Fully-Loaded GaAs Front-End Modules: Proven Winners for LTE and 3G Smartphones
By Phil Warder, Director of Strategic Marketing, TriQuint
Superior performance and small die size. Those are the key advantages that have made Gallium arsenide (GaAs) the go-to material technology for design engineers working on RF front-end designs for cellular phones. For years, using GaAs-based heterojunction bipolar transistors (HBTs) to build power amplifiers that deliver high efficiency in handset front-ends has meant long battery life in small form factors—particularly in the mid-range and high-end 3G/4G markets.
New Challenges for RF Front-end Designers
RF front-end design—generally encompassing all the components between the digital baseband transceiver and the antenna—has become significantly more complicated in recent years. With the transition to 3G- and 4G-enabled smartphones, design engineers are faced with several challenges: more stringent performance requirements, a rapidly increasing number of frequency bands and filters within the mobile devices, and the demand for increased performance from those filters to reduce the resulting interference issues.
In addition to the DC-to-DC conversion that is used to reduce the current drain at lower output powers, design engineers are looking to enhance the front-end and its basic building blocks (power amplifiers that boost RF signals, switches that direct the path of those signals, and filters that block unwanted noise) through the addition of complementary functions, such as envelope modulators and antenna tuning. And while there are many options available to designers today, GaAs-based power amplifiers still set the standard for current drain.
Performance vs. Integration
Although other technologies may promise single-chip solutions that incorporate the transceiver, PA, antenna switch and filters, they struggle to maintain GaAs’ efficiency levels at higher powers. As a result, there is often a tradeoff between integration and performance. Most handset designers are focused on optimizing the overall performance, size and cost of their front-ends across broad product lines, and they are unwilling to sacrifice performance for increased integration.
At the same time, delivering a complete RF solution is a key to success for III-V suppliers, which in many cases is an integrated module, not an individual die, using the best technology for each application. Leveraging both III-V and silicon technologies, for example, is one way to continue pushing performance and cost frontiers and recognizing that complete front-end solutions require advanced filtering technology.
A good example of combining the merits of GaAs and silicon is TriQuint’s multi-mode, multi-band power amplifier modules (MMPAs). These products combine high-performance GaAs PAs with a CMOS controller and silicon-on-insulator (SOI) switches. TriQuint’s MMPAs provide a highly-integrated approach for today’s increasingly complex RF design, and they equip designers with more room on the circuit board while minimizing engineering time and resources. MMPAs can also support more frequency bands than discrete architectures, while trimming board space by twenty percent. And these multi-band amplifiers feature a versatile design, allowing manufacturers to adopt a common platform for releasing new products at a faster pace, while keeping design and manufacturing costs in check.
The TQM7M9053, for example, is designed on TriQuint’s GaAs BiHEMT technology, with CuFlip® assembly offering state of the art reliability, temperature stability and ruggedness.
This fully-matched, multimode, multiband PA module supports quad-band GSM/EDGE Linear, W/CDMA & LTE Band 1, and low band WCDMA/LTE (externally tuned for either Band 5 or Band 8). The GSM PA output power is controlled by the input power coming from the transceiver in both GMSK and 8PSK modes. The 2-gain-state WCDMA PA operates in high power mode (HPM) and medium power mode (MPM) to maximize talk time over the entire range of operating conditions. It also includes a coupler and built-in regulator, ideal for today’s extremely small, data-enabled phones. Second-generation models provide 5 WCDMA/LTE bands, and future generations will feature broadband PAs that could support up to 10 WCDMA/LTE bands.
Comparing Technologies for Specific Components
To select the best parts for these high-performance modules, it is important to evaluate the relative merits of GaAs and other technologies on a component-by-component basis.
Power amplifiers: When it comes to PAs, GaAs continues to significantly outperform other technologies in terms of current drain and die size. GaAs (or GaAs-based MMPA modules that include silicon controllers and distribution switches, as mentioned above) will therefore continue to be widely used for mid-range and high-performance applications. For PAs targeting lower-end sockets where performance is not as important, other technologies such as silicon may be more suitable.
Switches: Another GaAs-based technology favored for years for its efficiency advantages in RF switches is pHEMT. But now that steady progress in SOI switches provides comparable performance, SOI switches are more widely used in mobile device designs. GaAs pHEMT switches tend to be reserved for applications where a superior cost or size tradeoff can be achieved by integrating the switch with a GaAs PA die, rather than using two separate die.
Filters: Filters play a crucial role in the RF front-end, because they selectively pass certain frequencies while rejecting unwanted noise. Unlike PAs, which can cover multiple bands, filters are band-specific, so growth in phone band counts leads directly to growth in the number of filters or duplexers within each device.
Many of the new bands allocated for LTE present significant filtering challenges. Amid a global spectrum crunch, new 4G bands are being squeezed next to preexisting bands, often with minimal guard bands. To mitigate the resulting interference issues, it is essential to employ advanced filter technology. Traditional surface acoustic wave (SAW) technologies have been adequate in the past, but the most challenging 3G/4G frequency bands need advanced filter technologies, such as bulk acoustic wave (BAW) or temperature-compensated SAW (TC-SAW).
Filters can be discrete components, or they can be integrated as filter banks or filter banks with switches. They also can be combined with active components to achieve higher levels of front-end integration that enhance performance and reduce overall size.
Other Components: Silicon is widely and successfully used for DC-DC converters and envelope trackers that further optimize battery current drain to improve the overall performance of both GaAs- and silicon-based RF architectures. Providing control and biasing circuits within amplifier modules is another area where silicon has been used for many years.
Power detectors, temperature sensors and regulators also have long silicon histories. These silicon circuits often comprise one die within a multiple-die module, and so module control has recently been transitioning from a few dedicated functional digital pins to a control bus architecture. This change is driven by the increasing number of bands and functions in front-end modules, as well as the desire to minimize the required control pins out of the transceiver or baseband. Silicon will remain the preferred choice for control buses as the MIPI front-end interface becomes more widely adopted.
Fully-loaded GaAs front-end modules are proven winners for providing superior performance, small size, and high efficiency for LTE and 3G smartphones. Leveraging the advantages of GaAs as well as the benefits of other technologies (e.g., silicon), is an effective way to maximize performance and cost-effectiveness while providing a complete front-end solution.
Products like TriQuint’s multi-mode, multi-band power amplifier modules (MMPAs) provide this sort of highly-integrated approach and meet today’s increasingly complex RF design challenges. By combining high-performance GaAs PAs with a CMOS controller and silicon-on-insulator (SOI) switches, these MMPAs consume less board space, minimize engineering time and resources, support more frequency bands than discrete architectures, and offer state of the art reliability, temperature stability and ruggedness.
Richardson RFPD offers the largest variety of RF GaAs components in the industry, with over a decade of design experience in GaAs power amplifiers, LNAs, switches, and other GaAs products.
TriQuint / Richardson RFPD
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