How do I design an outphasing or LINC transmitter architecture for linear amplification?
Outphasing/LINC Transmitter
The outphasing concept was invented by Henri Chireix in 1935 and revived in the 1990s as LINC. Despite decades of research, practical outphasing transmitters remain challenging due to the combiner problem.
| 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
Frequently Asked Questions
Is outphasing used in production?
Limited commercial adoption: outphasing has been demonstrated in research prototypes with excellent efficiency (50-70% at 6 dB backoff) but few production systems use it because: the Doherty architecture achieves comparable efficiency with simpler design, the outphasing signal decomposition requires high-speed digital processing (two DACs and two PA paths instead of one), and the mismatch sensitivity (any amplitude or phase imbalance between the two paths degrades the output linearity). Outphasing is used in some low-power transmitter ICs (for IoT and low-data-rate applications) where the small die area and high efficiency are attractive.
What is the Chireix combiner?
The Chireix combiner adds reactive elements (shunt stubs or lumped reactances) at the combiner junction to compensate for the reactive component of the PA load at specific outphasing angles. This creates two outphasing angles where the PA load is purely resistive and the efficiency peaks. The Chireix combiner provides 50-70% efficiency at 6-8 dB backoff (compared to 25-35% for an isolating combiner). Disadvantage: the efficiency peaks are at specific power levels, and the efficiency drops between the peaks. Extended Chireix designs use multiple reactive elements to create more efficiency peaks and flatten the efficiency curve.
How does outphasing compare to Doherty?
Doherty advantages: simpler architecture (one signal path, one DAC), well-understood design methodology, and proven in production at scale. Outphasing advantages: potentially higher efficiency at deep backoff (10+ dB), wider bandwidth potential (no quarter-wave impedance inverter), and can use identical PA devices (simpler supply chain). In practice: Doherty is the clear winner for base station and infrastructure applications. Outphasing remains a research topic for specialized applications (mmW, handset PAs, and highly efficient IoT transmitters).