This work describes a single-ended power amplifier, PA, behavioral model intended for in-band nonlinear distortion simulations. Starting from the identification and subsequent modeling of the small- and large-signal nonlinear distortion generation mechanisms that should be expected from a FET-based RF band-pass PA circuit, a more convenient low-pass equivalent behavioral model of the static and dynamic AM/AM and AM/PM distortions is derived. Beyond the previous approach that was mainly conceived for predicting the small-signal distortion behavior arising from the FET's drain-source channel current, the new proposed model now extends it to moderate and heavy gain compression and also includes the FET's reactive nonlinearities, namely the AM/PM arising in the gate-source, gate-drain and drain-source capacitances.