April 2008
Multi-Band Power Amp Modules
By Bruce Webber, Director of Marketing, Wireless, Anadigics, Inc.

Mobile phones are getting more features and functions all the time. The latest “world phones,” which let jet-setting executives carry just one handset wherever their business travels take them, are prime examples.
Creating one of these little marvels poses a real challenge. Designers and manufacturers have to deliver multi-band, multi-mode performance, including the latest 3G data and multimedia features, in a handset the size of a candy bar. Yet, with all these power-hungry features, the phone has to provide extended battery life, so users won’t have to look for an electric socket every few hours.

The transceiver sections in world phones and smartphones can use integrated circuitry to handle the various modes and frequencies. But transmitting a signal efficiently over the air is a more complicated matter. A phone’s RF circuitry requires components that are specific to each band and mode, including filters, power amplifiers (PAs), switches, and duplexers.

On a multi-band, multi-mode phone, that adds up to a lot of parts. At this point it is not possible to combine them into a single IC that accommodates all the modes and bands, while filtering out unwanted signal artifacts. So the issue is, how do you design an RF section that doesn’t occupy a lot of board real estate or draw too much power during operation?

Front-end modules (FEM) were an early approach to reducing RF design complexity. FEMs reduce the RF section’s overall board footprint and design complexity by combining dissimilar SAW filter, HBT devices, and other required technologies in a single package. However, this approach also shifts control of RF optimization from the phone designer to the module provider. To achieve cost and size targets, the module designer may choose a layout or components that do not provide optimal RF performance. And since the RF path is predetermined by the module’s design, an FEM actually limits the phone designer’s ability to create the optimum RF section for the phone.

In my opinion, a better approach to reducing RF section complexity and size is to combine similar technologies into modules, such as multi-band power amplifier and multi-band filter modules.

A multi-band module contains multiple PAs to handle different frequency bands, but does not incorporate filters. The phone designer can specify appropriate SAW filters for each band at the inputs of the appropriate PA modules. At their outputs, filters and duplexers can be integrated with a multi-port switch in a single module that feeds the antenna.

This approach offers several advantages for phone designers. Because the filters and duplexers are not integrated with the PAs, they can choose devices with characteristics that exactly match their requirements. In addition, a multi-band module offers a reduced footprint – always desirable in a device as small as a mobile phone – plus the ability to use the same supply voltages and logic lines as the rest of the circuitry.

In summary, mobile phone designers have many factors to consider when they select components and modules for their new products. The multi-band module approach gives them the size and performance characteristics they’re looking for in an RF section, without constraining their choices in other aspects of their designs.

Bruce Webber, Director of Marketing, Wireless, Anadigics, Inc.
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