High-frequency transistors are reaching new power levels and supporting the design of solid-state amplifiers with tube-like output capabilities.
Solid-state RF/microwave power was once limited to a few device options. But with everimproving semiconductor processes, transistors and transistor amplifiers are approaching output power levels that were once only possible with vacuum electron devices (VEDs), such as traveling wave tubes (TWTs). Devices and amplifiers based on silicon LDMOS and gallium nitride (GaN) are among the challengers to VED-based amplifiers.
On the GaN front, Nitronex, which was started 10 years ago by graduates of North Carolina State University's wide-bandgap program, made news earlier this year with the release of a 200-W high electron mobility transistor (HEMT). Built using the SIGANTIC NRF1 proprietary GaN-on-silicon technology, it serves applications to 1.2 GHz. The GaN HEMT spans DC to 1200 MHz. It promises to deliver high efficiency from 14 to 28 V. After combining losses, the power transistor achieves 200 W average output power at 3-dB gain compression at 63-percent efficiency with 18.3 dB gain at 900 MHz.
The NPT1007 can handle an output mismatch of 10:1 while in saturation. In application design AD-014, the company found that it delivered 90 W continuouswave (CW) power from 500 to 1000 MHz. The NPT1007 was developed to solve both size and efficiency problems for amplifier makers. It actually comprises two 100-W power transistors in an industry-standard, four-lead Gemini package. Thanks to this small footprint, the device makes it easier to combine both transistors into a compact, highpower- amplifier solution.
Work in GaN has been ongoing for some time at Toshiba America Electronic Components, Inc. In June, the firm announced three GaN semiconductor HEMTs. Among them is its first commercial C-band GaN HEMT for satellite-communication applications, which is known as the TGI 7785-120L. This transistor operates in the 7.7-to- 8.5-GHz range with output power of 120 W. Typically, the device features output power of +51.0 dBm with +44 dBm input power, linear gain of 11 dB, and drain current of 10 A.
Satcom applications also are the target of the extended Ku-band TGI1314- 50L, which operates from 13.75 to 14.5 GHz with output power of 50 W. Usually, the device features output power of +47.0 dBm with +42 dBm input power, linear gain of 8 dB, and drain current of 5 A. Toshiba's third transistor operates in the X-band from 11.3 to 11.5 GHz. The TGI1011- 50-771 boasts output power of 50 W with typical output power of +47 dBm and +41 dBm input power. The device generally provides linear gain of 9 dB with 5 A drain current. Interestingly, this product's targeted industrial applications include exciters for particle accelerators.
These impressive GaN developments have not eclipsed the feats of laterally diffused metal oxide semiconductor (LDMOS) technology. Freescale just entered the commercial- aerospace market with a 1-kW LDMOS field-effect transistor (FET) operating at L-band. The MRF6VP121- KH/HS covers 965 to 1215 MHz with 1 kW peak output power (100 W average). At 1030 MHz, this transistor provides 20 dB gain and 56-percent efficiency when operating with a 128-s pulse width and 10-percent duty cycle.
The firm's expanded offering also includes the MRF6V12500H/HS, which spans 965 to 1215 MHz with 500 W peak output power (50 W average). At 1030 MHz, it boasts 19 dB gain and 60-percent efficiency when operating with a 128-s pulse width at 10-percent duty cycle. The 50-VDC transistor was designed to operate at full rated peak power into a 10:1 voltage standing wave ratio (VSWR) load. Other members of this family include the MRF6V12250H/ HS spanning 965 to 1215 MHz with 275 W peak output power (27.5 W average). In addition, the MRF7S35120HS covers 3.1 to 3.5 GHz with 120 W peak output power (24 W average). These rugged devices are ideal for pulsed-signal applications, such as air traffic control, distance measuring equipment (DME), and weather radar.
To serve pulsed applications in the ultrahigh- frequency (UHF) and L-band radar markets, newcomer HVVi Semiconductors has turned to metaloxide- semiconductor field-effect-transistor (MOSFET) structures. The most powerful of these MOSFETs vows to deliver output power to 600 W. The HVV1011-600 device is a high-voltage-silicon, enhancement- mode RF transistor that operates from 1030 to 1090 MHz. With a 50-s pulse width at 2-percent duty cycle, the transistor typically delivers 645 W output power with 17.3 dB of gain. It exhibits 15 dB input return loss. The HVV1011-600 is extremely rugged, as it can withstand an output load mismatch corresponding to a 20:1 VSWR at rated output power over all phase angles and operating voltages across the frequency band of operation.
All of HVVi's devices leverage its High Voltage Vertical Field Effect Transistor (HVVFET) technology. The HVVFET is a discrete silicon RF power-transistor structure in which the device substrate is actually the drain of the transistor. As a result, the HVVFET depletes vertically into the substrate as voltage is applied to the drain. This novel device architecture approaches planar breakdown in the vertical drain region, thereby standing off the maximum voltage with minimum on resistance. According to the company, this structure provides performance characteristics that improve even as the device migrates to higher operating voltages.
MOSFETs also are a specialty of Microsemi Corp., which just introduced two devices operating to 175 and 100 MHz. Specifically, the VRF152 can operate to 175 MHz at 150 W with typical gain of 13 dB on a 50-VDC supply. This gold-metallized silicon, n-channel RF power transistor provides a 130-V breakdown voltage for improved ruggedness and 200-M on resistance for optimized efficiency. It offers typical gain of 22 dB at 30 MHz and 13 dB at 175 MHz. The transistor provides a VSWR of 30:1. The VRF152 is nitride passivated with gold metallization for improved reliability.
Its sibling, the ARF477FL, is a 500-V (BVdss) push-pull, matched-pair transistor product that operates to100 MHz in the Industrial, Scientific, and Medical (ISM) band. It is capable of delivering 400 W CW power to 80 MHz. In pulsed applications, the transistor will produce 1500 W peak power at 5 ms at a 25 percent duty cycle. At 65 MHz, it provides typical gain of 17 dB in Class AB at 300 W output operating from a 125-V supply. The ARF477FL is housed in the T3 "flangeless" package, which utilizes Microsemi's patent-pending lid for improved thermal contact to heatsinks.
AN ALTERNATIVE TO TWTS
Transistors like the previous examples form the basis of high-power amplifiers. Using GaN HEMTs, for example, Fujitsu Laboratories Ltd. just announced the development of a 101-W, X-band amplifier that achieves 53-percent efficiency. Compared to conventional amplifiers employing gallium-arsenide (GaAs) HEMTs, it is anticipated that this new GaN-HEMToperating based amplifier can achieve a range that is 2.6X as great at C band. As a result, it may serve as an alternative to traveling-wave-tube (TWT) amplifiers in high-output applications. The company developed this GaN HEMT amplifier for use in both the X and C bands. It is comprised of two transistor chips, which allows the inherent highoutput performance of GaN HEMTs to come through even at high frequencies. To accommodate those frequencies, the amplifier's gate length was reduced to 0.25 m and the gate-drain gap was optimized (Fig. 1).
For application to X-band, Fujitsu Laboratories further optimized the manifold I/O path structure that was designed for the C-band GaN HEMT that it developed last year (Fig. 2). That structure now eliminates phase discrepancies introduced to the input signal within the chip for X-band as well as C-band, enabling uniform performance for a GaN HEMT with highoutput- power density and efficiency. By suppressing thermal interference between the two chips, the performance degradation caused by chip heating was suppressed as well. At X-band, this resulted in efficiency of 53 percent and high output of 101 W. According to comparisons made by Fujitsu, the 101-W output that was generated was roughly 4X greater than output generated with GaAs HEMT amplifiers, allowing reach to be extended by up to 2X. At 343 W at C-band, Fujitsu asserts that this amplifier outperforms GaAs-based amplifiers by a factor of seven. Reach can therefore be extended by potentially a factor of 2.6X.
Although this innovation from Fujitsu Laboratories certainly has the potential to be a disruptive technology, TWT amplifiers are firmly entrenched in many current designs. An array of high-power TWT amplifiers is available from IFI. The firm provides impressive specifications for many of its models, which cover 10 kHz to 40.0 GHz. For instance, the PT-KW series amplifiers deliver 3 to 8 kW of pulsed power from 1 to 18 GHz with a duty cycle to 10 percent. The PT128-3, which spans 8.2 to 12.4 GHz, delivers at least 3.0 kW of output power with a minimum of 65 dB saturated gain and a noise figure to 35 dB.
AR Modular RF (www.arwwmodularrf.com) is known for providing rack-mounting and table-top amplifier systems and modules spanning 0.01 to 6000 MHz with output power to 5000 W. To help Softronics Ltd. support complex waveforms in base-station applications, it designed the model KAW4040M13 in both 100- and 500-W versions. This amplifier serves as a replacement for Softronics' legacy base-station amplifier, although more applications are certainly possible.
The 100-W model KAW4040M13, which operates from 225 to 400 MHz, was designed for use with modern modulation schemes. It includes a breakout port that allows the insertion of a tracking filter into the amplifier chain to enhance noise performance. Its sibling, the 500-W KAW4040M13, covers 225 to 400 MHz. This amplifier is a rack-mounted "drop-in replacement" for an existing but unsupported amplifier system. It offers an automatically leveled linear output power of 500 W with input power ranging from 10 to 100 W. Although radar usually comes to mind when one thinks of today's high-power designs, this example shows that such power is increasingly in demand throughout much of the microwave industry.