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$|\omega _{2}{\,}-{\,}\omega _{1}|$(IF1), must be short-circuited to avoid bias voltage fluctuation. This letter demonstrates that this assumption might not be the best 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 IF1impedance can become either inductive or capacitive. In these cases, shorting the IF1impedance leads to performance degradation of the PA. To demonstrate this effect, a hardware prototype with controllable IF1impedance is designed and evaluated. Measurements confirm that concurrent-mode drain efficiency and output power improve under the analytically derived IF1impedance, in comparison to the short IF1impedance case, validating the developed theory. Additionally, the adjacent channel leakage ratio (ACPR) measurement also demonstrates enhanced linearity when the optimal IF1impedance is applied.