How does load modulation affect the linearity of a Doherty power amplifier?
Doherty PA Linearity and Load Modulation Effects
The Doherty architecture is the dominant PA topology for 4G/5G base stations because it provides 2x the efficiency of Class-AB at backed-off power levels typical of OFDM signals. Understanding and managing its inherent nonlinearity is critical for meeting spectral emission standards.
| 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
Frequently Asked Questions
How much linearity does a Doherty PA sacrifice compared to Class AB?
Without DPD: a Doherty PA typically has 5-10 dB worse ACLR than a Class-AB PA at the same average output power. The Doherty's AM-AM and AM-PM variation is 2-3x larger. With DPD: both architectures achieve similar ACLR (-45 to -55 dBc), but the Doherty maintains 40-55% efficiency vs. 25-35% for Class AB. DPD correction capability is the enabling technology that makes Doherty commercially viable.
What happens at the peaking amplifier turn-on point?
The transition where the peaking amplifier begins conducting is the most challenging region for linearity. There is typically a gain step (1-3 dB), phase discontinuity (5-15 degrees), and a change in the gain slope. Modern GaN Doherty designs use: soft turn-on bias (the peaking amplifier is biased at a low Class C point rather than deep pinch-off), analog input signal shaping (modifying the drive level to the peaking path), or digital DPD with separate main/peaking models to smooth this transition.
Does the Doherty combiner bandwidth limit linearity?
Yes. The quarter-wave impedance inverter operates perfectly at only one frequency. Across a wide bandwidth (> 10% fractional), the impedance transformation ratio and phase shift deviate from ideal, degrading both efficiency and linearity at the band edges. Wideband Doherty designs use: post-matching compensation networks, stepped impedance inverters, or offset-line technique to extend the combiner bandwidth to 20-30% fractional bandwidth.