Digital Sequential PA for Flexible Efficiency Tuning Over Wide Power Back-Off Range

This paper presents the first digitally designed sequential power amplifier (DSPA) module setup for the 900 MHz range. The demonstrator applies an H-bridge (peak PA) as well as a single-ended digital power amplifier (main PA) both utilizing a 4-stage GaN-based voltage-mode class-DIS PA MMIC. Maximum output power of H-bridge is 6.7 W while the main PA delivers 4.9 W for 40 V final-stage drain supply voltage (V_DD). The amplifiers are combined with a simple 3 dB power combiner to achieve sequential PA operation for high efficiency over a wide power back-off (PBO) range. While varying V_{DD_peak} and applying digital Doherty operation to the peak PA the DSPA reaches several maxima in drain efficiency over a 12 dB PBO range. The same is true for V_DD-tuning of the main PA. The flexible efficiency peak tuning makes the DSPA suitable for operation with different modulated signals varying in encoded PBO and signal bandwidth. At 5 dB, 8 dB and 10.5 dB PBO the DSPA achieves a final-stage drain efficiency of 67%, 74% and 71%, respectively. Due to the digital design of both PAs the very compact DSPA can be easily reconfigured for other frequency bands by only changing output filters and coupler.