Load-Modulation Network for High-Efficiency 5G Power Amplifiers

The high data rates achieved by 5G wireless networks are enabled by high power amplifier (PA) efficiency. 5G NR waveforms operate at an average power of 11 dB from the peak output power and the PA is less efficient at the average power. Load modulation improves the backoff efficiency of PAs by exploiting the relationship between load impedance, efficiency, and output power. However, conventional approaches require multiple amplifiers and often utilize precise phase and amplitude control on the inputs, making them difficult to implement at millimeter-wave (mm-wave) frequencies. Dynamic load modulation (DLM) optimizes output power efficiency, but most approaches require additional capacitors or varactors, which limits the scaling of DLM to mm-wave bands. Additionally, large banks of switches are generally only feasible in highly scaled CMOS processes, placing limits on the power handling for the network at the output of the PA.

Researchers at University of California, Santa Barbara have developed a passive outphasing load modulator (POLM) that allows for higher efficiency and linearity of the power amplifier (PA) with a single power amplifier device. This technology switches across a transmission line ring where only one switch is required for each state, enabling low loss and efficient device layout at high frequency. The GaN process’ high linearity and enhanced power handling serve to minimize distortion in transmitting — all with a relatively simple network, only requiring one PA for an inherently wideband network exceeding 2.5GHz.