Freescale Unleashes New RF Power Transistors for a Wide Array of Applications
By Freescale Semiconductor
Among the deluge of new products announced at IMS2012, Freescale Semiconductor was unusually prolific, introducing devices for public safety radios, Doherty-based amplifiers for cellular infrastructure, extremely wideband devices covering 1 MHz to 2 GHz, and its first gallium nitride (GaN) RF power transistor.
Of the new Freescale devices, the most versatile in terms of potential uses are two wideband LDMOS FETs that can serve applications ranging from 1 MHz to 2 GHz, which covers an enormous array of services. The 25 W MRFE6VS25N and the 100 W MRFE6VP100H (Figure 1) can deliver their full CW rated RF output power over the entire operating frequency range. The products offer a combination of broadband capability, ruggedness, and wide frequency range at lower cost than GaN devices over this range. Target applications include HF and UHF transmitters and transceivers, television transmitters, white space data transceivers, aerospace/defense systems, test equipment, and radar systems.
The devices are extremely rugged, and can handle an impedance mismatch (VSWR) of at least 65:1 while also remaining highly linear. The wideband LDMOS FETs are designed to work flawlessly under extremely harsh conditions, where survivability and availability are critical. The devices are housed in Freescale’s low-thermal-resistance packages that minimize internal temperature rise and improve long-term reliability while reducing thermal management issues.
The devices also integrate internal networks that enhance circuit stability over a wide range of operating conditions, simplifying external circuitry. In order to assure optimal ruggedness, Freescale tests the devices under conditions far beyond what they would experience in normal operation, evaluating them at 20% beyond their rated operating voltage and twice their rated RF input power into a 65:1 VSWR.
With such a wide operating frequency range, the MRFE6VS25N and MRFE6VP100H allow designers to use a single amplifier to cover multiple bands, such as 1.8 to 54 MHz and 30 to 512 MHz. The result is a reduced bill of materials, smaller amplifier size and weight, lower switching losses, and less cooling overhead.
Specifications for the MRFE6VS25N and MRFE6VP100H include:
MRFE6VS25N: 25 W CW, gain of more than 26 dB from 1.8 to 30 MHz, more than 25 dB at 512 MHz, and efficiency of 50 to 74.5%.
MRFE6VP100H: 100 W CW, gain of 26 dB at 512 MHz, more than 19 dB from 30 to 512 MHz, and efficiency of 40 to 70%.
The MRFE6VS25N is housed in Freescale’s TO-270-2 over-molded plastic package and the MRFE6VP100H/HS transistors are housed in Freescale’s NI-780-4 and NI-780S-4 air cavity packages. Both devices are now in production and are supported by reference designs and other tools.
New Airfast Devices
Freescale also introduced 28-VDC Airfast LDMOS RF power transistors that like the other members of the Airfast family are designed to increase the efficiency, peak RF output power, and bandwidth of next-generation wireless base stations. The new devices expand the Airfast line to include at least one solution for small- and large-cell base stations in every cellular band.
The AFT09S282N is designed for use in multi-carrier GSM and LTE macro cells operating from 720 to 960 MHz. It delivers the highest peak power in its class for a device in an over-molded plastic package. Measured in a two transistor symmetric Doherty circuit, peak power is 700 W (58.5 dBm), average power is 140 W, gain is 18 dB, drain efficiency is 48%, and ACPR is -30 dBc. The device is housed in an OMNI OM-780-2 package.
The AFT18S230S is optimized for digital predistortion (DPD) circuits and Doherty amplifiers in cellular base station amplifiers operating from 1805 to 1880 MHz. Peak power is 50 W (47 dBm), average power is 80 W, gain is 17 dB at 8 dB output back-off, and drain efficiency is 32%. Such specifications are typically achieved only by more complex and expensive solutions employing asymmetric-only Doherty techniques. The AFT18S230S provides even higher efficiency when employed in asymmetric Doherty amplifiers. It supports 35-MHz linearized signal bandwidth for multicarrier GSM and has greater negative gate-source voltage range for improved Class C operation. The device is housed in a NI-780S-6 package.
The AFT21S230S (Figure 2) is designed for use in symmetric or asymmetric Doherty cellular base station amplifiers operating from 2110 to 2170 MHz. Peak power is 500 W (57 dBm), 80 W average power (49 dBm), gain 15.5 dB, drain efficiency 45%, video bandwidth up to 100 MHz. It supports 35-MHz linearized signal bandwidth and has a wide negative gate-source voltage range for improved Class C operation. It is optimized for DPD circuits and Doherty amplifiers and is housed in a NI-780S-6 package.
Finally, the AFT18HW355S is an advanced high-performance “in-package” asymmetrical Doherty amplifier designed for high-power PCS and DCS cellular base station amplifiers requiring very wide instantaneous bandwidth from 1805 to 1995 MHz. The device rivals much more expensive GaN-based solutions and is capable of operating at either 1805 to 1880 MHz or 1930 to 1995 MHz. It uses Freescale’s enhanced video bandwidth technology to enable full-band, multi-carrier operation. Typical Doherty single-carrier W-CDMA performance is 63 W average power with an input signal PAR of 9.9 dB at 0.01% probability on CCDF, and efficiency of 48% at 8% back-off.
Devices for Land Mobile
Freescale has been continuously manufacturing RF power transistors for land mobile radios and infrastructure since 1962, when Motorola (from which Freescale was spun off in 2003) introduced the first fully-transistorized two-way radio called the Handie-Talkie HT200, “affectionately” known by users as “The Brick.” RF power transistor technology today is orders of magnitude more advanced, but some of the many issues faced by The Brick remain today.
Specifically, public safety portable and mobile VHF and UHF transceivers take punishment more akin to their military battlefield counterparts, from rough handling (including being used to fend off attackers) to more subtle technological sources such as under-and overvoltage conditions, rapid changes in ambient temperature, input power overdrive and sometimes all three.
In the last half century Freescale has probably had customer discussions about every conceivable type of hostile operating condition, and its current LDMOS devices incorporate all of the lessons learned along the way. The company’s three new devices in the Airfast family designed for land mobile (and other) applications provide levels of ruggedness and stability that have previously been unattainable by LDMOS or other RF power transistor technologies for use in these applications. For example, the new 12-VDC Airfast devices will operate into a VSWR of at least 65:1 at 17 VDC with 3-dB input overdrive with no damage or degradation in performance.
Two of the new devices are designed for operation in the VHF and UHF public safety bands from 136 to 520 MHz and one is optimized for operation in the 700 to 900 MHz bands. They integrate much of the circuitry required to maintain stability within the devices themselves (rather than externally), simplifying design and maintaining stability under a wider range of conditions. They are tested to deliver their rated output power over their operating frequency range at an ambient temperature of -40º C with 20% undervoltage and an RF drive level twice their rated input power into a VSWR of 3:1.
The new devices include the AFT05MP075 (136 to 520 MHz) shown in Figure 3 that delivers more than 75 W P1dB RF output power (85 W P3dB), efficiency of 68%, and gain of 19 dB, and can achieve full power with as little as 1 W of drive. The AFT05MS031 (136 to 520 MHz) delivers 31 W P1dB RF output power, efficiency of 71%, gain of 18 dB, and achieves full RF output power with as little as 500 mW of drive. The AFT09MS031 (764 MHz to 941 MHz), delivers more than 31 W P1dB RF output power, efficiency of 71%, and gain of 17 dB, with only 600 mW of drive.
All three have integrated ESD protection for increased resistance to stray voltage during assembly and better performance when operated in Class C mode. The AFT05/09MS031N is available in Freescale’s two-lead over-molded plastic TO-270-2N (AFT09MS031N) and TO-270-2GN “gull wing” packages (AFT05/09MS031GN). The AFT05MP075 is available in the four-lead TO-270 WB-4 (AFT05M075N) and TO-270-WB-4 “gull wing” plastic package (AFT05MP075GN).
The devices also have performance and cost advantages over amplifier modules. Performance, particularly efficiency and ruggedness, is significantly better than industry standard modules. For example, the typical efficiency of an 800 MHz module is 25 to 33%, while the AFT09MS031N achieves efficiency of over 65%. This difference can cut cooling requirements by 70 to 80%, enabling smaller, more cost efficient terminals. To minimize engineering, Freescale provides complete amplifier reference designs that are both compact and enable low-cost mass production. These designs are simple and require only a few external inductors and capacitors to compliment the Airfast device, and result in an amplifier that maintains the same footprint as a module with higher performance at a lower cost, even when required external components are taken into consideration.
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