Power Amps Boost Multimode, Multiband Signals

Dec. 17, 2009
A new generation of multimode, multiband PA modules is supporting 3G/4G mobile devices to manage complex radio signals spread across multiple frequency bands.

Modern wireless handsets must handle cellular signals with complex modulation as well as other radio signals, like Bluetooth, WiFi, WiMAX, and Global Positioning System (GPS). To meet consumer demands for small size and long battery life, the handset requires a single broadband RF power amplifier (PA) capable of high linearity and low noise. Among the key manufacturers driving this trend are Anadigics, Avago Technologies, RF Micro Devices, Skyworks Solutions, STMicroelectronics, and TriQuint Semiconductor.

Over two years ago, Anadigics foresaw this demand and launched its pentaband, multimode third-generation (3G; GPRS/EDGE/HSDPA) PA module. Utilizing the company's second-generation, high-efficiency-at-low-power (HELP2) BiFET process technology, the AWT6223 PA module significantly reduced average power consumption in wideband code division multiple access (WCDMA) as well as GSM modes. By leveraging fourth-generation HELP4 technology, Anadigics is now readying a generation of multimode, multiband PA modules for 3G/fourth-generation (4G) applications (Fig. 1). According to Mahendra Singh, Director of Business Development and Applications, "The InGaP-Plus-derived BiFET process integrates both RF switches and PA on the same die along with a voltage regulator. And it enables the fabrication of multi-gain-state PAs with each gain-state optimized for linearity and efficiency."

The manufacturer has developed a number of PAs that promise to tackle multimode, multiband signals for next-generation handsets. The ALT6704, for example, is designed to cover bands 3, 4, and 9 from 1710 to 1785 MHz. It can support CDMA, WCDMA/UMTS, and Long Term Evolution (LTE) modes. Singh states that the efficiency at ~+28 dBm output power is greater than 40 percent. At ~+16 dBm output power, it is greater than 30 percent. The PA provides −40 dBc (5 MHz ACLR) linearity over the frequency range while quiescent current, Icq, is only 3 mA. "Unlike others, HELP4-based PAs don't require DC-DC converters because the internal switch provides the optimum load line at different power levels," asserts Singh. The firm also is working toward a true converged 3G/4G PA module, which will support numerous frequency bands and air-interface modes.

A true converged PA module also is on the drawing board at TriQuint Semiconductor, Inc. In cooperation with selected transceiver/baseband chipset makers, it is developing a scalable 3G/4G converged RF architecture for multimode, multiband mobile devices. Called TriQuint Unified Mobile Front-end (TRIUMF), it is architected to support numerous frequency bands and air interfaces used in 3G mobile devices like GSM, EDGE, WCDMA, and HSPA. This convergence of functionality into one PA module should offer up to a 50 percent size reduction over today's discrete approach, notes Shane Smith, TriQuint's Senior Director of Marketing. Using its expertise, the firm is designing both passive and active RF elements to create a broadband RF amplifier architecture. As a result, TRIUMF will support modes like GSM/GPRS/EDGE for voice and lower-data-rate applications and WCDMA/HSPA/LTE for high-speed data. On the multiband side, it will handle traditional quad bands (GSM850/900/DCS1800/PCS1900) unified with options for 3GPP-designated bands 1 through 17. In doing so, it will enable worldwide WCDMA/HSPA/LTE coverage.

This architecture also promises to provide scalability and system-level validation. According to Smith, the TRIUMF architecture is being designed in close alignment with industry-leading 3G-chipset suppliers. Key benefits of TRIUMF include long battery life, reduced bill of materials, and miniaturization of the RF system. The company emphasizes that a single converged PA module integrated with antenna switching, a mode/band switch, and duplexers will reduce front-end board area significantly. While the RF switches will be implemented using GaAs pHEMT devices, the PA will utilize InGaP-based heterojunction-bipolar-transistor (HBT) devices. In fact, the enabling technology is the mixed-mode BiHEMT process.

Another semiconductor supplier moving in the direction of a converged solution is RF Micro Devices. The firm has unveiled a 3G/4G front-end platform that supports as many as nine cellular bands across the 2G/2.5G/3G/4G mobile standards. In addition to flexibility in the front end, this converged architecture improves functional density to simplify design, reduce cost, and accelerate the implementation of 3G and 4G multimode mobile devices, says Ben Thomas, Marketing Director for 3G/4G Products. Labeled the RF6460, RFMD's converged platform comprises the following: the RF6260 multimode, multiband PA module; the RF6360 antenna switch module (ASM); and the RF6560 front-end power-management IC (Fig. 2). The PA is crafted so that it can operate in saturated GMSK and linear EDGE architectures during 2G operation and in an efficient, optimized linear mode during 3G/4G operation. To eliminate isolators and maintain both broadband and VSWR-tolerant performance, the RF6260 PA module uses a load-insensitive, balanced (dual-quadrature) architecture. For multiband configuration and 2G and 3G/4G operation, it integrates an SP4T/SP3T-mode switch. The scalable RF6260 supports as many as five bands.

Designed to work with the PA, the front-end power-management chip comprises a boost/buck DC-DC converter and auxiliary charge pump. Responding rapidly to load and line transients, the converter supplies an output voltage to the PA with minimal ripple over a wide voltage range. As a result, it can dynamically control PA operating conditions for optimum efficiency and linearity. For its part, the RF6360 ASM switch provides a single-pole, eight-throw (SP8T) pHEMT switch that supports five WCDMA bands (1, 2, 4, 5, 8) as well as two high-band and two low-band 2G/2.5G paths. A three-wire SDI interface permits the user to select the ASM active path.

Late last year, Skyworks Solutions, Inc. launched its first multimode, multiband frequency- division-duplexing/time-division-duplexing (FDD/TDD) PA module for 4G applications. According to the firm's Senior Director of Engineering for LTE development, Gene Tkachenko, the SKY77441 is a fully matched, surface-mount module developed for LTE FDD band 7 and LTE TDD bands 38 and 40.

Implemented in an InGaP-bipolar-FET (BiFET) process, the SKY77441 delivers over +26 dBm linear output power with full resource-block allocation with quadrature phase-shift keying (QPSK) or 16-state quadrature-amplitude- modulated (QAM) signals. It provides over +28 dBm linear output power with WCDMA modulation. Aside from power-amplification stages with power detection, the PA module integrates input and output matching networks. The supplier also is developing variants of the SKY77441 to support more bands for new applications.

To successfully combine bands and modes in an amplifier, Avago Technologies notes that a physically larger structure will be required. Such structures often require additional, somewhat costly circuitry like a DC-DC converter. There is also a significant penalty in efficiency with this kind of architecture due to the switch losses after the PA, the less-optimized load line, DC-DC power consumption, and other hurdles. Given today's multiple-frequency-band demands, manufacturers are essentially saving the cost of one or maybe two single-band PAs at a penalty of 10 to 20 percent more current draw. Because a single PA is usually less than one percent of the bill of materials (BOM), Avago's designers wonder if this trade off is worth it.

In the future, the maker predicts that three or more PAs will be eliminated from a design. That should be a crossover point for taking a single-broadband PA-module approach for multimode, multiband applications.Avago has plans to develop multimode, multiband PA structures and is working toward that approach. The firm's designers are investigating multiple architectures with some of them showing promise. Architecturally, however, it is still trying to determine the advantages that will clearly outweigh cost and performance.

While the majority of developers have taken the gallium-arsenide (GaAs) and associated compound-semiconductor route for their active devices, RF CMOS suppliers also are eying this lucrative market. Leveraging a novel PA architecture, startup Black Sand is readying a 3G CMOS PA. Although the first version is band- and mode-specific, a multiband, multimode CMOS PA is on the company's roadmap. Because silicon has the potential to integrate digital-controller and power-management circuitry on the same die, the developer is exploring architectures that will allow a core CMOS PA to be dynamically reconfigured for the desired band and mode. Ideally, it also will be possible to dynamically optimize the bias for optimum performance.

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