Highly Linear High-Power 802.11ac/ax WLAN SiGe HBT Power Amplifiers With a Compact 2nd-Harmonic-Shorted Four-Way Transformer and a Thermally Compensating Dynamic Bias Circuit

This article presents the design of a highly linear high-power silicon-germanium (SiGe) heterojunction bipolar transistor (HBT) 802.11ac/aχ wireless local area network (WLAN) power amplifiers (PAs). The challenges associated with electrothermal effects on the dynamic operation of WLAN PAs are first discussed. We then propose the design methods that take into account the electrothermal transient effect to improve linear output power (P _οUT ) and dynamic error vector magnitude (DEVM). A compact four-way output transformer balun is proposed to achieve efficient power combining, and a built-in 2nd-harmonic short is demonstrated by using a novel multi-layered metallization scheme. A thermally compensating dynamic bias circuit that improves the DEVM and reduces memory effects is designed with an integrated temperature sensor. Different SiGe HBT array layouts, laterally and vertically arranged, of the output stage of the PA are also investigated. With the 802.11ac MCS9 VHT80 test signals, the DEVM of a PA with a laterally arranged output stage is lower than that of its vertical counterpart. This suggests the importance of the layout on the transistor electrothermal transient effect. The PA with the laterally arranged output stage shows a P _οUT of 22.5/23.6/23.2 dBm (DEVM = -35 dB) with 10.0/12.2/11.2% power-added efficiency (PAE) at 5210/5530/5855 MHz under an 802.11ac MCS9 VHT80 test signal at 50% duty cycle. Good DEVM performance was measured under the test signal with various duty cycles, indicating that the proposed PA is thermally robust. The design supports the 802.11ax MCS11 VHT80 signals with 19.4-dBm P _οUT , satisfying a DEVM of -40 dB.