RF power amplifier (PA)

The last and hungriest stage of a transmitter — the muscle that takes a milliwatt-level modulated signal and shoves enough power into the antenna to reach across town. If the LNA's obsession is noise, the PA's obsession is efficiency and linearity: it burns the lion's share of a phone's battery during a call, so every percent of efficiency saved is talk-time gained.

The cruel trade-off: the most efficient PA classes (B, C, switching D/E/F) are the least linear, and modern modulations like 64-QAM and OFDM demand high linearity or the constellation smears and adjacent channels get polluted. Designers fight this with back-off (running below saturation, sacrificing efficiency), and with tricks like Doherty PAs and digital pre-distortion. A typical handset PA delivers +27 dBm (0.5 W) with maybe 40% efficiency; mm-wave 5G PAs in CMOS struggle to reach even that.