What is envelope tracking and how does it improve power amplifier efficiency?
Envelope Tracking Architecture
In a conventional PA with a fixed supply voltage Vdd, the efficiency at backed-off power is poor because the output swing is small relative to the supply voltage. The transistor dissipates the difference as heat. Envelope tracking reduces Vdd in proportion to the signal envelope, maintaining a small margin between the signal swing and the supply rail at all times. This keeps the transistor near compression (high efficiency) regardless of the instantaneous signal amplitude.
| 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 |
Frequently Asked Questions
ET vs Doherty: which is better?
For handsets: ET is preferred because it works with a single PA (simpler, smaller). For base stations: Doherty is more mature and slightly higher efficiency at high power. The combination of Doherty + ET achieves the highest efficiency (50-60% average PAE). The choice depends on frequency, power level, and signal bandwidth.
What is the modulator bandwidth requirement?
The ET modulator must track the signal envelope, which has a bandwidth of 3-5× the RF signal bandwidth (due to the squaring operation in envelope detection). For a 100 MHz 5G NR signal: the ET modulator needs 300-500 MHz bandwidth. This is the primary challenge for wideband 5G ET implementations.
Does ET affect linearity?
Yes. The dynamic supply modulation introduces AM-AM and AM-PM distortion that must be corrected by digital pre-distortion (DPD). The DPD must model both the PA nonlinearity and the ET supply path nonlinearity. ET-aware DPD algorithms are well-developed and achieve ACPR < -50 dBc with ET.