CMOS PA Paves The Way For A Single-Chip Cell Phone

Oct 17, 2008 11:42 AM, By Donald McClymont, Axiom Microdevices Inc.

Who said you can’t put a cell-phone power amplifier on a CMOS chip?

Although integration of small signal circuitry on the same die as the PA is now possible, there are still challenges to overcome. At its maximum operating point, the PA produces RF voltages in excess of 20 V, which couple to almost every point on the die. Careful design and layout were required so the small signal circuitry may operate reliably in this environment. It was like trying to jump on a trampoline at the same time as a 600-pound gorilla.

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All voltage regulation to provide stable supplies for the power control is also integrated on the same die to allow the device to operate all circuitry directly from the battery supply. Low-power biasing circuitry had to be created to provide references to the voltage regulators generated from the battery supply, which could still withstand high battery voltages without breaking down.

In the realization of this device, Axiom found that traditional simulation techniques weren’t sufficient to predict the large signal RF performance of complex passive and active structures. Computationally intense simulation tools to predict passive performance, which had been traditionally deployed for non-IC applications, had to be integrated into a more conventional IC design flow.

With requirements for slim form factors becoming more important, Axiom selected a low-profile micro lead frame (MLF) package, which is only 0.9 mm in height. This package also is very low cost by virtue of its simple lead-frame chip carrier. It has excellent RF and thermal properties as well, as it was originally designed with a high-power millimeter-wave monolithic IC (MMIC) application in mind.

The DAT-based CMOS PA technology is now proven within a standalone GPRS product, through the large-volume production shipments of Axiom’s AX502 device. Axiom has now shipped tens of millions of this device to handset customers.

An added bonus of designing in CMOS as compared to a GaAs module, and having strictly followed the foundry design rules, is the device reliability achieved. Axiom’s design team carried out extensive reliability testing and found zero failures. This reliability has also carried forward into production shipment, with customers so far also reporting zero failures.

Having the CMOS power amplifier proven as a standalone product provides the potential for future integration, which will continue to help the semiconductor industry continue to address and fulfill the demands of manufacturers and designers alike for chipsets that are smaller and more cost-effective while offering broader feature sets.