Transistors Energize Solid-State Amplifiers

April 11, 2011
Higher-power discrete transistors translate into fewer active devices and their associated matching circuits, power combiners, and other components for a given power level in an amplifier circuit.

Early high-power RF/microwave amplifiers relied on vacuum electronics devices, such as traveling-wave tubes (TWTs). But applications where size and weight are critical, such as airborne avionics systems, have motivated amplifier designers to seek higher and higher power levels from transistors. The result has been the development of a wide assortment of high-power RF/microwave discrete transistor technologies, from older silicon bipolars to the latest high-power silicon-carbide (SiC) and gallium-nitride (GaN) power transistors.

Silicon bipolar transistors were once the transistor of choice for high-power pulsed radar systems. Because these devices exhibit a positive temperature coefficient, however, they draw an increasing amount of bias current as temperature rises. The increased current consumption also causes the device junction temperature to rise, causing a dangerous spiral as part of a condition called "thermal runaway." As a result, bipolar power amplifiers have typically incorporated elaborate temperature-compensation circuitry to protect against over-current conditions.

By adapting FET device structures with silicon epitaxial materials, devices such as metal- oxide-semiconductor FETs (MOSFETs) and laterally diffused metal-oxide-semiconductor (LDMOS) transistors can provide equivalent power levels as a silicon bipolar, but with numerous benefits. These devices have a negative temperature coefficient and tend to be stable with temperature. They also offer somewhat more gain than bipolar transistors designed for a similar frequency range. As a result, silicon LDMOS transistors have gained wide acceptance in power amplifiers for cellular base stations, as well as in pulsed applications in various types of radar systems.

An online product matrix provided by Richardson Electronics is an excellent starting point when searching for LDMOS transistors at different frequencies and power levels. The product guide includes transistors from leading LDMOS device suppliers, including Freescale Semiconductor, Infineon Technologies, M/A-COM Technology Solutions, ST Microelectronics, and TriQuint Semiconductor.

For example, model MR-F377HR3 is a high-power LDMOS transistor from Freescale Semiconductor designed for boosting digitally modulated signals in communications and digital television systems. It is rated for 235 W output power at 1-dB compression from 470 to 860 Mhz with better than 18 dB gain at 860 MHz. More typically, the device will provide 45 w average output power with 16.7 dB over that frequency range when working with complex 64QAM signals. It operates with a +32-VDC supply.

The same firm also offers their model MRF6VP121KHR6 for pulsed signal applications from 965 to 1215 MHz. Because of the shorter duration of the signals, this silicon LDMOS device can deliver 1 kW peak output power when operating from a +50-VDC supply. The output power level is based on a signal with 128-s pulse width at 10-percent duty cycle. The transistor yields 20- dB power gain with 56-percent drain efficiency.

One LDMOS device supplier not found on the richardson list is NXP Semiconductors, which offers high-power LDMOS transistors such as the model BlA0912-250R for pulsed avionics applications. The device, with is designed for maximum supply voltage of +36 VDC, provides more than 12 dB gain and 250 W output power with 100-s, 10-percent-duty-cycle pulses from 960 to 1215 MHz. The silicon LDMOS transistor, which is supplied in an SOT-502A flange-mount package with ceramic cap, is suitable for TCAS and Mode-S systems from 1030 to 1090 MHz and JTIDS systems from 960 to 1215 MHz.

The model IXZ2210N50L from IXYS RF is a silicon MOSFET capable of 550 W CW output power with 14-dB gain at 175 MHz. Ideal for broadcast applications, the +150-VDC device offers minimum drain efficiency of 50 percent.

For amplifier designers comfortable with silicon bipolar transistors, model MRF10502 from M/A-COM Technology Solutions is a high-power device designed for applications in TCAS, TACAN, and Mode-S transmitters. It is usable from 1025 to 1150 MHz, with 9 dB gain at 1090 MHz. When boosting a 10-s pulse at 1-percent duty cycle, the bipolar transistor produces 500 W peak output power. It is supplied in a hermetic ceramic package and runs on supply voltages to +65 VDC.

When higher solid-state output power is needed at higher frequencies, silicon must be abandoned in favor of more exotic materials, such as SiC and GaN. Last year, Microsemi Corp., an innovator in high-power SiC pulsed radar transistors, surpassed the performance of its earlier 1500-W model 0405SC-1500M UHF (406-to450-MHz) transistor by introducing model 0405SC- 2200M, a UHF device rated at 2200 W output power for weather radars and overthe- horizon (OTH) radars. Its output power is determined with 300-s pulses at 6-percent duty cycle from 406 to 450 MHz. As with its 1500-W counterpart, it is designed for high supply voltages, to +125 VDC, to minimize peak current demand. The pulsed SiC transistor achieves 8-dB typical power gain with 55-percent drain efficiency at 450 MHz and is supplied in a hermetic flange-mount package.

Although not yet at the pulsed output levels of SiC, GaN devices have been developed for higher-frequency use than SiC, including the recently announced RF3934 GaN device from RF Micro Devices. Although it is sold as a "GaN amplifier," it is actually an unmatched GaN HEMT die in a hermetic flange-mount ceramic package (see figure). It can be optimized for use from DC to 3 GHz, and is rated for 13-dB gain at 2 GHz and as much as 140 W output power at 3-dB compression. The device is designed for supplies to +48 VDC and is available in die form (unpackaged) as model RF3934D.

Model CGH21240F is a GaN HEMT from Cree designed for high-power use in fourth-generation (4G) cellular communications base-station amplifiers. The unmatched device can be optimized for use from 1800 to 2300 MHz and can generate as much as 240 W output power. When operating with the modulated signals of LTE and WiMAX systems, the device can provide 40 W average power with 33-percent efficiency. It offers 15.1-dB gain at 2.2 GHz and is supplied in a ceramic/metal flange-mount package. It is designed for +28-VDC supply voltage and can achieve 65 percent drain efficiency running in pulsed operation.

Because of the challenges of properly dissipating heat from such a small heat source as a transistor, GaN pioneer Nitronex aimed at achieving low device thermal resistance in its latest generation of GaN transistors. Last year, the firm announced model NPT1010, a +28-VDC GaN HEMT that can be impedance matched for use from DC to 2 GHz. It is capable of 100 W output power at 900 MHz when operating at 3-dB compression, and delivers typical small-signal gain of 19.7 dB with drain efficiency of typically 64 percent. The GaN HEMT exhibits a thermal resistance of 1.4C/W to aid in thermal management at high power levels.

Regarding the company's new generation of GaN devices, Ray Crampton, Vice-President of Engineering, has this to say: "We focused our efforts on reducing thermal rise and developed a complete plan to attack all the key factors: FET design, die thickness, die attach methods, and package materials. We recognized early on that the contribution of the substrate is secondary to the contribution of other factors, particularly the FET design. By combining improvements from several areas, we achieved a 22-percent improvement in thermals compared to our last generation products."

In terms of pure bandwidth, GaAs FET devices still reach well into the microwave region while most GaN power transistors level off at about 3 GHz. But TriQuint has developed some impressive devices by fabricating GaN HEMT devices on SiC substrates, including their model TGF2023-20 with usable bandwidth of DC to 18 GHz. The 90-W discrete power transistor is suitable for broadband wireless, defense, and aerospace applications and provides +49.6 dBm saturated output power at the lower end of its operating- frequency range with 17.5-dB gain. It delivers power-added efficiency of 52 percent when supplied with +28 VDC.

Still, power GaAs FETs are well established in many different applications, notably for narrower telecommunications bands and broader EW use. Last year, long-time leader in power GaAs FETs, Toshiba America Electronic Components, announced additions to their Ku-band line of power GaAs FETs with their models TIM1213-18L and TIM1213-30L devices, with 18 and 30 W output power, respectively, from 12.7 to 13.2 GHz. The company's earlier devices in that frequency range provided output levels from 2 to 15 W. The former device offers +42.5 dBm output power at 1-dB gain compression with typical power gain of 6 dB and 28 percent PAE across the frequency range. The latter provides +45.0 dBm output power at 1-dB gain compression with typical power gain of 5.5 dB and PAE of 23 percent.

Another long-time power GaAs FET supplier, Fujitsu, has undergone some changes in business approach with its devices, creating a joint venture called Eudyna in 2004 with Sumitomo Electric Device Innovations, perhaps better known at that time for its GaAs foundry services. Fujitsu would eventually sell its 50-percent interest in the collaboration to Sumitomo in 2009. As a result, all of the high-power GaAs FET devices for telecommunications bands, such as 3.7 to 4.2 GHz and 5.9 to 6.4 GHz, formerly available as Fujitsu devices, are now supported, fabricated, and supplied by Sumitomo.

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