Compact RF Power Amplifiers Exploit Full Potential of GaN
By BC Systems

Gallium-nitride (GaN) has become the RF power amplifier device technology building block for a wide array of defense applications, ranging from IED jammers to broadband communications systems, and radar. The increasing popularity of GaN HEMT devices has been stated as “necessary” for specific DoD programs, primarily because of its high power density, ability to operate at much higher temperatures and voltages than gallium arsenide (GaAs), and its broad operating bandwidths. BC Systems has been using GaN devices extensively in its RF and microwave power amplifiers, and is exploiting their performance in ruggedized military amplifiers that are far smaller than their output power would suggest.

The company has just introduced two RF power amplifiers that exemplify the development efforts made to “get the most out of GaN.” Both are designed for specific military applications. The DA Module (Figure 1) is extremely compact and lightweight, measuring only 4.5 x 3 x 0.6" and weighing less than 1 lb. This makes it well-suited for manpack and man-portable communications and electronic warfare systems. It combines a broad frequency range of 25 to 1000 MHz with an RF output power of at least 20 W.

The amplifier is designed to be operated in Class AB mode, and has very fast blanking speed of less than 5 µs to achieve excellent noise performance and low standby power consumption. Its efficiency is at least 30%. The DA Module’s custom discrete design DC-to-DC converter allows the amplifier to operate from a 22 to 34 VDC power source.

The Model RF40015 amplifier (Figure 2) was developed to meet the need for applications that require broadband frequency coverage of 20 to 305 MHz and RF output power of at least 100. It measures only 5.5 x 4.5 x 1.6" and weighs less than 2 lb. Like the DA Module, it has a very fast blanking speed of less than 5 µs with efficiency performance of at least 30%. Its DC-to-DC converter is highly efficient and allows the amplifier to operate from a 26 to 30 VDC power source.

Both amplifiers are based on the latest Gallium Nitride (GaN) RF power transistors from multiple vendors, are fully protected for over-current and over-voltage conditions, and can be specified with an integrated low-power sampling port for RF output monitoring as well as a directional coupler. They can deliver their rated power continuously into a VSWR of 2.5:1 and are unconditionally stable over all combinations of load impedance and phase conditions. In addition, the amplifiers have an operating temperature range of -20º C to +85º C and storage temperature range of -45º C to +95º C. The modules can be customized to meet various military standards as well as for parameters such as operating power, frequency range, mounting configurations, and connector type.

Power Supply Considerations
The company has an unusual advantage in a fiercely competitive amplifier marketplace in that it has been manufacturing power supplies for military systems for more than 25 years. Design, development, and manufacturing are all conducted in-house, using a discrete component design approach that allows the power supplies to be matched to amplifier performance requirements, providing a more highly integrated product.

While this capability may not seem important when related to RF and microwave power amplifiers, just the opposite is true because an amplifier can perform to its fullest potential under varying signal conditions only if its power supply is an integral part of the overall amplifier design. When the DC-to-DC converter and amplifier are designed together, the overall result is a unit that is typically smaller and weighs less than if a COTS “brick” power supply was used. The integrated design approach also enables greater power supply-to-amplifier isolation, and is more reliable as well. The issue of isolation has taken center stage of late as the Next Generation Jammer (NGJ) program, for example, will attempt to replace the current traveling wave tube amplifiers (TWTAs) used in the EW systems it replaces, and requires very high levels of noise reduction.

Spectral regrowth, for example, can occur because of interaction of the amplifier’s input signal with component nonlinearities, as well as noise or voltage ripple in the power supply. If the amplifier design does not provide a high level of power supply rejection, noise from the supply will typically have a negative effect on the amplifier’s output waveform. Noise can result from coupling of RF and analog signals to the supply through bond wires and other parasitics, thermal noise from devices, digital switching, or voltage ripple from switching regulators.

A good example of a BC Systems AC-to-DC converter designed to provide power for solid-state RF amplifiers in airborne Identification Friend or Foe (IFF) applications is the BC-751 (Figure 3), an 800 W AC-to-DC unit with eight outputs. It accepts a three-phase 155 VAC, 400 Hz input with a power factor of at least 0.88 and maximum inrush current of less than 13 A, and has overall efficiency of 77%. The converter provides eight outputs: +3.3 VDC at 45 A and +5 VDC at 40 A with ripple and noise of less than 100 mVpp, -5 VDC at 600 mA with ripple and noise of less than 50 mVpp, +51 VDC at 10 A and +24 VDC at 1 A, and +12 VDC at 8 A, -12 VDC at 5 A, and +15 VDC at 3 A.

The converter provides full protection for over-current, over-voltage, and over-temperature conditions, operates over a temperature range of -40º C to +85º C (-55º C to +85º C non-operating), and is designed for use with forced-air cooling. The BC-750 Series of power supplies all meet CE101 and CE102 conducted emissions requirements per MIL-STD-461, and shock and vibration to IAW MIL-STD-810D. They measure 8.51 x 8.43 x 3.69" and weigh less than 12 lb. The BC-750 series are custom AC-to-DC converters and many of their specifications as well as mounting and configuration can be modified by BC Systems to meet specific customer requirements.

Summary
The potential of GaN is just beginning to be realized as more and more defense systems demand the performance characteristics that only it can provide. The BC Systems amplifiers and DC-DC and AC-DC converters described in this article are only a few examples from the company’s entire product line. For more information, visit www.bcpowersys.com.

BC Power Systems
www.bcpowersys.com
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