What is the memory effect in a power amplifier and how does it affect linearization?
PA Memory Effects
A memoryless PA has a fixed relationship between input and output: the same input amplitude always produces the same output amplitude and phase. A PA with memory has a relationship that depends on the recent history of the input signal. This means the AM-AM and AM-PM curves measured with CW signals differ from those effective with modulated signals, and the difference increases with modulation bandwidth.
| 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 |
- 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
How do I identify memory effects?
Measure the PA's two-tone intermodulation products at closely spaced and widely spaced tone separations. If the IM3 products are equal at all tone spacings, the PA is memoryless. If IM3 changes with tone spacing (especially asymmetry between upper and lower IM3), memory effects are present.
Can I reduce memory effects?
Wideband bias networks with low impedance at all envelope frequencies reduce electrical memory. Using bypass capacitors with multiple values (1 μF, 100 nF, 10 nF, 1 nF) in parallel on the drain supply provides wideband decoupling. Thermal memory is reduced by improved thermal management (lower junction temperature rise).
How many memory taps do I need in DPD?
Depends on the memory depth of the PA. For well-designed bias networks: 3-5 memory taps suffice. For PAs with strong thermal memory or narrow bias decoupling: 10-50 taps may be needed. The memory depth in samples equals the memory time divided by the sample period.