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 |
Bias and Operating Point
The ET system consists of an envelope detector (or digital envelope generation from the baseband), a wideband power supply (ET modulator) that converts the envelope signal into a dynamic supply voltage, and the PA. The ET modulator is typically a combination of a linear regulator (for wideband tracking accuracy) and a switching converter (for high efficiency at the low-frequency content of the envelope).
Stability Considerations
ET has been widely adopted in mobile handsets (starting with Samsung and Qualcomm's QET implementation) for LTE and 5G sub-6 GHz transmitters. The power savings extend battery life by 10-20% compared to fixed-supply average power tracking (APT). For base station PAs, ET is competing with Doherty as the preferred efficiency enhancement technique.
- 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
Thermal Management
When evaluating envelope tracking and how does it improve power amplifier efficiency?, engineers must account for the specific requirements of their target application. The optimal choice depends on the frequency range, power level, environmental conditions, and cost constraints of the overall system design.
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.