Analysis and Design of Millimeter-Wave Power Amplifier Using Stacked-FET Structure

A new analysis methodology for millimeter-wave stacked-FET power amplifier design is proposed with a focus on the output power improvement by adjusting the complex load-admittance of each stacked-transistor. From this analysis, it is shown that there exist fundamental limitations on the maximum FET-stacking number and the operation frequency with given FET characteristics. Moreover, comprehensive analysis is performed to understand the effect of each FET parameter in limiting the number of stacks and output power. Based on this, a simple design method of C gs compensation is proposed to further increase the number of the stacks and enhance the output power. To verify analyses, various high-frequency stacked-FET PA MMICs are designed and fabricated at different frequencies with pHEMTs, mHEMTs, and CMOSFETs, which all have different maximum transition frequencies ( f T's). This paper presents comprehensive analysis to identify the limitation of the stacked-FET amplifiers at millimeter-wave frequencies and presents a new design methodology to further improve the output power performance at high frequencies.