How do I select a power amplifier for a transmitter design based on output power and efficiency?
PA Selection Guide
Power amplifier selection is driven by the application requirements across multiple dimensions. The output power determines the PA technology: sub-watt for handsets (GaAs HBT, SiGe, CMOS), 1-10W for small cells and tactical radios (GaN, GaAs), 10-100W for macro base stations (LDMOS, GaN), and 100W+ for broadcast and radar (GaN, vacuum tubes).
| 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
Efficiency is critical because the PA consumes 60-80% of a transmitter's total DC power. Class AB amplifiers achieve 30-40% PAE at backed-off power levels needed for linear modulation (OFDM, QAM). Advanced architectures (Doherty, envelope tracking) improve backed-off efficiency to 40-55%. Class E/F switching amplifiers achieve 60-80% efficiency but require constant-envelope signals (FM, GMSK).
- 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
Stability Considerations
Linearity is measured by ACPR (adjacent channel power ratio) and EVM (error vector magnitude). For LTE/5G, typical requirements are ACPR < -45 dBc and EVM < 3% for 256-QAM. These requirements force the PA to operate 6-8 dB below its P1dB compression point (backed off), reducing efficiency. Digital pre-distortion (DPD) linearizes the PA, allowing operation closer to compression and recovering 3-5 dB of efficiency.
Frequently Asked Questions
GaN or GaAs?
GaN for: high power (>5W), high voltage (28-50V), high frequency (>6 GHz), wideband, and high-temperature operation. GaAs for: moderate power (0.1-5W), lower voltage (3-8V), and established supply chains. GaN costs more per device but its higher power density often reduces the total PA module cost.
What about CMOS PAs?
CMOS PAs are emerging for handset applications (sub-1W at 2-6 GHz) because they can be integrated with the transceiver on a single die. Their efficiency and linearity are improving but still lag GaAs and SiGe for demanding specifications. CMOS is the long-term cost leader for high-volume consumer applications.
How do I handle the heat?
A PA operating at 40% efficiency with 10W output dissipates 15W as heat. The thermal design must keep the junction temperature below the technology limit (150°C for GaN, 175°C for GaAs). Use a metal-backed PCB, thermal vias under the device, and a heat sink or cold plate. Thermal simulation (FEA) is essential for high-power designs.