A radio for the 24 to 29.5 GHz band must achieve both performance and commercial targets. Evaluating antenna options, the system architecture could use a large active array with SiGe RFICs or a passive array driven by a very high power GaN amplifier, though both lack the flexibility to adapt quickly and cost-effectively to the fast-changing telecoms market.
SIAE MICROELETTRONICA chose a third option for its 24 to 29.5 GHz radio system: developing a custom GaAs chipset comprised of up- and down-converters with integrated variable gain and low noise amplifiers (an IF from DC to 7 GHz), with a medium power amplifier for the user terminals and a high power amplifier (HPA) for the network equipment.
The HPA design required major effort to achieve the target specifications using a low-cost 0.25 µm GaAs MMIC process. The 36 dBm output power at 1 dB compression and 44 dBm OIP3 were challenging for a process limited to 6 V maximum operating voltage. At the targeted output power, the DC power consumption is approximately 19 W, meaning the power device draws 3.2 A. Handling such high current would have caused several problems and reduced the efficiency of the system. So a stacked FET topology was chosen, with the stacked FET biased at 12 V and drawing 1.6 A. This approach is typical of Si CMOS PAs, where the process ft is more than 6x the operating frequency; however, it is challenging for a 0.25 µm GaAs process with a ft of 65 GHz, where device parasitics can degrade the theoretical performance.
Nonetheless, the MMIC, packaged in a 6 mm QFN, achieves more than 36 dBm at 1 dB compression and 25 dB small-signal gain from 24 to 29.5 GHz.