How do I design a Doherty power amplifier for improved efficiency with modulated signals?
Doherty PA Architecture
The Doherty amplifier was invented in 1936 for AM broadcast transmitters and has experienced a renaissance in cellular base station PAs because modern digital modulation formats (LTE, 5G NR) have high peak-to-average power ratios (PAPR) of 6-10 dB. A conventional Class AB PA operates most of the time at backed-off power levels where efficiency is poor. The Doherty architecture maintains high efficiency across the backed-off range by dynamically adjusting the load impedance.
| 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 |
- 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 much efficiency improvement is typical?
For a 7 dB PAPR LTE signal: conventional Class AB achieves 15-25% average PAE. A symmetric Doherty achieves 30-40%. An asymmetric Doherty (with a larger peaking amplifier) achieves 35-45%. Digital Doherty with DPD achieves 40-50%. The improvement depends on the signal statistics and the Doherty back-off range.
What are the bandwidth limitations?
The quarter-wave impedance inverter is inherently narrowband (approximately 10-20% bandwidth). Wideband Doherty designs use modified inverters (stepped impedance, lumped-element approximations) or output transformer combiners for 20-30% bandwidth. Multi-band Doherty PAs use digitally controlled matching networks.
What is an asymmetric Doherty?
An asymmetric Doherty uses a peaking amplifier larger than the carrier amplifier (typically 2× the carrier power). This extends the high-efficiency range from 6 dB to 9-10 dB back-off, better matching the PAPR of modern signals. The 2:1 power ratio is the most common asymmetric configuration.