C/W. Liquid cooling achieves 0.05-0.2°C/W. For dissipation above 50W, forced air is typically required. Above 500W, liquid cooling becomes practical.
How does altitude affect cooling?
Air density decreases with altitude, reducing convective cooling effectiveness. At 3000m (10,000 ft), air cooling efficiency drops by approximately 20%. Active cooling systems must be derated for altitude. Conduction-cooled chassis are unaffected by altitude.