Technical Insight
CMOS poised for power amplifier takeover
As Nujira unveils its highest performing envelope tracking chip for mobile handsets yet, vice president Jeremy Hendy asserts the technology is ready for GaAs and CMOS power amplifiers. Compound Semiconductor reports.
Will CMOS power amplifiers and envelope tracking become mainstream in LTE handsets?
In the last few weeks UK-based Nujira has broken its own world record for envelope tracking (ET) linearity on a GaAs power amplifier (PA) and unveiled its highest performing ET modulator yet, primarily designed for a GaAs PA.
The double-whammy confirms that 4G smartphone volume shipments are almost upon us, but this doesn't signal pay day for manufacturers of GaAs PAs.
“Envelope tracking gives manufacturers of CMOS power amplifiers an opportunity to get a grip on the high-end smart phone market,” says Jeremy Hendy, vice president of sales and marketing at Nujira. “I'm not sure what percentage of power amplifiers will be CMOS in three to five years time, but my guess it will be a lot and could well be 50%.”
Nujira has spent just over a decade developing its envelope tracking power supply technique to boost the efficiency and linearity of PAs for RF front ends. Device makers can replace the normal DC-DC converter with a power modulator that varies the supply voltage to the PA to track the amplitude of the transmitted RF signal, keeping it in compression across the entire modulation cycle, not just the peaks.
This not only boosts PA performance but enables full-power transmission from handsets, a huge bonus for LTE network operators desperate to expand the coverage area of a base station.
But while envelope tracking can take GaAs PAs to linearities that CMOS alternatives can only dream of, handsets do not demand this level of performance, not even 4G LTE devices. What's more, Nujira's ET modulators now push CMOS PA performances to 4G requirements, as laboratory tests on prototype devices showed earlier this year. And according to Hendy, ET technology is just as easy to use with CMOS as it is GaAs.
“There's no real difference in how you use envelope tracking for CMOS compared to GaAs and we've been saying for a couple of years now that this could be key to unlock the performance of CMOS in high-end smart phones,” says Hendy. “The technology levels the playing field as you are no longer reliant on the inherent linearity of the transistor.”
In response, GaAs manufacturers have continued to buy CMOS PA manufacturers. On the tail of RFMD acquiring Amalfi, Avago bought 3G CMOS PA pioneer, Javelin Semiconductor in April, while Murata recently partnered with CMOS RFIC company, Peregrine Semiconductor, to develop CMOS PAs.
And as the GaAs heavyweights build up CMOS technical reserves, these businesses are still a step-ahead on optimising PA design to make the most of envelope tracking. “A lot of CMOS power amplifier companies tried to linearise the power amplifier through clever circuit design... but they went off in the wrong direction for using envelope tracking,” says Hendy. “We're now seeing the first GaAs vendors really get to grips with designing a good envelope tracking GaAs power amplifier.”
But the CMOS businesses aren't hanging around, as evidenced by Qualcomm's RF360, the first CMOS PA with ET, released earlier this year. As Hendy highlights, this GaAs-based multi-band multi-mode PA alternative indicates CMOS PA manufacturers are certainly “thinking about the two in parallel”. Indeed, some industry players anticipate a high-tier handset containing a Qualcomm CMOS PA to surface early next year.
But will other chip-set manufacturers jump? Yes, and Hendy reckons a key driver will, of course, be integration.
“Look inside some of the latest handsets with GaAs power amplifiers and you see four different GaAs dies for different bands, different modes and switches. There's a CMOS controller, multi-chip modules... and it's all a big mess,” he says. “With CMOS you integrate a lot of this onto the same die and get a much cleaner module.”
Commercial pressures could also prompt a switch to CMOS. As Hendy asserts: “Broadcom, Qualcomm, Intel and Nvidia; they all buy bucket loads of CMOS from TSMC.”
“These companies would much rather be in control of the supply chain that they already have, than be reliant on weird and wonderful GaAs companies such as Skyworks and RFMD to always provide them the [components] they want on time,” he adds.
So where does this leave the GaAs PA? Safe for now.
Hendy asserts the lion-share of first 4G smartphones, set ship late this year or early 2014, will come with a GaAs PA. However, he also believes CMOS with ET will “inevitably” capture market share from GaAs in the RF-front-end market.
“When you have a big player such as Qualcomm into this, everyone else will be looking into it,” he concludes.
Jeremy Hendy is certain chip-set manufacturers will adopt CMOS power amplifiers with envelope tracking.
