GaN-based Amplifiers, Compact Power Supplies Combine for Defense Applications
By Vincent LaRose, Director of Operations, BC Systems

At first glance, RF and microwave power amplifiers used in defense applications such as electronic warfare, avionics, radar, and secure communications appear to be similar to those used in the base station transceivers of commercial wireless networks. However, the resemblance fades quickly upon further inspection, since they are designed to meet greater levels of ruggedness and performance “under duress”, and are very conservatively rated. They also make extensive use of gallium nitride (GaN) RF power transistors, which have risen from the speculative stage to “must have” status by the Department of Defense in only two years or so. Their performance must also be tightly coupled to the performance of their power supplies. The development of GaN devices, which was given a major boost by the Defense Advanced Research Projects Agency (DARPA), has been championed by several device manufacturers, and the technology is developing rapidly. The latest amplifiers and power supplies from BC Systems exemplify how all of these requirements can be met in small, lightweight enclosures.

Their differences aside, RF and microwave power amplifiers for commercial wireless and military communications applications (for example) are both required to deliver their rated performance when amplifying signals using higher-order modulation schemes (such as Orthogonal Frequency Division Multiplexing, or OFDM) that have peak-to-average ratios up to 12 dB, higher than previous generations of communications systems. As a result, the RF power transistors and amplifiers must maintain exceptional linearity in order to keep intermodulation distortion in check. Laterally-Diffused Metal on Silicon (LDMOS) FETs have stepped up to this challenge and today are the devices of choice for all commercial wireless base station amplifiers. While not inexpensive, they remain substantially less expensive than GaN devices.

However, AlGaN/GaN devices have intrinsic advantages not possessed by LDMOS devices that make them especially appealing in broadband, high-power applications in defense systems operating at higher frequencies. These attributes include higher gate-to-drain breakdown voltages and current densities that together deliver overall power densities several times that of LDMOS devices. This allows them to operate at high DC voltages and makes amplifier designs less complex, since die and packaged devices can be made smaller. They also have higher device impedances, which make them considerably easier to match over broad operating bandwidths.

As a result, GaN RF power devices have progressed from the developmental to production stage, as reliability, one of the major impediments to GaN’s acceptance, has been dramatically improved. Proof of that is the intense interest in GaN by the Department of Defense, long known for seeking out promising technologies – but not employing them unless thoroughly proven. While GaN’s cost penalty remains high, it continues to drop as requests grow for larger production quantities of GaN devices and because they are often fabricated on large silicon substrates rather than those of silicon carbide (SiC) or sapphire. Its unique intrinsic benefits are almost certain to make GaN a choice even for cost-sensitive applications in years to come.

Harnessing GaN’s Potential
The new BC Systems amplifiers, which are employed in EW/ECM, radar, avionics and applications including jammers for improvised explosive devices (IEDs), make extensive use of GaN HEMTs. They are the result of extensive development that was necessary to meet the stringent requirements which the customer’s specifications demand. A good example is the BCPA-1000-3000-80, which is designed for use in IED jammers. It produces at least 80 W at up to 30% efficiency between 1 and 3 GHz in Class AB mode, weighs less than 2.5 lb., and measures only 6.5 x 6.5 x 1.5 in.

The amplifier provides extremely fast blanking of less than 5 µs and has an integrated DC-to-DC converter that can accommodate power supply voltages between 18 and 36 VDC. Harmonic rejection is typically greater than 20 dBc and operating temperature range is -40°C to +85°C. The BCPA-1000-3000-80 meets MIL-STD-810 specifications for three-axis vibration (“Wheeled and Tracked Vehicles”) as well as functional shock.

The BCPA-25-1000-20 delivers at least 20 W of RF output in Class AB with greater than 30% efficiency from 25 to 1000 MHz, and is well suited for jammers as well as a driver stage for higher-power amplifiers. Input range is 22 to 34 VDC, harmonic rejection is greater than 20 dBc, and operating temperature range is -20°C to +85°C. The rugged amplifier can operate indefinitely into a 2.5:1 VSWR (impedance mismatch) at rated power without damage. The amplifier module weighs 0.6 lb. and measures 4.5 x 3 x 0.6 in.

Both amplifiers are unconditionally stable over all VSWR and signal phase conditions, and provide a low-power RF sampling output for measurement and monitoring purposes. Protection is provided for over-voltage, over-current, and over-temperature conditions and they are housed in fully EMI-shielded aluminum enclosures. Like all BC Systems RF power amplifiers, specification requirements of the BCPA-1000-3000-80 and BCPA-25-1000-20 can be customized to meet unique customer specifications.

Clean Power Personified
One of BC Systems’ inherent advantages is its ability to match power amplifiers to their DC supplies, thanks to its more than 20-year history as a military power supply manufacturer. Both the amplifiers and power supplies are designed, manufactured, and tested in the same facilities, which makes it possible to achieve a level of functional integration and amplifier performance difficult to realize and costly at best when using off-the-shelf switching power supply “bricks”. These “COTS” power supplies, while often delivering excellent performance in their own right, can significantly compromise the performance and customization potential of integrated power supply/RF power amplifier solutions. Cooling, which is a prime issue when the end use of a system is a UAV or other space-and-power-constrained platform, can also be more efficiently implemented when the power supply can be matched to a specific amplifier. Greater overall efficiency is possible as well.

The BC Systems power supplies (patent pending) meet stringent requirements for conducted and radiated emissions, which allows them to operate together with the amplifier without interfering with each other and degrading performance. In addition to extensive emissions control in the power supplies, the RF amplifiers also reject power supply noise caused by the coupling of RF and analog signals through bond wires, and from thermal device noise, digital switching, or voltage ripple produced by switching regulators.

The BC Systems power supplies also have the unique ability to automatically bias the power transistors so that they always operate under optimum conditions, which cannot be easily done when using standard commercial versions. This feature is available throughout the product line and even in the tiny models designed for manpack applications. BC Systems effectively addressed all of these problems through careful design and by tight integration of amplifier and power supply.

The BC-75 power supply is one of the company’s best examples of these efforts and occupies less space and delivers greater performance than could be achieved using a “brick” approach. It measures 2 x 1.1 x 0.5 in. and weighs just 16 g. The supply has overall efficiency of at least 93.5% and accepts an input voltage of 22 to 34 VDC (80 W) to deliver its full primary output of 26 to 29 VDC at up to 75 W, and is adjustable via an RS-232 Interface.

The BC-75’s secondary output is +5 VDC at 200 mA and is switched via the blanking input. Four bias outputs of 0 to -5 VDC are available to bias the GaN RF power transistors, and they can be adjusted through an RS-232 interface and upgraded via firmware to modify device bias for better amplifier performance over varying temperature conditions. The BC-75 operates over a temperature range of -20°C to +85°C and is qualified to MIL-STD-202 Method 213 for shock and MIL-STD-202 Method 214 for vibration.

The BC-385 is another one of the company’s small but potent power supplies, occupying only 5 x 3 x 1 in., weighing 130 g, and achieving efficiency of at least 95%. It accepts an input voltage of 26 to 30 VDC (385 W) to deliver its primary output of 26 to 32 VDC at up to 365 W, secondary output of +8 VDC at 500 mA switched with the blanking input, and the four outputs of 0 to -5 VDC for biasing the RF power transistors. Like the BC-75, the power supply’s outputs are adjustable via RS-232 and programmable through changes in firmware to optimize device bias, and it meets MIL-STD-202 Method 213 for shock and MIL-STD-202 Method 214 for vibration.

Conclusion
Defense applications place unique requirements on both RF power amplifiers and the power supplies that serve them, delivering very high performance and reliability when deployed in hostile environments rather than the comparatively benign confines of a wireless base station. The new amplifiers and power supplies from BC Systems uniquely address these challenges through the use of proprietary design techniques, GaN RF power transistors, and the ability to match amplifier and power supply to achieve optimum performance and efficient space utilization. More information can be obtained at www.bcpowersys.com.

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