This work presents a theoretical analysis of the frequency dependence of the load impedance seen from the transistor’s intrinsic current source and the package plane for designing class-J*/B/J power amplifiers (PAs). The presented results are possible thanks to the new analytical expression for calculating the output current. Moreover, the proposed analysis permits the study of the relationship between the maximum output current and the frequency assuming a nonlinear output capacitance; the information allows designers to evaluate the capability of the transistor for designing class-J*/B/J PA at predetermined drain efficiency and frequency. The proposed impedance equations with load-pull data at different frequencies, help to identify a design space that makes the design of output matching networks (OMN) for broadband cases flexible. Experimental data of a designed and fabricated class-J*/B/J GaN PA for 2–3 GHz with drain efficiency and output power higher than 60% and 40 dBm, respectively, validate the theoretical results of the proposed analysis.