FBH research: 29.06.2016

Digital microwave power amplifier with energy recovery

Digital PA system with energy recovery concept and realized digital PA module
Digital PA system with energy recovery concept (l.) and realized digital PA module (r.) including 2 GaN DPAs, wideband 180° hybrid coupler and RF-DC rectifier, size: 85 x65 mm2
Measured Pout and final-stage drain efficiency
Measured Pout and final-stage drain efficiency ηD w/ and w/o energy recovery, VDD = 30 V

Next-generation wireless communication infrastructure demands for high flexibility, low cost, and high efficiency. These requirements are complemented by the growing needs towards wideband, multi-band, and multi-standard features. As the RF power amplifier (PA) is the last analog part in the base-stations, extensive research efforts have been devoted to digital PA realizations within the last years.

One of the major problems on the way to a fully digital transmitter is the low overall power-added efficiency (PAE) of digital microwave PAs (DPAs) even at full-scale. Hence, they are not yet competitive to analog concepts like envelope tracking (ET). One approach to overcome this problem is energy recovery. The binary pulse sequences in a DPA contain a lot of spectral power outside the desired band, particularly in the back-off case. This power can be converted to DC and then reused to assist in the supply of the PA, thus improving efficiency at power back-off (PBO).

Recently, FBH presented at the IEEE International Microwave Symposium 2016 in San Francisco a DPA system for 900 MHz applying energy recovery. The output of two digital PAs is combined with a rat-race coupler. The out-of-band energy is collected at the isolated port. A charge-pump voltage-doubler rectifier circuit (RF-DC rect.) converts this signal to DC and this power can assist in supplying the PA.

The DPA system reaches a maximum Pout of 7.5 W and a peak PAE of 42%. Without energy recovery, final-stage efficiency ηD drops from 70% for full-scale (0 dB PBO) to 13% at 15.2 dB PBO. Due to digital Doherty operation, a second peak in efficiency with 58% at around -6 dB PBO is achieved. By applying energy recovery the module now reaches ηD values of 74% and 52% at -6 dB and -10 dB PBO, respectively (23% at -15.2 PBO).

These results demonstrate the significant improvement in final-stage drain efficiency when applying energy recovery technique in digital PA systems. Performance will improve even more if truly digital modulated signals are applied.

Publication:

A. Wentzel, W. Heinrich, “Digital Microwave Power Amplifier with Energy Recovery”, IEEE MTT-S International Microwave Symposium Digest 2016, THIF2-1, San Francisco, USA.