Field Guide · hardware

Also known as: power amplifier, PA, RF PA

A power amplifier (PA) is the final gain stage of a transmitter, raising a modulated signal to the wattage needed to drive an antenna across the intended range.1 Unlike a receive-side amplifier, a PA is judged mainly by its output power, efficiency, and linearity rather than its noise — and those last two pull in opposite directions, which is why PAs come in a family of classes.

input drive output power compression linear region backoff
A PA is linear only well below saturation; a peaky signal must be backed off from the compression point to stay clean.

Overview

The core tension in PA design is that the most efficient operating point — driving the transistor hard toward saturation — is also the most nonlinear. A signal pushed into compression develops harmonics and intermodulation products that splatter energy into adjacent channels. How much distortion is tolerable depends on the modulation: constant-envelope schemes can run a PA flat-out, while amplitude-varying schemes must keep the PA in its linear region.

How it works

Amplifier classes describe how much of each RF cycle the active device conducts, which sets the efficiency/linearity balance:

  • Class A — device conducts for the full cycle. Most linear, worst efficiency (theoretical max 50%, often ~20–30% in practice).
  • Class AB / B — conducts for roughly half to somewhat more than half the cycle. A practical compromise widely used for linear RF; class B tops out near 78.5%.
  • Class C — conducts for less than half the cycle. Efficient but nonlinear, suited only to constant-envelope signals (FM, CW).
  • Switching classes D, E, F — the device acts as a switch rather than a linear element, reaching 80–90%+ efficiency, but they amplify constant-envelope or specially shaped waveforms rather than arbitrary linear signals.

In practice — backoff and PAPR

Modern digital waveforms (OFDM, and the shaped single-carrier schemes in land-mobile radio) have a high peak-to-average power ratio. The peaks, not the average, are what drive a PA into compression, so the amplifier must be operated with its average power set well below saturation — backoff equal to roughly the signal’s crest factor / PAPR. More backoff means cleaner output but lower efficiency and wasted DC power. The usable ceiling before distortion grows is anchored by the PA’s 1 dB compression point; designers also watch third-order intermodulation, since that is what lands in neighbouring channels. Techniques such as digital pre-distortion, envelope tracking, and Doherty amplification recover efficiency while keeping linearity.

Relevance to SDR

Power amplifiers live on the transmit side, so they matter for SDR transceivers — HackRF, LimeSDR, PlutoSDR, USRP — and for the base stations and mobiles that GopherTrunk listens to. The choice of PA class explains a signal’s spectral shape at the receiver: a splattering, over-driven transmitter widens occupied bandwidth and raises the noise a nearby receiver sees.

GopherTrunk is a receive-only decoder and contains no power amplifier or transmit chain. PA behaviour is relevant to it only indirectly, as one physical cause of the adjacent-channel interference and signal impairments its DSP must cope with.

Sources

  1. Power amplifier classes — Wikipedia, on conduction-angle classes A–F and their efficiency/linearity trade-offs. ↩

See also