In this paper, a new method is proposed to calculate transconductance ratio of the main and the peaking transistor of a Doherty power amplifier (DPA) which employs output matching circuits and offset lines. Cascade ABCD two-port parameters are used to analyze output section of the DPA. It is shown that output matching circuits has impact on the magnitude and the phase of the transconduction ratio. The effect of the input phase mismatch on the output power and efficiency is analyzed analytically. A 100W dual drive Gallium Nitride (GaN) Doherty power amplifier is realized to experimentally verify the results. The drain efficiency measured as 68% at the correct phase, 65% for 30 degrees input phase mismatch and 56% for 45 degrees input phase mismatch.