How does the supply modulation bandwidth affect the linearity of an envelope tracking PA?
Envelope Tracking Supply Modulation Bandwidth
Envelope tracking provides a significant efficiency advantage over fixed-supply PAs by dynamically adjusting the drain voltage to follow the signal envelope, keeping the PA near saturation at all signal levels. However, the supply modulator's bandwidth and efficiency are critical to achieving this advantage.
| 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
Can ET PAs handle 5G NR 100 MHz bandwidth signals?
It is challenging. A 100 MHz 5G NR signal requires 200-300 MHz supply modulator bandwidth. Current state-of-the-art hybrid supply modulators achieve 100-200 MHz bandwidth with 70-80% efficiency. At these bandwidths, the ET efficiency advantage over Doherty narrows. For 5G sub-6 GHz base stations, Doherty (with DPD) remains the dominant architecture for signals wider than approximately 60 MHz. ET is more common in handsets (narrower signals, smaller PAs) and in mmW 5G where the signal bandwidths are 50-200 MHz but the PA power levels are lower.
Is ET or Doherty better for 5G base stations?
For sub-6 GHz with > 60 MHz bandwidth: Doherty with DPD provides approximately 45-55% average efficiency and handles wide bandwidths easily. ET provides approximately 50-60% efficiency for narrower signals (< 40 MHz) but efficiency drops for wider signals. For handsets: ET is preferred because the PA output power is low (< 2 W), the signal bandwidth is typically 20-40 MHz per carrier, and the ET supply modulator can be integrated compactly. For mmW 5G: both approaches are being explored; ET is particularly attractive because mmW PAs have lower power levels (0.1-1 W) where supply modulator efficiency is higher.
What is the minimum supply modulator bandwidth for acceptable linearity?
The minimum bandwidth for < 2 dB ACLR degradation (compared to ideal ET) is approximately 2x the signal RF bandwidth. For < 0.5 dB degradation: approximately 3x. These are empirical guidelines; the exact requirement depends on the PA's sensitivity to drain voltage variation (AM-AM and AM-PM vs. V_dd characteristics) and the acceptable EVM degradation. Simulation with the actual PA model and modulated signal is the most reliable method to determine the minimum bandwidth.