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A theoretical validation of the offset lines method for design of Doherty power amplifiers is presented for the first time. The analysis is carried out considering the simplified unilateral active device model classically adopted in Doherty amplifier theory. In particular, it is proved that, in narrow band conditions, properly designed offset lines preserve the ideal load modulation across the full power sweep range. The demonstration is independent from the Doherty strategy adopted, e.g., in terms of output power back-off, even or uneven architecture. A simple analytical formula to calculate the proper length of the lines is given, and validated through a simulation example.