Compact and Wideband Spatial Power Combining Module for mm-Wave High-Power Amplifiers

Licentiate thesis, 2020

The continued growth of data traffic in wireless communication applications demands to launch next-generation communication services at millimeter wave frequencies. Despite the obvious advantage of a large available spectrum, using millimeter wave bands is accompanied by many technological challenges. Mainly, this is due to the increased propagation and material loss, as well as limitations of the existing semiconductors, which deliver less power at these frequencies. The latter could be partly overcome by combining the power of multiple active devices on each antenna array element. However, conventional on-chip power combining networks have inherently high insertion losses, which significantly increase with the number of interconnected devices.

This work presents a novel power combining solution where an array of multiple power amplifiers (PAs) are interfaced to a single substrate integrated waveguide (to be integrated with an antenna element) in the context of high-power array transmitters. Its operation principle is based on the direct excitation of TEm0 modes by an array of electrically short and coupled microstrip lines. This allows one to directly excite waveguide modes with high power, and hence, realize a desired spatial power combing functionality, which obviates the need for a potentially lossy on-chip power combiners. The proposed structure has wide impedance bandwidth (50%) and low insertion losses (0.4 dB) while offers a small form-factor. Moreover, the insertion losses are virtually independent from the number of interfaced PAs in contrast to conventional power combining techniques. The work also evaluates the approximate scalability bounds of such a structure as well as discusses the critical effects of coupled non-identical PAs. These undesired effects are reduced by employing on-chip isolation load resistors, which make the proposed configuration comparable with the classic Wilkinson power combiner in terms of the sensitivity to a non-uniform excitation.

The direction of the ongoing work is a realization of a Watt-level PA, which combines power of multiple PA-cells integrated in the same compact module. For this purpose, a single cascode differential PA-cell is designed and implemented in a SiGe:C BiCMOS technology. It has a wideband performance (22—34 GHz) with both high efficiency (30%) and high output power (24.2 dBm), which outperforms the state-of-the-art single-cell silicon-based PAs.