How do I design a power combiner for combining the output of multiple power amplifiers?
RF Power Combiner Design
Power combining is the standard technique for generating RF power levels beyond what a single transistor or MMIC can produce. The combiner's efficiency directly determines the system's overall power efficiency.
| 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 |
- 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
- Margin allocation: include sufficient design margin to account for manufacturing tolerances and aging effects
Frequently Asked Questions
How many PAs can I combine?
The practical limit depends on the combiner loss per stage: for Wilkinson (0.2 dB per stage): 2-way: 0.2 dB loss. 4-way: 0.4 dB. 8-way: 0.6 dB. 16-way: 0.8 dB. 32-way: 1.0 dB. Beyond 16-32 PA combining: the combiner loss becomes significant relative to the power gained by adding more PAs. At 64-way: the combiner loss is approximately 1.2 dB, meaning 24% of the power is wasted. For N greater than 16-32: consider a radial combiner (0.3-0.5 dB for 16-32 way) or spatial combining.
What about phase matching between PAs?
All PA outputs must be phase-matched for efficient combining. Phase error reduces the combining efficiency: for phase error delta_phi: combining loss = -10log10(cos²(delta_phi/2)). For 10 degrees phase error: loss = 0.07 dB (negligible). For 30 degrees: loss = 0.6 dB (significant). For 90 degrees: loss = 3 dB (half the power is wasted). Phase matching is achieved by: using equal-length transmission lines from all PAs to the combiner, matching the PA modules' group delay, and phase-trimming individual channels if needed.
What happens when a PA fails?
In a well-designed Wilkinson combiner: when one PA of N fails: the output power drops by more than 1/N because: the failed PA contributes zero power, and the power from the working PAs that would have combined with the failed PA's signal is absorbed by the isolation resistors. For a 4-way combiner with one PA failed: P_out = (3/4)² × 4 × P_PA = 9/4 × P_PA = 2.25 × P_PA (compared to 4 × P_PA for all working). The output drops by 2.5 dB (not 1.25 dB). Graceful degradation is better with: more PAs (the impact of one failure is smaller) and higher isolation (less interaction between working and failed PAs).