This paper presents behavioral modeling of an envelope tracking (ET) power amplifier (PA) excited by an slewrate limited envelope. In order to relax the slew-rate requirements of current efficient envelope amplifiers (EA), we can drive the ET PA with a slower version of the signal’s envelope. However, the resulting amplified signal suffers from nonlinear distortion due to the time-variant gain effects that arise when using the slower version of the signal’s envelope. In this paper this time-variant effects are presented and characterized. A new envelope-dependent (ED) behavioral model is presented and compared with a simpler static AM-AM and AM-PM modeling. Experimental results showing the performance of the proposed ED model were obtained with an ET PA based on a GaN transistor.