First Class-S Power Amplifiers for the Microwave Frequency Range

Energy consumption of the wireless communication infrastructure has become a major concern, particularly when introducing bandwidth-optimized modulation schemes with high peak-to-average power ratios. Consequently, concepts for highly efficient power amplifiers have become the subject of intensive research. These efforts are presently focusing on switch-mode operation (class-E, F, J …) which inherently offers a higher power-added efficiency (PAE) than conventional analog amplifier classes. One of the most advanced switch-mode amplifier concepts is the so-called class-S approach, which uses digital signals for all components except for the filter at the output.

A given analog signal is converted to a 1-bit digital bit stream using a band-pass delta-sigma modulator (BPDSM). The digital signal is then amplified using a highly efficient power switch. An analog band-pass filter at the output of this switching stage reconstructs the amplified signal at the signal frequency f₀. For ideal devices, the class-S concept avoids any overlap of output voltage and current in time and thus yields high efficiency, independently of the power back-off. Though very popular at audio frequencies, class-S has not been used in the microwave frequency range so far. One reason is, that this concept needs very fast and broadband power transistors. In this regard, GaN technology is very promising as it combines high breakdown voltage with high speed.

FBH is working on the microwave S-class amplifier since several years now, also in the framework of a BMBF-funded project with industrial and academic partners. Recently, class-S amplifiers in both current- and voltage-mode topology for the 450 MHz band were presented at the leading international microwave conferences. The amplifiers were realized in hybrid technology with dedicated broadband power-switch MMICs based on the FBH 0.5 µm GaN-HEMT process delivering output powers up to 19 W. The results achieved significantly advance the state-of-the-art in microwave class-S and yielded excellent international visibility for FBH in this field.

Publications:

[1] A. Wentzel, C. Meliani, W. Heinrich, „RF Class-S Power Amplifiers, "RF Class-S Power Amplifiers: State-of-the-Art Results and Potential", IEEE MTT-S Int.Microw.Symp.Dig., Anaheim, CA, May 25-27, pp. 812-815 (2010).

[2] A. Wentzel, C. Meliani, W. Heinrich, "A Voltage-Mode Class-S Power Amplifier for the 450MHz Band", Proc. European Microwave Conf. (EuMC 2010), Paris, France, September 28-30, pp. 640-643 (2010).

FBH research: 05.11.2010