How do I design a broadband power amplifier that covers an octave or more of bandwidth?
Wideband PA Design
Narrowband PAs achieve high efficiency because the output matching network can be precisely optimized for the optimum load impedance at a single frequency, including harmonic terminations. As bandwidth increases, the matching network must compromise between frequencies, and harmonic termination becomes impractical because the second harmonic of the low end of the band falls within the operating band.
| 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
What efficiency is achievable?
For octave-band Class AB PAs: average PAE of 20-35% with modulated signals, compared to 35-50% for narrowband. The efficiency loss comes from the inability to optimize harmonic terminations (they fall in-band) and the matching network losses at the band edges.
Can I use DPD with broadband PAs?
Yes, and it is essential. Broadband PAs have more nonlinearity variation across the band, requiring wideband DPD models. The DPD bandwidth must cover the entire PA operating band including intermodulation products, which can be challenging for very wideband designs.
What about GaN for broadband?
GaN is excellent for broadband PAs because its higher Ropt (compared to LDMOS at the same power) means less impedance transformation is needed. A 10W GaN PA at 28V has Ropt ≈ 31 Ω (close to 50 Ω), while a 10W LDMOS PA at 28V has similar Ropt but at lower frequency. GaN's wider bandwidth transistor characteristics also help.