Improved linearity is one of the requirements for an effective wireless commuications transition from 2G series to 3G/4G systems, and high-performance RF transistors are vital to the amplifiers in these newer systems. As network operators transition from 2G services to 3G/4G systems, the role of highly efficient, highly linear RF transistors with higher operating voltages in base station amplifier designs become critical. Consequently, the technology that was well suited for 2G system amplifiers is inadequate for the nextgeneration platforms. To ensure that gallium arsenide (GaAs)-based RF power transistors are in the race for sockets in high-power amplifier designs for 3G/4G infrastructure applications, TriQuint Semiconductor has developed highvoltage heterojunction bipolar transistors (HV-HBTs). Launched late last year, the first members of this new GaAs HV-HBT amplifier family were tested in a Doherty configuration commonly used by base station amplifier manufacturers. According to the manufacturer, the HV-HBT devices delivered an efficiency level of 57 percent, surpassing the efficiencies available using competing technologies like laterally diffused metal oxide semiconductor (LDMOS) transistors or more- expensive gallium nitride (GaN) devices.

Since the HV-HBTs are designed for the output stages of the base station amplifier, the company has also readied low-power HBT pre-drivers and drivers for these devices, which were unveiled at the recent IEEE MTT-S International Microwave Symposium. Combining the HBT drivers with HV-HBTs, the company has developed a complete RF high-power amplifier (HPA) solution for the 3G/4G infrastructure applications.

In fact, using these HBTs in a three-stage configuration (predriver, driver, and output stage), the supplier demonstrated a symmetrical Doherty amplifier reference design that was capable of delivering 200 W P1dB or 50 W average WCDMA output power with an overall power-added efficiency (PAE) of 42 percent and 40 dB gain (Fig.1). The two-output stage HV-HBTs in this Doherty design included the new 2.14 GHz 100 W (P1dB) TG1H214100-FL, which the company is sampling to key customers and is slated to go into pre-production in the fourth quarter. The 50 W P1dB driver used in this reference is the new TG1H214050-FL, preceded by the new HBT pre-driver.

The new HBT pre-drivers integrate two stages in a single package. This allows designers to reduce the number of discrete amplifier components in a system. When used in a typical base station HPA design, a 25 percent cost reduction and a PCB area savings of 12 cm2 can be achieved compared to designs using two separate discrete amplifier stages, states Doug Slansky, TriQuint's product marketing manager. The new amplifiers' high linearity minimizes additional signal distortion for repeater applications when used in final amplifier stages, and reduces back-off power requirements to minimize distortion from high peak-to-average ratio (PAR) signals in 3G/4G mobile base stations. This translates into reduced overall system costs and improved efficiency, which can lower HPA power consumption and improve operational expenditures (OpEx) for multicarrier 3G mobile infrastructures, notes Slansky. The new high-dynamic range 2-stage 28 V HBT amplifier drivers AP631 (4 W) and AP632 (7 W) are available now.

In addition, for WCDMA or multicarrier GSM designers, the manufacturer is also sampling the 1842.5 MHz 100 W (P1dB) TG1H184100-FL part to key users. Meanwhile, the supplier is also developing second-generation versions with almost twice the output power capability. Plus, a 2650-MHz, 100-W (output at P1dB) version is also in the works for WiMAX and LTE applications, according to TriQuint.

BROADBAND HPA
To extend the reach of solid-state amplifiers into the power levels of traveling- wave-tube (TWT) and vacuum electron device (VED) domain, CAP Wireless is exploiting the attributes of spatial combination. Using spatial power combining techniques, the developer has given solid-state amplifiers in general, and GaAs MMICs in particular, the needed boost. In this scheme, output signals from multiple low-power devices are coherently combined, typically in a constrained, guided wave environment, to provide high output power without incurring the losses associated with printed circuit combiners.

Thus, taking a total of 16 1.5-W GaAs MMIC amplifiers with reasonable efficiencies and combining their individual outputs via spatial combining technology, CAP has crafted a 10-W instrumentation-grade amplifier with a bandwidth of 2 to 20 GHz. According to the manufacturer, the 2-to-20-GHz, 10-W amplifier, labeled GT-1000A, provides the bandwidth and output power of a TWT amplifier but with the reliability of solid-state electronics, safe low-voltage operation, minimal aging characteristics, and excellent fault tolerance. By offering the 2-to-20-GHz bandwidth in one unit and eliminating the band switching typical of multiple amplifiers from older technology, the new rack-mount broadband amplifier saves cost and time and increases reliability. The amplifier also features high linearity, a noise figure of better than 8 dB, lower than -30 dBc harmonics, and less than -60 dBc spurious content.

Interestingly, test equipment supplier Giga-tronics has incorporated the GT- 1000A amplifier into its broadband frequency synthesizers to achieve 10 W of leveled output test-signal power over a wide frequency range. In fact, the GT-1000A microwave power amplifier has been paired with Giga-tronics 2400B or 2500A frequency synthesizer to deliver +40 dBm of leveled output power from 2 to 20 GHz. It provides all the power needed for overcoming cable and switching loss to a device under test (DUT) and for providing the power levels needed for pulsed test applications, according to CAP.

Aimed at EMI/EMC testing, the GT- 1000A is tailored as a general-purpose R&D lab amplifier, as an exciter for high-power transmitters, and can be rack mounted for ATE and system integration for manufacturing test, the developer said. The broadband microwave power amplifier with Spatium technology was demonstrated at last month's IEEE International Microwave Symposium in Atlanta, GA. The company has dubbed the proprietary spatial-combining technology Spatium.

Another solid-state proponent expanding into the TWT turf is RF Micro Devices, Inc. The device supplier has unwrapped a 400-W HPA that exploits the attributes of internally developed GaN-on-silicon-carbide high electron mobility transistors (HEMTs). According to RFMD, the 400-W gallium nitride (GaN) HPAs are designed for air traffic control radar and shipborne or ground-based pulsed S-band surveillance radar applications. The 400-W HPAs operate over a frequency range of 2.9 to 3.5 GHz, from a 65-V supply delivering 10.5-dB gain. Placed in a thermally efficient, ceramic hermetic package measuring only 24 x 17.4 mm, the 400-W GaN HPAs deliver power density and size advantages over competing silicon bipolar technologies, asserts RFMD. The drain efficiency of the GaN transistor is 50 percent across the band.

In S-band radar applications, the HPAs are combined in larger 2.5-kW "pallet" amplifier assemblies with as many as eight or more HPAs in each pallet. TWT technology, traditionally used in these applications, is prone to reliability issues resulting in field failures and expensive replacement costs. With a mean-time-to-failure (MTTF) goal of 1E6 hours at a 200 degrees C junction temperature, RFMD's GaN-on-SiC technology will deliver superior reliability, resulting in a considerably lower total cost of ownership for customers, asserted RFMD.

GAN RF POWER TRANSISTORS
Concurrently, rapid improvements in performance, reliability and manufacturing is attracting HPA designers toward GaN RF power transistors. For instance, Merrimac Industries, Inc. recently inked a development agreement with Nitronex Corporation to develop highly integrated power amplifiers using Merrimac's proprietary Multi-Mix multilayer circuit technology and high-power density GaN-on-silicon transistors.

Similarly, UK's RF power efficiency specialist Nujira is collaborating with Nitronex to create a power amplifier reference design for WiMAX base stations. Combining 28-V 90-W (Psat) GaN devices (NPT25100) from Nitronex with high-accuracy-tracking (HAT) technology from Nujira and conventional digital predistortion (DPD) techniques, the partners have been able to realize more than +44 dBm of linear power with 45 percent efficiency at a linearity of 55 dBc using a four-channel WCDMA waveform. Under a demanding WiMAX waveform with 20-MHz video bandwidth and 8.2-dB peak-to-average power ratio (PAPR), the same solution demonstrated +43.2-dBm linear power with 43-percent power-added efficiency, according to the developers.

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Nujira's HAT is an envelope tracking technology where bias voltage of the PA is changed dynamically to ensure that the output power transistors remain in the optimum part of their operating curve, and thereby keep power losses to a minimum. Hence, Nujira believes that HAT can squeeze the best performance in terms of efficiency and broadband from GaN power transistors. In fact, according to Nitronex's manager Ray Crampton, by going from Doherty architecture to envelope tracking using class AB design, the amplifier is showing a 10-percent improvement in efficiency. Since GaN transistors are also suitable for switch-mode PAs, the two partners are also exploring envelope tracking using switch-mode topology.

Meanwhile, for higher-output power, Nitronex is also readying higher-voltage GaN-on-Si HEMTs. While new 48-V devices are in the works, the manufacturer is also taking existing parts and qualifying them to run at 30 V. General sampling of the NPT2000 series will begin in the late second half of this year. Simultaneously, using two 90-W (CW) devices in pushpull configuration, the company is also prepping a 180-W part for broadband military applications below 1 GHz.

Likewise, years of work on the Defense Advanced Research Projects Agency (DARPA) contract for high-power, high-frequency GaN transistor-based amplifiers has resulted in commercial versions as well for TriQuint. As a result, the company has released its first line of high-frequency discrete die-level GaN devices that boast up to 2.5-times the power density of high-voltage GaAs devices. The discrete transistors operate to 18 GHz with 55-percent PAE and can produce as much as 90-W output power. Unlike Nitronex, TriQuint prefers GaN on SiC substrates.

Another GaN proponent eyeing WiMAX infrastructure applications is Cree. Like TriQuint, Cree also prefers building GaN transistors on SiC substrates. The supplier claims to have produced the industry's first GaN HEMT products for use in 5-GHz WiMAX applications covering the 4.9-to-5.8-GHz frequency band. The new transistors, models CGH55015F and CGH55030F, offer 15- and 30-W output power, respectively, with linearity of better than 2.5-percent EVM at average power under a WiMAX signal at 25-percent drain efficiency. Both transistors are available with "reference design" amplifier platforms. To complement the 5.8-GHz band, Cree has also released a 60-W GaN HEMT for 3.5-GHz WiMAX applications.

Japan's Eudyna Devices, Inc. also has taken the SiC route. One of the earlier entrants into this space, Eudyna continues to squeeze more power from its GaN HEMTs. It has demonstrated a 650-W pulsed peak-power GaN HEMT transistor for WCDMA base station applications. In essence, the device combines the output of two EGN21B180IV devices operating at 65 V to realize the higher output power.

ALTERNATIVE SILICON
As power amplifier designers weigh cost, efficiency, linearity, thermal performance, and peak power in RF transistors, Phoenix, AZ-based startup company HVVi Semiconductors, Inc., is giving engineers a silicon alternative. The company has developed a high-frequency, high-voltage vertical FET (HVVFET) architecture that delivers frequency bandwidth, voltage and power levels to radar and avionic applications that exceed the capabilities of bipolar and LDMOS transistors. According to the developer, this revolutionary patent-pending technology allows HVVi to achieve performance levels comparable to non-silicon technologies at a more attractive cost.

Exploiting its innovative architecture, the supplier has initially unwrapped three parts aimed at high-power, pulsed RF applications at L-band such as IFF, TCAS, TACAN and Mode-S systems. Leveraging the inherent benefits of the HVVFET process to deliver high output power and high gain in an extremely compact package, all three transistors are designed to operate at 48 V. Since the vertical structure allows the heat to be extracted from the hottest spot of the device directly to the heatsink, it enables more efficient thermal management for better reliability and improved MTBF (Fig. 2).

To overcome the limitations of older vertical designs like parasitics associated with silicon substrates, the HVVi uses a novel edge termination structure and a gate-drain faraday shield to minimize these effects. In addition, the HVVFET's small gate length translates into smaller capacitance and higher-frequency performance. According to HVVi, the first products offer radar and avionics system designers a 30 percent reduction in power consumption, a 100 percent increase in gain, and a tenfold increase in ruggedness.

From a system's perspective, HVVFET's performance advantages in terms of gain, efficiency and power density offer designers an opportunity to eliminate amplification stages in power amplifiers, reduce parts count, and shrink PCB space requirements, notes Brian D. Battaglia, HVVi's senior RF applications engineer. At the same time, the technology's higher- rated ruggedness allows radar and avionics designers to eliminate bulky and costly isolators and, in the process, significantly reduce system weight, size, and cost, continues Battaglia.

All three products operate over a wide supply voltage range of 24 to 48 V. For pulsed applications at L-band from 1030 to 1090 MHz, model HVV1011-300 operates at 48 V and delivers more than 300 W pulsed output power with typically 15-dB gain and 48-percent efficiency under pulsed signal conditions with a pulse width of 50 s and a pulse period of 1 ms. The device is specified to withstand a 20:1 VSWR at all phase angles under fullrated output power.

Likewise, the models HVV1214-025 and HVV1214-100 are enhancementmode RF transistors for L-band pulsed radar applications in the 1.2-to-1.4-GHz frequency range. Both devices operate with a 48-V supply and produce 25- and 100-W output power, respectively. Under test conditions that include signals with a pulse width of 200 s and a pulse duty cycle of 10 percent, the HVV1214-025 offers 17.5-dB typical gain and the HVV1214-100 provides 19.5-dB typical gain. Both transistors are capable of withstanding an output load mismatch corresponding to a 20:1 VSWR at rated output power and nominal operating voltage across the entire frequency band of operation.

Sampling now, the HVVFETs are slated for production in the third quarter. The two high-power products are in an industry-standard flanged package, while the 25-W driver is housed in an innovative surface-mount package. Evaluation kits are also available. Meanwhile, higher-power versions to S-band are expected to be released in the fourth quarter of 2008.

In the meantime, using the 25-W driver and two 100-W HVVFETs, Daico Industries has developed a fully qualified Aerostat L-band radar amplifier that offers 100 W output at 28 V and 200 W at 48 V. This dual-voltage L-band amplifier was demonstrated at last month's MTT-S.

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NEXT -GENERATION LDMOS
Speaking of the entrenched LDMOS, the RF power transistor continues to advance to keep the hold on the infrastructure applications and simultaneously expand into higher-frequency territory. Thus, based on its Gen7 LDMOS technology, optimized for high-power use and symmetrical Doherty amplifier designs, NXP Semiconductors has launched the 3.8-GHz BLC7G22L(S)-130 base station power transistor that offers a 20-percent improvement in power density and 2-percent improvement in power efficiency over the previous generation. Plus, the thermal resistance (Rth) is lower by 25 percent, as well as the output capacitance, as compared to the earlier generation. Using two BLC7G22L(S)-130 devices in a Doherty configuration with digital-predistortion (DPD) correction, NXP has crafted a Doherty reference design that offers average output power of +47 dBm at 2.14 GHz with PAE of 43 percent and gain of 15.7 dB at 28 V.

On the packaging front, LDMOS backer Infineon Technologies has encased a new line of RF power transistors in an innovative open cavity plastic package with a copper base. Offering RF performance that is comparable to traditional ceramic packages but at lower cost, the enhanced plastic open-cavity (EPOC) package delivers a 12-percent improvement in thermal resistance due to better thermal conductivity of copper (Fig. 3). The operating frequencies of the LDMOS devices in the new EPOC packages range from 920 to 1990 MHz with average output power levels from 25 to 50 W. Operating at 28 V, the PTFA091201GL and PTFA091201FL provide a typical gain of 18.5 dB with 44-percent efficiency at 50-W average output power under EDGE signal conditions. Additionally, Infineon has released two more parts for use from 1805 to 1880 MHz (models PTFA181001GL and PTFA181001HL) with typical gain of 17 dB and 27.5-percent efficiency at 25-W average output power under WCDMA signal conditions.

Freescale Semiconductor. continues to push the limits of LDMOS by unveiling what it claims is the world's first 50-V LDMOS RF power transistor for L-band radar. Implemented in sixth-generation, very high voltage (VHV6) LDMOS technology, the MRF6V14300H is capable of producing pulsed RF output power of 330 W at 1400 MHz and 300-s pulse width and 12-percent duty cycle. Other features include 17 dB gain and 60-percent drain efficiency. The company has added the MRF6V10010N driver, with 8-W peak output power at 1400 MHz, 300-s pulse width and 12-percent duty cycle, 22-dB gain, and 60-percent drain efficiency. To reduce susceptibility to electrostatic events on assembly lines, the devices have electrostatic-discharge (ESD) protection.