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Power-Combining Techniques for Millimeter-wave Silicon Power Amplifiers


Emerging millimeter-wave applications, including high speed wireless communication using 5G standards, favor silicon technologies, both CMOS and SiGe, for transceiver design, due to the high level of integration at reduced cost and availability of high speed transistors. Efficient, linear and reliable high power amplifiers with broad bandwidth are needed at the transmitter front-ends to enable high data rate links at long distances. But the low breakdown voltage of CMOS FETs due to gate length scaling and other transistor non-idealities make the design of high power mm-wave amplifiers in deeply scaled CMOS nodes difficult. Circuit techniques like FET stacking provide a compact and efficient way of implementing high power mm-wave amplifiers reliably. Other power combining techniques such as on-chip and spatial power combining can be used along with FET stacking to achieve even higher output power levels. This thesis investigates the design of high power mm-wave power amplifiers at frequencies from 28 GHz to 94 GHz, using multiple power combining techniques.

This work extends the use of FET stacking for high power PA design to 94 GHz. A 3-stack PA designed in 45 nm CMOS SOI with 17 dBm output power and 9% efficiency is presented. Using this PA as front-end, a CMOS PA-antenna array is designed, to additionally provide spatial power combining. The CMOS chip has a 2 x 4 array of pseudo-differential power amplifiers along with the signal distribution networks and pre-drivers. A quartz wafer with a 2 x 4 array of differential microstrip antennas deposited on it is placed on top of the CMOS chip, electromagnetically coupled to the PA outputs on the CMOS chip. The spatially power combined PA-antenna array achieved a measured equivalent isotropic radiated power (EIRP) of 33 dBm and an estimated output power of 24 dBm at 94 GHz. Modulated data measurements at 3 Gbps (375 MS/s, 256 QAM) speed using digital pre-distortion are demonstrated with the PA-antenna array.

A novel layout style is introduced for stacked FET design at low mm-wave frequencies. A small multi-finger FET is laid out with fingers connected in series to create the stacked FET. The gate capacitors are realized around the FET with the back-end-of-line metal available in the CMOS process. Multiple multigate cells are interconnected to implement the stacked FET PA. A PA designed in this style in 45 nm CMOS SOI process achieved 24.8 dBm of output power and 29% PAE at 28 GHz with high reliability. This PA is very broadband and linear as shown by the modulated data measurements achieving a data rate of 36 Gbps (6 GS/s, 64 QAM) at 14 dBm with 9.3% PAE, with no digital predistortion.

NFETs and PFETs available in nano-scale CMOS processes are compared and it is shown that in deeply scaled processes, PMOS devices are a viable alternative to NFETs due to their cut-off frequencies similar to those of NFETs, and higher breakdown voltages than NFETs. The first exclusively PMOS mm-wave PA design is reported. This 3-stack PA, made in 32 nm CMOS SOI process, achieved a maximum output power of 19.6 dBm and maximum efficiency of 24% at 78 GHz.

All the designs reported in this thesis achieved either the highest output power or the highest PAE for a CMOS PA at their respective frequencies.

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