A departure of an amplifier's input‑to‑output transfer from a linear gain relationship such that output amplitude and phase are nonlinearly related to input, producing distortion products (harmonics, intermodulation), AM‑AM and AM‑PM conversion, increased error vector magnitude (EVM) and possible violation of spectral masks.

Principle

Nonlinearity generates new spectral components whose amplitudes depend on input amplitudes and the amplifier transfer function; as input drive approaches compression and saturation regions, in‑band distortion and out‑of‑band spectral regrowth increase nonlinearly, degrading modulation fidelity and creating adjacent‑channel interference.

Demonstration

Illustrative scenario → A transmitter's PA is driven toward its compression region to maximize radiated power. Recognition → Measured constellation shows increased EVM and adjacent‑channel power rises. Action → Reduce drive level or apply digital predistortion (DPD) linearization. Consequence → EVM and adjacent leakage return toward compliant levels.

Misapplication

Assuming any increase in bit‑errors or adjacent‑channel interference is due to channel noise or misconfiguration rather than amplifier nonlinear behavior; this mistakes additive noise effects for deterministic distortion products tied to transmitter amplitude.

Consequence

Degraded demodulation and higher BER, increased retransmissions, potential interference to adjacent channels and regulatory non‑compliance, and reduced spectral efficiency.

Reversal

Linearity requirements can be relaxed in narrowband or analog systems that tolerate distortion, or compensated by linearization methods (DPD, feedback), or avoided by operating with sufficient back‑off; some applications intentionally exploit nonlinearity (e.g., power‑efficient constant‑envelope modulation).

Boundary

Clearly within: power amplifier stages near saturation that produce harmonics and intermodulation. Boundary case: mixers and frequency converters that are nonlinear by design but are accounted for in system analysis. Clearly outside: purely linear baseband digital processing that introduces quantization noise but no RF spectral regrowth.

Semantic Tension

Linearity ↔ Efficiency — increasing linearity (operating with back‑off, DPD) reduces spectral regrowth but often decreases power‑efficiency and increases cost/complexity.

Synthesis

Amplifier nonlinearity is a deterministic distortion mechanism linking input amplitude distributions to predictable spectral and modulation degradations; system design balances linearity and efficiency through hardware choices and digital compensation techniques.