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 |
Bias and Operating Point
The carrier amplifier is biased in Class AB and operates at all power levels. The peaking amplifier is biased in Class C and only turns on when the signal amplitude exceeds a threshold (typically at the 6 dB backed-off point). The quarter-wave transmission line between the carrier and the combining point inverts the impedance, so the peaking amplifier's contribution looks like a load modulation to the carrier amplifier.
Stability Considerations
At full power: both amplifiers deliver maximum output, each seeing the nominal load impedance (typically 25 Ω for a 50 Ω combining). At 6 dB backed-off power: only the carrier operates, and the impedance inverter transforms the load to 100 Ω (4× nominal), which keeps the carrier near compression (high efficiency) even at reduced output power.
Thermal Management
When evaluating design a doherty power amplifier for improved efficiency with modulated signals?, engineers must account for the specific requirements of their target application. The optimal choice depends on the frequency range, power level, environmental conditions, and cost constraints of the overall system design.
Gain and Linearity Trade-offs
When evaluating design a doherty power amplifier for improved efficiency with modulated signals?, engineers must account for the specific requirements of their target application. The optimal choice depends on the frequency range, power level, environmental conditions, and cost constraints of the overall system design.
- 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
System-Level Integration
When evaluating design a doherty power amplifier for improved efficiency with modulated signals?, engineers must account for the specific requirements of their target application. The optimal choice depends on the frequency range, power level, environmental conditions, and cost constraints of the overall system design.
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.