Baseband Impedance Design Space for Concurrent Band PA Operation

Concurrent dual-band power amplifiers (PAs) often experience significant efficiency degradation due to the strong baseband components generated under two-tone excitation. Conventional wisdom dictates that the baseband impedance, in particular at |w2 - w1| (IF1) must be short-circuited to avoid bias voltage fluctuation. This paper demonstrates that this assumption is not the unique solution for the output matching. A broader analytical formulation of the device operation shows that, when different fundamental impedances are selected at each operation band, the desired IF1 impedance can become either inductive or capacitive. In these cases, shorting the IF1 impedance leads to performance degradation of the PA. To demonstrate this effect, a hardware prototype with controllable IF1 impedance is designed and evaluated. Measurements confirm that concurrent-mode drain efficiency and output power improve under the analytically derived IF1 impedance, in comparison to the short IF1 impedance case, validating the theory.