The phase and amplitude responses of a high power amplifier have a significant role in linearity performance. Analytical analysis of a high power solid-state device at linear and saturation regions can provide initial information about the overall performance before design and implementation. This study presents the analytical analysis of the amplitude and phase sensitivity of a GaN HEMT. The single-parameter and first-order analysis was considered to calculate sensitivity functions regarding the nonlinear components of the transistor. A simplified equivalent circuit model was used for the analysis. The analytic results were verified with a high power Class-AB amplifier at X-band while the maximum output power of 37 dBm at 8.2 GHz. The results show that the single-parameter and first-order sensitivity analysis can be used to evaluate the phase response of the amplifier to estimate the initial parameters before the final design and implementation of the amplifier.