How do I measure the efficiency of a power amplifier under modulated signal conditions?
PA Efficiency Under Modulation
Measuring PA efficiency under modulated conditions reveals the real-world efficiency that determines the thermal design, power supply requirements, and operating cost of the system.
| 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
Frequently Asked Questions
Why is modulated efficiency so much lower than CW?
Under CW: the PA operates at a constant output power, near its peak efficiency point (Psat or P1dB). Under a modulated signal: the output power varies continuously; the PA spends most of the time at the average power (7-12 dB below Psat), where the efficiency is 10-25% for a linear PA. The peaks (which reach near Psat) occur only briefly (0.01-1% of the time) and do not significantly contribute to the average efficiency. The average efficiency is dominated by the backed-off (low power, low efficiency) portion of the signal.
How do I report the measurements?
Report: the waveform used (standard, bandwidth, PAPR/CM), the PA's operating frequency and bias point, the average output power (dBm and watts), the peak output power (if measured), the DC supply voltage and average current (for each supply rail), PAE and/or drain efficiency (%), EVM and ACLR (to confirm the PA is meeting the linearity specification at the measured efficiency), and the PA's junction temperature (to confirm thermal equilibrium).
What about time-domain efficiency measurement?
For signals with significant duty cycling (radar, TDMA): the instantaneous efficiency varies dramatically between on and off periods. Time-domain measurement: use a high-speed digitizer to capture the RF output envelope and DC current waveform simultaneously. Compute the instantaneous efficiency at each time sample. This reveals: the peak efficiency during the on period, the quiescent (standby) power during the off period, and the overall average efficiency including the standby power. A PA with 50% peak efficiency and 50% duty cycle has an average efficiency of approximately 25% if the quiescent power is significant.