The cell phone market has become one of the most competitive arenas in the semiconductor industry, with manufacturers producing more than 1 billion units annually.
The argument has been made that specialty processes, such as gallium arsenide (GaAs), laterally diffused MOS (LDMOS) or silicon germanium (SiGe) bipolar CMOS (BiCMOS), with less-precise geometries, may offer the short-term cost advantage and linear modulation demanded by manufacturers and designers. The economies of scale inherent in CMOS, however, have driven the industry to make significant investments in this process, creating volumes that have outlasted, and will continue to outlast, any niche process offerings.
For example, transceiver blocks previously realized in specialty BiCMOS processes from vendors such as Infineon, NXP and Skyworks have long since been implemented in CMOS--and, in some cases, integrated with the handset's main processor inside a system-on-chip. Designers have repeatedly found that implementing an analog block in standard CMOS has paid off in the long run, despite the hurdles posed by challenging circuit blocks in a less-forgiving process.
One area that the CMOS process has not been able to successfully penetrate, however, is the power amplifier (PA) block, which remains a key element within the cell phone.
Until now, the PA block has been developed using a specialty GaAs or LDMOS process coupled with a hybrid module packaging technology--in total an expensive manufacturing flow, which has made the PA a substantial part of a cell phone's bill of materials. The specialty semiconductor process is required to provide a high-gain, high-frequency transistor element with a high-breakdown voltage. The hybrid packaging technology provides high-Q passive components to generate the 50-ohm matching circuit.
Implementing in standard CMOS means the designer has to live without enhanced transistors and high-Q passives, making the development of a fully integrated PA extremely challenging.
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Recently, a new technique has emerged that makes it possible to design a PA using CMOS processes, such that it can be housed in a simple plastic package. Distributed active transformer (DAT) technology, as it is called, employs a geometry that enables a relatively low-Q semiconductor metal to be used to provide a transformer-based matching circuit, eliminating the need for a module package. Distributing the PA core into several blocks and combining the power using the transformer structure means, in conjunction with several other patented techniques, that a high-breakdown transistor is not required.
Originators of the DAT technique built on research by the California Institute of Technology in Pasadena, Calif. Further innovation at Axiom Microdevices, the commercial producer of DAT-based PAs, perfected the productization of a General Packet Radio Service (GPRS) PA device, solving application-specific problems, such as receive band noise and operation from a high-voltage battery pack.
Unlike GaAs equivalents, with the power core now on CMOS, the small-signal control circuitry required to bias and regulate the power of a GPRS-type PA may now be integrated in the same die as the main power stages, further reducing costs.
Integration of the PA core also provides more flexibility in supporting linear modulation schemes. Traditionally, designers have used one of two methods--the "brute force" method, which employs a linear modulator and PA, or the more elegant "envelope reconstruction" method, in which a nonlinear, highly efficient PA becomes the core of a polar modulator.
In the case of the latter, using a traditional PA construction technique poses significant challenges. For example, a PA core implemented in a specialty process will have a different response to variations in temperature and process than its linearization circuitry. This usually leads to the extremely undesirable requirement that the end user, or handset manufacturer, must increase his factory calibration times to compensate, thus increasing costs. Conversely, having the PA core on the same die as its controller provides access to multiple points either for monitoring or control actuation. This gives the designer more options when developing a transmission architecture with best-in-class performance and cost structure.
The next logical step is to further integrate the PA with other cell phone components, such as the baseband and transceiver modules, which, as previously mentioned, are already being implemented using the CMOS process.
The DAT-based CMOS PA technology has recently proved to be viable for use within a standalone GPRS product through the large-volume production shipments of Axiom's AX502 device. It also has the potential for future integration, which will help the industry continue to address and fulfill the demands of manufacturers and designers for chip sets that are smaller and more cost-effective, and that offer broader feature sets.
David Kang (firstname.lastname@example.org) is vice president of engineering at Axiom Microdevices Inc. (Irvine, Calif.) Kang has more than 15 years' experience in product design and development.