Theory and Design of Doherty Power Amplifier With Active Harmonic Injection

This paper analyzes the performance of the Doherty power amplifier (DPA) with the injection of the second harmonic component generated by the carrier power amplifier (PA) into the peaking PA and vice-versa, using a harmonic injection network (HIN). A mathematical analysis is presented to estimate the second harmonic loads seen by the carrier and peaking PAs in the presence of the HIN. Furthermore, a step-by-step design guide to designing a harmonic injection DPA (HI-DPA) is also presented. It is noted that the passive load provided by the output matching network (
$Z'_{c2}$
and
$Z'_{p2}$
), the electrical length of the HIN (
$\theta _{inj}$
), and the phase difference between the currents of the carrier and peaking PAs (
$\theta _{p}$
) at the second harmonic are crucial to present an active load to either the carrier or peaking PAs. In the HI-DPA, it is not possible to achieve an active second harmonic load for the carrier and peaking PAs simultaneously. Due to the interaction between the carrier and peaking PAs at the second harmonic, if the carrier PA sees an active load, the peaking PA sees a passive load, and vice-versa. The HI-DPA is designed with the HIN, and its performance is compared with the high-efficiency passive harmonic-tuned DPA. It is demonstrated that both the carrier and peaking PAs cannot see an active impedance at the second harmonic simultaneously. However, it is possible to increase the efficiency in the Doherty load-modulation region with the harmonic injection technique using the HIN. This would be translated into an average efficiency increase when the PA is excited with the modulated signal.