Design of a Load Modulated Balanced Amplifier with a Two-Stage Control Power Amplifier

While an ideal Doherty power amplifier has a linear response, the load modulated balanced amplifier (LMBA) has a compressive response under ideal conditions. This inherent nonlinear characteristic is due to the lower power contribution of the single auxiliary device as the balanced amplifier transistors approach compression. This article presents an LMBA with a two-stage control signal amplifier in place of the single auxiliary device. The idea is to preserve a high and constant gain across the high- and low-power regions by tuning the two-stage gain control signal to match the balanced amplifier gain. An optimal load trajectory can be found for a high-efficiency design by appropriately terminating the second harmonic while ensuring an optimal impedance match in all devices. At the same time, by setting an optimal output power from the auxiliary device, sufficient power is provided to linearize the response of the main power amplifier beyond the output back-off power boundary. As proof of concept, a prototype is designed and implemented. The experimental measurements demonstrate a drain efficiency of 59%–64% at maximum output power and 46%–52% at 7.5 dB output back-off power over the target frequency range of 3.3–3.8 GHz.