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This contribution presents the theoretical analysis and design guidelines to increase the average efficiency of a Doherty power amplifier (DPA), accounting for the device on-resistance. Starting from a simplified device model, closed-form equations for the estimation of both design parameters and obtainable performances are reported. Moreover, advantages and disadvantages of the approach are deeply investigated through a comparison with the standard implementation of a DPA, i.e., based on constant knee voltage behavior. Finally, as experimental support for the developed theoretical analysis, two X-band monolithic microwave integrated circuit DPAs, based on the same GaAs technology, have been designed, realized, and tested. The first one was based on the standard methodology, while the other one has been optimized exploiting the device knee voltage behavior. Measurement results validated the developed analysis, confirming what is theoretically expected for the main DPA features. In particular, both DPAs have 29 dBm of output power with 7.2 dB of power gain in 6 dB of output power back-off (OBO). The efficiency is larger than 35% for the standard DPA and 42% for the one designed exploiting the device on-resistance, in the same OBO region.