Envelope Tracking
Understanding Envelope Tracking
Envelope tracking addresses the fundamental efficiency problem of linear PAs with high-PAPR signals. A constant-supply Class AB PA is only efficient near compression; at the average power (6-10 dB below peak), efficiency drops dramatically. ET solves this by reducing the supply voltage at low signal levels.
ET Architecture
- Baseband processor generates the envelope signal from the I/Q data.
- Envelope modulator (high-efficiency supply modulator) generates a time-varying supply voltage.
- PA supply voltage tracks the signal envelope in real-time.
- PA operates near compression at every amplitude level.
ET vs Doherty
| Parameter | ET | Doherty |
|---|---|---|
| Efficiency (OFDM) | 40-55% | 35-50% |
| Bandwidth | Wide (envelope modulator limited) | Narrow (lambda/4 combiner) |
| Complexity | High (supply modulator + DPD) | Moderate |
| Application | Handsets, small cells | Base stations |
Frequently Asked Questions
What is envelope tracking?
ET dynamically modulates the PA supply voltage to follow the signal envelope, keeping the PA near compression at all levels. This achieves 40-55% efficiency for OFDM signals vs 15-25% for fixed-supply operation.
Why is ET important for 5G?
5G OFDM signals have 8-10 dB PAPR. With fixed supply, the PA operates 8-10 dB below peak most of the time, wasting power. ET keeps the PA efficient at all levels, reducing power consumption and heat in handsets and small cells.
What is the bandwidth limitation of ET?
The supply modulator must track the signal envelope, which has bandwidth equal to the RF signal bandwidth (or wider). For 100 MHz 5G NR signals, the supply modulator needs ~200 MHz bandwidth. This is the primary design challenge.