A Tapped Transmission Line Ring Power Amplifier for High Peak and Average Efficiency

We propose an alternative approach to load modulation based on switching between high-power (HP) and low-power (LP) amplifiers that, respectively, offer high power-added efficiency (PAE) under peak and average output power. Rather than adding a switch at the output of the amplifiers, the power amplifiers (PAs) share a common transmission line ring network that simultaneously tunes for both load conditions. We describe the design methodology and present a prototype PA that operates from 26 to 31 GHz with 6-dB output backoff range. Both the HP and LP PAs are realized from four-stack FETs fabricated in a 45-nm RF-SOI CMOS technology. The HP mode achieves 19-dBm saturated output power ( $P_{\text {sat}}$ ) and 35.1% PAE, and the LP mode achieves 15.2-dBm $P_{\text {sat}}$ and 35.7% PAE. The HP mode exhibits an average output power ( $\text {P}_{\text {out,avg}}$ ) of 11.4/11.8/11.1/11.2 dBm, average PAE ( $\text {PAE}_{\text {avg}}$ ) of 10.3%/11.3%/9.6%/9.8%, and rms error vector magnitude (EVM) −25.5/−26.4/−26.7/−24.7 dB in 64-QAM 100M/250M/500M/1G Sym/s measurement. The LP mode exhibits an average output power ( $\text {Pout}_{\text {avg}}$ ) of 8.2/8.5/7.7 dBm, an average PAE ( $\text {PAE}_{\text {avg}}$ ) of 20%/21.5%/17.8%, and rms EVM −25/−25.4/−25 dB in 64-QAM 250M/500M/1G Sym/s measurement without using digital predistortion (DPD).