Amplifiers Cultivate Greener Base Stations

June 17, 2010
As network operators look to lower power consumption while raising capacity, amplifier makers are responding with diverse and cutting-edge solutions.

"GREEN"basestationswould have been considered a camouflage technique a decade ago. With the "greening" of engineering and the desire to conserve energy to lower costs, however, base-station designers are seeking more integrated, power-efficient, and smaller components. At the same time, mobile data traffic is predicted to double every year for the next few years. Capacity must therefore rise as power consumption decreases. Amplifiers are being spotlighted as key components for lowering power consumption without limiting network capacity. Whether they are low-noise amplifiers (LNAs), power amplifiers (PAs), or driver amplifiers, they are being designed for decreased size and power consumption while enabling greater reliability, capacity, and efficiency.

The need for increased capacity is only expected to grow going forward. According to Fawad Maqbool, President of Amplitech, Inc., "Wireless applications and demands are increasing daily. More and more bandwidth is needed to satisfy the next generation (4G, 5G) of wireless products including handheld PDAs and land-based communication links. This demand is also steadily pushing up the frequencies of operation as well as the speed of the data, which requires the receiving systems to provide lower noise, lower distortion, higher dynamic range, and broader coverage area."

A slew of amplifiers are being introduced to satisfy these needs. For example, RFMD just unveiled high-linearity, digitally controlled (6-b) variable-gain amplifiers (DVGAs). The RFDA DVGA family provides a noise figure below 5 dB over the entire gain range. Each VGA has a gain range of 31.5 dB with 0.5-dB step resolution. The family offers a selection of components with gain to 38 dB, output third-order intercept point (IP3) to +43 dBm, and operation to 4 GHz. For instance, the RDA1005L covers 10 to 4000 MHz while providing 18.5 dB of gain in 0.5- dB steps. It provides +21.0 dBm output power at 1-dB compression and a +35.0-dBm output IP3. Its sibling, the RFDA2026, spans 1800 to 2400 MHz while exhibiting 32.0 dB gain in 0.5-dB steps. It offers +24.0 output power at 1-dB compression and an IP3 of +43.0 dBm. These components are suitable for both receiver and transmitter designs and are offered in both parallel and serial interface versions.

Wireless infrastructure also is among the targets of RFMD's RF5633 2.2-to- 3.8-GHz WiMAX PA integrated circuit (IC). The IC, which features indium-gallium- phosphide (InGaP) heterojunctionbipolar- transistor (HBT) technology, integrates a three-stage PA and power detector. It delivers 2.5 percent error vector magnitude (EVM) with +28 dBm output power from 3.4 to 3.6 GHz and +27 dBm from 3.6 to 3.8 GHz. The PA's bias may be controlled to accommodate a 22-dB gain step to increase the system's dynamic range. The RF5633 offers 34 dB gain.

A new 0.5-W HBT driver amplifier from M/A-COM Technology Solutions targets both cellular and WiMAX base stations. The MAAM-009286 covers 250 to 4000 MHz with an output IP3 of +42 dBm biased at 5 V and 155 mA. At 2140 MHz, it provides 15.5 dB of midband gain. In addition, cellular base stations are among the suitable applications for the firm's 70-to-3000-MHz miniature gain-stage amplifiers.

These gallium-arsenide (GaAs), monolithic-microwaveintegrated- circuit (MMIC) amplifiers employ one-stage, self-biased designs (Fig. 1). They feature 50-O input/output impedance to minimize the number of external components required. M/A-COM's MAAL-009120 offers a typical noise figure of 1.4 dB at 900 MHz and IP3 performance of +35 dBm from 500 to 3000 MHz. Its sibling, the MAAL- 010200, delivers a 1.3-dB noise figure at 900 MHz and +36 dBm IP3 from 500 to 3000 MHz. Both products offer typical gain of 14 dB at 900 MHz. According to Jack Redus, Product Manager at M/A-COM Technology Solutions, "Increased data rates and more complex modulation schemes are driving wireless infrastructure to demand increased linearity from driver amplifiers. These trends are accompanied by a constant demand to reduce overall power consumption and manufacturing complexity. While integrating amplifier, switch, attenuator, and other functions on a single die can simplify manufacturing, it is often at the expense of performance of each functional block. Multifunction integration of dissimilar technologies in a single package will both optimize performance and manufacturing, leading to the eventual replacement of single-function components. Some of our single-function amplifiers available today will be integrated into future multifunction, multichip solutions in surface-mount packages."

WiMAX and 4G LTE applications from 2.3 to 2.8 GHz also are among the target applications of a GaAs InGaP HBT MMIC PA from Hittite Microwave Corp. The HMC755LP4E provides gain to 31 dB with an output IP3 of +43 dBm. It achieves 28-percent power added efficiency (PAE) at +33 dBm saturated output power. Three power control pins can be used to reduce the RF output power or power down the PA to conserve DC power. With orthogonal frequency division multiplex (OFDM) signals, the HMC755LP4E achieves output power of +25 dBm (64 QAM, 54 Mb/s) with EVM of 2.5 percent.

Although Freescale Semiconductor is well known for its LDMOS expertise, the firm recently unveiled MMICs that are optimized for transmitter and receiver functions in base stations, femtocells, and cellular repeaters. The devices address both low-noise amplifiers and transmit power amplifiers. The MML09211H, for example, is an enhancement-mode pHEMT MMIC LNA that is well suited for applications ranging from WCDMA base stations in the 865-to-960-MHz band to the high-data-rate networks currently being implemented in the 728-to-768-MHz band. The device offers a low noise figure of 0.6 dB from 400 to 1400 MHz. At 900 MHz, it delivers 20 dB small-signal gain with +21 dBm output power at 1-dB compression. The amplifier exhibits isolation of 35 dB with output IP3 of +32 dBm. Its sibling, the MMA20312B, is a two-stage InGaP HBT power amplifier designed for use in wireless base stations as well as repeaters and femtocells. The amplifier covers 1800 to 2200 MHz. At 2140 MHz, it delivers +31 dBm output power at 1-dB compression with a smallsignal gain of 26 dB.

The two other new broadband MMIC amplifiers are equally suited for use as driver amplifiers in the transmit chain or as second-stage low-noise amplifiers in the receive chain. The MMG15241H pHEMT device covers 500 to 2800 MHz with a noise figure of 1.6 dB at 2140 MHz. It boasts +24 dBm output power at 1-dB compression with an IP3 of +39 dBm and small-signal gain of 15 dB. In contrast, the MMG20271H LNA covers 1500 to 2400 MHz. It provides a noise figure of 1.8 dB at 2140 MHz, +27 dBm output power at 1-dB compression, IP3 of +42 dBm, and small-signal gain of 15 dB.

Two new GaAs MMIC LNAs from Skyworks Solutions, Inc. promise to satisfy the demanding noise and linearity requirements for multiple cellular-infrastructure receiver applications including GSM, CDMA, WCDMA, and LTE base stations and repeaters (Fig. 2). The SKY67100-396LF covers 1.7 to 2.0 GHz while the SKY67101-396LF spans 0.7 to 1.0 GHz. These enhancement-mode pseudomorphic high-electron mobility transistor (pHEMT) LNAs are designed for a low noise figure down to 0.49 dB while providing an output third-order intercept point of +34 dBm. An internal active bias circuitry vows to provide stable performance over temperature.

At 1.95 GHz, the SKY67100-396LF offers a noise figure of 0.61 dB. It exhibits input return loss that is better than 20 dB with a +34-dBm high output third-order intercept point. In contrast, the SKY67101-396LF delivers a noise figure of 0.49 dB at 0.9 GHz. It exhibits input and output return loss above 20 dB at 0.9 GHz. The LNA delivers a high third-order intercept point of +34.1 dBm at 0.9 GHz.

By covering 1500 to 2300 MHz and 2300 to 4000 MHz, two devices from Avago Technologies complete the firm's next-generation LNA series covering all cellular bands for GSM, CDMA, UMTS, WiMAX, and LTE. The LNAs target base-station RF front-end design. At 1900 MHz with a typical operating condition of 5 V/51 mA, the firm's proprietary GaAs enhancement-mode pHEMT process technology provides a noise figure of 0.48 dB and an OIP3 of +35 dBm. It delivers +21 dBm output power at 1-dB compression with 17.8 dB gain. At 2500 MHz with a typical operating condition of 5 V/56 mA, however, the technology offers a noise figure of 0.59 dB with OIP3 of +35 dBm. It boasts 17.5 dB gain with +22 dBm output power at 1-dB compression. Thanks to built-in active bias circuitry, Avago's LNA operating current is adjustable. Designers can therefore make tradeoffs between operating current and output linearity as measured by OIP3 while maintaining an optimum noise figure.

The HBT amplifier line from Aeroflex/Metelics (s) is well suited for cellular, PCS, 2.5/3G, MMDS, WLL, and other types of wireless-infrastructure applications. The firm offers both low and medium power amplifiers as well as Darlington gain blocks. The HBT low and medium PAs are specifically designed for highefficiency, Class A driver devices. They provide output power from 0.25 to 4 W with gain ranging from 100 MHz to 2650 GHz. The MMA717-3030, for example, is designed to provide moderate power levels from 100 MHz to 2.5 GHz. Typically, it provides +31 dBm output power with a high output IP3 of +50 dBm. It delivers a typical dynamic range of 97 dB.

For their part, the Aeroflex/Metelics InGaP HBT MMIC amplifiers are specially designed as broadband Darlington gain blocks that combine different levels of small signal gain, noise, and bandwidth. They offer a flat gain response from 100 MHz to 12 GHz. At 2 GHz, for example, the MMA708 spans DC to 4000 MHz while offering 13 dB of gain. It features +20 dBm output power at -1 dB with a typical IP3 of +38 dBm. The amplifier provides a noise figure of 6 dB with input and output VSWR of 1.40:1 and 2.00:1, respectively.

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At the IEEE's International Microwave Symposium (IMS) last month, NXP demonstrated a range of RF and IF amplifiers based on silicon germanium:carbide (SiGe:C) technology. Ranging from LNAs to fixed and variable-gain amplifiers, these devices promise to enable a higher level of integration in wireless infrastructure TRx radio design. The firm offers a range of Doherty PAs covering 400 to 3500 MHz. Among these are a three-way Doherty amplifier and a single-package 600-W Doherty PA based on the firm's 50-V LDMOS process, which both run at 900 MHz. The three-way Doherty circuit achieves +52.7 dBm peak power (+44.1 dBm average power) with high efficiency of 49.2 percent. The 600-W (+57.8 dBm) single-package circuit achieves more than 43 percent efficiency at +49.2 dBm output power across the band.

According to RFHIC Corp., having higher efficiency throughout the 30-MHz bandwidth and six channels were a challenge for LDMOS. In addition, both the LDMOS transistor package and input/output matching patters were larger. As a result, the firm opted for a gallium-nitride (GaN) approach. The 80-W PA for LTE and WCDMA applications covers 2110 to 2140 MHz with a 30-MHz bandwidth (Fig. 3). By utilizing a Doherty design and DPD technique, it promises to show 35 percent or higher efficiency. The PA provides 50 dB gain at 48 V. This GaN amplifier is already being deployed together with Korean mobile-service provider SK Telecom in selected sites. RFHIC is targeting 40 percent efficiency with its next generation.

Beyond the amplifiers themselves, firms like Nujira provide ancillary products and services to support the evaluation and development of base-station and remote radiohead PAs. For example, the NCT-E9001 Evaluation and Development Platform Package includes waveform transmission and analysis, multi-mode digital predistortion (DPD), and a complete RF transmit line-up. Control and analysis tools are provided by the EvalSys suite. Users are able to incorporate their own PA designs, DPD algorithms, and DPD hardware for reference comparison and development. So far, reference PA output stages are available with 28- and 48-V RF power transistors in LDMOS and GaN. Others are under development, however. The firm also offers the Coolteq-h high-power modulator, which promises to raise the efficiency of PAs by as much as 60 percent.

Another interesting approach is taken by AmpliTech, which offers a variety of cryogenic amplifiers and systems that promise to vastly improve all parameters and range of these wireless systems while addressing the demanding needs of future wireless technologies for base stations and multipoint links. For example, the firm's 80K cryogenically cooled LNA assembly offers noise figures down to 0.05 dB at GPS and GSM bands and 0.6 dB in Ka-band. This cooled LNA/assembly is located directly at the antenna feed via a 3-m umbilicus. With no microphonic sidebands, it reduces digital-signalprocessing (DSP) post-processing.

Transistor developments are helping to raise efficiency in base-station amplifiers. For instance, TriQuint's TriPower GaAs HV-HBT radiofrequency integrated circuits (RFICs) promise to reduce the electricity needed to power network base-station amplifiers while allowing operators to more easily increase network capacity and speed. Today, a system with 2000 amplifiers might be used to cover one medium-sized city and surrounding area. According to TriQuint, a TriPower-based network would cut CO2 emissions by about 340 tons each year.

When used in a symmetric Doherty amplifier application for maximum efficiency boost, two TriQuint TG2H214120 120-W devices can deliver over 60 W of average WCDMA power with 55 percent efficiency. Due to TriPower's very high efficiency, operators can place larger amplifiers onto existing cell site towers without a corresponding increase in size or weight. Higher-power amplifiers, in turn, deliver higher data rates to all users in the cell. Clearly, the amplifier is the main key to making base stations greener. Yet a host of related technologies are helping amplifiers answer the call for more capacity from less power.

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