Flexible and Efficient Microwave Power Amplifier for Future Mobile Base Stations

The continuously increasing demands on today’s wireless communication infrastructure (more subscribers, more applications, higher data rates) require broadband channels (e.g. LTE) and spectrum-efficient modulation schemes, with an optimized exploitation of the available (limited) bandwidth. One consequence is that the RF power amplifier (PA) has to operate at high peak-to-average power ratios (PAPR), which significantly reduces energy efficiency when using a common linear PA (e.g. class-AB). In order to improve efficiency at usual UMTS PAPR (6 - 8 dB), one commonly applies the Doherty concept or envelope tracking.

Beyond this , the H-bridge class-D PA topology has been considered as a proper candidate to achieve high efficiency. It uses two complementary voltage-switching class-D PAs with two final stage transistors each acting as a switch. The realizations so far show either fully integrated solutions with output powers well below 1 W or use technologies with f_T and power density limitations lower than GaN.

At FBH a GaN-based H-bridge class-D PA for the 900 MHz band has been developed and realized. The figure shows the concept and the realized PA module.

The demonstrator achieved a maximum efficiency of 50% and a saturated output power of 8 W. Increasing the PAPR to 10 dB efficiency is degraded to 22% with a special pulse-width modulated signal. Also, other modulation approaches, e.g. band-pass delta-sigma modulation, have been tested, but did not improve the performance. The H-bridge PA represents a compact, efficient and flexible alternative for future mobile base stations, which allows to expand the digital part within the base-station architecture. The realized PA advances the state-of-the-art in mainly two ways:

• This is the first microwave H-bridge realization in the 10 W of peak output power range.
• The H-bridge topology offers new degrees of freedom with regard to ternary coding schemes, which is a key issue for efficiency and thus PAPR performance of the PA.

The results confirm FBH's leading role in this field of research. They were presented at the IEEE International Microwave Symposium in Montreal..

Publication

A. Wentzel, C. Meliani, G. Fischer, W. Heinrich, ”An 8 W GaN-based H-Bridge Class-D PA for the 900 MHz Band Enabling Ternary Coding”, IEEE MTT-S International Microwave Symposium Digest 2012, WEPG-14, Montreal, Canada.

FBH research: 14.08.2012