This paper presents the full wave analysis of a microwave amplifier by the extended finite difference time domain method. The device-wave interaction is characterized and incorporated into the FDTD time-marching scheme. The equivalent current source is used to model the amplifier and all the electric field components at the active sheet are updated by FDTD. Central-difference approximation is used to discretize a set of state equations. The results obtained by the present method show good agreement with those of the frequency domain circuit analysis and serenade v8.5.