PA Technology Tracks Digital Handset Designs

Amplifier manufacturers turn to modules to meet the performance/size requirements being dictated by today's handset designers

By Janine Sullivan, Wireless Design Online

Designers of mobile phones for digital communications systems know that time to market is still one of their most critical concerns. As a result, many handset manufacturers are exploring the option of using power amplifier (PA) modules to speed engineering and manufacturing time, particularly in their new digital handset designs.

Power amplifier modules, in fact, are not a new idea. Back in the 1960's, Motorola Semiconductor (Phoenix) teamed with E.F. Johnson to provide the first power amplifier module. It served the 4 to 5 MHz range and was designed for use in land mobile radios. Early designs started as discrete transistors embedded in passive components.

Modern modules

Today's power amplifier modules, such as the ones newly announced by Anadigics (Warren, NJ) and RF Micro Devices (RFMD; Greensboro, NC) are significantly different from their historical predecessors. These designs feature monolithic die embedded into components within a module, and offer higher performance, smaller packages, and modern surface mount technology.

"Over the last few years, handset manufacturers have started asking for power amplifier modules because they are easier to use than integrated circuit solutions," explains Curtis Gong, Applications Engineer for the RF/IF Division of Motorola Semiconductor. "Power amplifier modules already contain the 50W matching circuitry, which requires less engineering expertise to implement and optimize."

This is especially true for first-generation designs, particularly in the CDMA market. Since all handset manufacturers are feeling time to market pressure, slicing time off of a design cycle by using a PA module is attractive. However, PA modules require more design and manufacturing time than ICs; therefore they cost PA manufacturers more to produce. As a result of this inherent price difference, it is likely that as handset designs enter their second and third generations, handset manufacturers will move to ICs to reduce overall cost.

In order to satisfy customer demands, Anadigics and RFMD are offering both PA modules and monolithic IC solutions. Both companies are sampling module products now, with production expected later this year or early next year.

The parts

Anadigics' most recent PA module offering, the AWT6101 (Figure 1) is one of the first parts to incorporate the company's integrated DC/DC converter technology, a development that allows GaAs MESFETs to operate from positive supply voltages without additional supporting components in system designs. This allows the company to address the cost issue of PA modules, "We chose GaAs MESFET processing for our PA modules because it allows us to target low costs while still meeting industry requirements of power and linearity," says David Osika, chief scientist with Anadigics' Strategic Technologies Group.

Anadigics AWT6101 combines the DC/DC converter and power amplifier functionality within a module

"For the most part, customers are asking for lower priced PAs, and would prefer modules," says Mark Williams, applications manager for the RF/IF Division of Motorola's Semiconductor Products Sector (SPS). Motorola is currently targeting Ics, but is considering producing PA modules for new digital handset designs.

"Current PA modules are using traditional technology. The future success of modules depends upon finding a new technology that will reduce their size and cost. Of course, the success of this new technology will depend on how fast it moves down the price learning curve," Williams says.

PA performance

One of the biggest challenges in designing power amplifiers (ICs or modules) for the CDMA/TDMA market is the tradeoff between power, efficiency, and linearity. Of course, designers must do this under the umbrella of cost, performance objectives, external components, and size considerations-the driving forces for all devices in the wireless market. (Most manufacturers agree that cost is perhaps the biggest factor influencing new designs.)

Linearity is a new concern for PA designers. "In older AMPS/GSM designs, the primary concerns were efficiency at a given power out," Gong says. In an effort to satisfy the linearity needs of new digital designs, and the desire for single source operation, Motorola has moved from GaAs MESFET to pHEMT GaAs technology with its most recent power amplifier design, the MRFIC1856 (Figure 2) for dual-band TDMA designs.

pHEMT products, such as Motorola's MRFIC1856 PA, provide better linearity than commonly used MESFET devices

As a subset of GaAs MESFET technology, pHEMT technology is suited for higher performance applications. "In pHEMT processing, you are working with higher electron mobility. Products are more difficult to make, but are better suited for a higher performance structure," Williams says.

HBT technology

Motorola is focusing on pHEMT technology now, but recognizes that heterojunction bipolar transistor (HBT) technology is probably the next step in the processing evolution. HBT processing leads to very low on-resistance, allowing for improved gain and efficiency. And, in small signal devices, it leads to lower noise figures.

RFMD recently introduced its RF3101 PA module, and is using GaAs HBT IC technology-a process that the company has been using in its monolithic PA solutions for several years. RFMD selected this technology because it can be used with a single-positive voltage supply, offers high performance in power and efficiency, and allows for smaller die sizes than FET-based devices.

Designing PAs into modules offers additional challenges for PA manufacturers. "When designing PA modules, one of the biggest challenges is controlling the impedance while working with the other components within the module. All the while, you must minimize size and cost while maximizing functionality," Osika says.

"Thermal issues are also very important for PA modules," adds Victor Steel, director of PA products at RFMD. While any PA package technology has thermal issues, modules typically do not have copper slugs (as many current IC packages do). "In modules, extra care must be taken to ensure that there is low thermal resistance from the device junctions to the PCB in order to guarantee long-term lifetimes in the millions of hours at elevated temperatures," explains Steel.

Choosing a PA

For purchasers of power amplifiers, one of the biggest considerations beyond performance (linearity, efficiency, power, and gain) is the total cost of use. It is important to consider design, engineering, manufacturing, tuning, and optimization time, as well as the cost of managing additional required off-chip components. This also includes the cost of the number of solder joints required and the subsequent reliability statistics. All of these considerations have driven the PA market over the years away from discrete solutions and towards highly integrated solutions, such as ICs and now, it seems, into modules.

Other concerns for the PA consumer include the manufacturers' ability to deliver parts in high volume and overall reliability. In the past, some handset manufacturers have been limited in their ability to deliver product because of difficulty in obtaining PAs.

Those who select a PA module also need to recognize that there is more variation than with monolithic solutions because of additional components in the module besides the IC. "With IC solutions, this variation is accommodated by the handset manufacturer because the matching components are on the phone board," Steel explains. "With a module, the matching is contained within the module and thus must be absorbed by the PA manufacturer. It will give the customer a more robust solution, since the PA module is fully tested to spec prior to shipping, but customers must also realize that the variation of those matching components is still present, albeit within the PA module."

The future of PAs

The future of PAs is likely to be marked by increased integration, greater functionality, and reduced overall footprint.

"We can expect PA solutions to continue to increase in integration," Anadigics' Osika predicts. "Eventual integrated functions many include filtering, switching, and power control."

RFMD's Steel agrees, and adds that modules are more likely than ICs to enable this multi-function trend, explaining that "with modules, technologies can be mixed, allowing isolation to be added."

While the footprint of the PA module may not decrease too much, increased integration and functionality will decrease the overall solution size because of a reduction in external components.

Article originally published in Wireless Design Online, 29 July, 1999