4.4 A Highly Linear High-Power 802.11ac/ax WLAN SiGe HBT Power Amplifier Using a Compact 2nd-Harmonic-Shorting Four-Way Transformer and Integrated Thermal Sensors

Increasing demand on high-speed mobile access requires a spectrum-efficient wireless connectivity platform. With dense modulation and MIMO techniques, 802.11ac WLAN has been widely deployed as a viable solution. The emerging 802.11ax standard promises increased network capacity and higher data-rates. As 802.11ac/ax imposes stringent linearity requirements on power amplifiers (PAs), a high peak-to-average power ratio, operation at large power back-off, and reduced PAE are unavoidable. As a result, III-V processes [1] have dominated the development of high-output-power (P OUT ) WLAN PAs. To replace them with lower cost Si-based PAs, several linearization schemes [2], [3] have been proposed, such as digital pre-distortion (DPD) and a spatial power combining, at the cost of increased complexity in the digital domain. In [4], a 2nd-harmonic short is used for linear Class-AB PAs, but an inductor on the supply lane occupies Si area and causes ohmic losses, degrading P OUT and PAE. SiGe HBTs [5] are suitable for WLAN PAs due to their high power density, ruggedness, and CMOS compatibility. However, as current gain ($\beta $) drops with junction-temperature (T J ) rise, this distorts PA linearity and power gain.