RF Power Amplifier Design
Power amplifiers (PAs) are among the most challenging RF circuits to design because efficiency and linearity pull in opposite directions. A perfectly linear PA wastes most of its supply power as heat. A highly efficient PA distorts the signal. Modern PA design — especially for cellular and wireless standards — is the art of managing this tradeoff.
PA Classes
PA classes are defined by the transistor's conduction angle — the fraction of the RF cycle during which the device conducts current:
| Class | Conduction angle | Max efficiency | Linearity | Use |
|---|---|---|---|---|
| A | 360° (always on) | 50% | Excellent | Small-signal, wideband, instrumentation |
| AB | 180°–360° | 50–78% | Good | Most practical linear RF PAs (LTE, Wi-Fi) |
| B | 180° | 78.5% | Moderate | Push-pull audio and some RF |
| C | < 180° | > 78.5% | Poor | Narrowband constant-envelope (FM, CW radar) |
| D | Switch | 100% (ideal) | Needs filtering | Audio switching amplifiers, some RF |
| E | Switch + resonant | 100% (ideal) | Needs filtering | High efficiency narrowband RF |
| F | Resonant harmonics | 100% (ideal) | Narrowband | High efficiency RF (mobile base stations) |
Drain Efficiency vs Power-Added Efficiency (PAE)
PAE is the more meaningful figure for RF PAs because it accounts for the drive power required. For a PA with 15 dB gain, the difference between the two is small. For a PA with only 6 dB gain (common at mmWave), PAE is significantly lower than drain efficiency.
1-dB Compression and Saturation
As input power increases, the output eventually saturates. The P₁dB (1-dB compression point) is where the gain has dropped by 1 dB from its small-signal value. Above P₁dB the device is nonlinear and generates harmonics and intermodulation products. For linear operation, PAs are backed off from P₁dB by 6–10 dB, accepting lower efficiency.
Linearity Requirements by Modulation
| Modulation | PAPR | Typical back-off | Notes |
|---|---|---|---|
| CW / FM | 0 dB | 0 dB | Constant envelope — no linearity needed |
| BPSK/QPSK | 3–4 dB | 3–6 dB | Low PAPR, tolerates moderate nonlinearity |
| 16-QAM | 6–7 dB | 6–8 dB | Moderate requirements |
| 64-QAM | 8–9 dB | 8–10 dB | LTE typical; high back-off needed |
| OFDM (Wi-Fi/5G) | 10–12 dB | 10–12 dB | High PAPR; DPD or Doherty needed for efficiency |
PAPR = Peak-to-Average Power Ratio.
Doherty Architecture
The Doherty PA is the dominant architecture for modern base station and handset PAs because it maintains high efficiency at back-off. It uses two amplifiers — a "main" (carrier) PA that operates in class AB/B, and a "peaking" PA that only turns on near peak power. At back-off, only the main PA operates (high efficiency). At peak power, both combine, restoring efficiency. A Doherty PA can achieve > 50% efficiency at 6 dB back-off, compared to 12–15% for a class A PA.
Digital Predistortion (DPD)
DPD applies an inverse of the PA's nonlinearity digitally, before the signal reaches the PA, so that the output is linear. It requires a feedback path to measure and adapt to the PA's actual behaviour. DPD allows a PA to operate closer to compression (higher efficiency) while meeting EVM and spectral mask requirements. It is standard in modern cellular base stations.
Output Matching
PA transistors are typically low-impedance devices that need an output matching network to present the optimal load impedance for maximum power and efficiency. The load line resistance \(R_{opt} = V_{dd}^2 / (2 P_{out})\) sets the ideal load for a class A stage. For a 28 V, 10 W PA: \(R_{opt} = 784/20 = 39.2\) Ω — conveniently near 50 Ω, which is why GaN on 28 V supply is so popular for base station PAs.