How do I linearize a power amplifier using digital predistortion?
DPD Implementation
Without DPD, a PA must operate well below its compression point (6-10 dB back-off) to meet linearity specifications. DPD allows the PA to operate 2-4 dB closer to compression while maintaining the same ACPR specification. This higher operating point improves efficiency significantly because the PA converts more DC power to RF output rather than wasting it as heat.
| 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
Frequently Asked Questions
How much ACPR improvement is typical?
15-25 dB for memoryless DPD. 25-35 dB for memory polynomial DPD with adequate memory depth. The improvement is limited by the PA's residual non-idealities (noise, thermal drift) that the DPD model cannot capture. For most base station applications, DPD achieves ACPR < -50 dBc.
What about DPD for 5G NR?
5G NR signals with 100 MHz bandwidth require DPD bandwidth of 300-500 MHz, demanding very fast DACs and ADCs. Multi-band DPD (concurrent correction of signals in different frequency bands) adds further complexity. These requirements push DPD processing to advanced FPGA or custom ASIC platforms.
Does DPD work with GaN PAs?
Yes, and GaN PAs benefit more from DPD than GaAs because GaN PA nonlinearity is more predictable and memoryless at moderate back-off. GaN also has stronger memory effects at high power, which specialized memory DPD models handle effectively.