GaN Breaks Barriers—RF Power Amplifiers Go Wide and High
The increasing demand for higher data rates and higher resolution is pushing the frequency of operation. This article will briefly describe the state of the semiconductor technology.
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Many wireless electronic systems operate over wide frequency ranges. In telecommunications, base stations operate from 450 MHz to ~3.5 GHz and continue to increase as the need for more bandwidth continues. Satellite communications systems operates from mainly C-band to Ka-band.
Instrumentation used to measure these different electronics needs to work over all required frequencies to be universally accepted. As a result, the systems engineer faces challenges trying to design electronics to cover the entire frequency range. Given the possibility of having one signal chain cover the entire frequency range, most systems engineers and procurement folks would be very excited. There are many advantages to having one signal chain cover the entire frequency range, including simpler design, faster time to market, less component inventory to manage, and more.
The challenge with the one signal chain approach is always related to the performance degradation that comes with a wideband solution vs. a narrow-band solution. At the heart of this challenge is the power amplifier, which commonly has superior performance in terms of power and efficiency when tuned over a narrow bandwidth.
Advantages in Semiconductor Technology
In years past, traveling wave tube (TWT) amplifiers have dominated higher power electronics as the output power amplifier stage in many of these systems. There are some nice attributes to TWTs, including capability of kWs of power, operation over octaves or even multiple octaves of bandwidth, high efficiency in back off condition, and good stability over temperature. There are some drawbacks to TWTs that include poor long-term reliability, lower efficiency, and the need for very high voltage to operate (~1 kV or higher). Given the long-term reliability of semiconductor ICs, there has been a push toward these electronics for many years, starting with GaAs.
When possible, many systems engineers have worked to combine multiple GaAs ICs to generate large output power. Entire companies have been created based entirely on combining technology and doing it efficiently. There are many different types of combining technologies, such as spatial combining, corporate combining, etc. These combining techniques all suffer from the same fate—combining has loss and, ideally, you would not have to use these combining techniques. This motivates us to use high power electronics to start the design.
The easiest way to increase the RF power from a power amplifier is to increase the voltage, which has made gallium nitride transistor technologies so attractive. If we compare the various semiconductor process technologies, we can see how the power generally increases with high operating voltage IC technology. Silicon germanium (SiGe) technology uses a relatively low operating voltage of 2 V to 3 V, but is very attractive for its integration benefits.
GaAs and GaN technology
GaAs has been used widely for power amplifiers for many years in microwave frequencies and has operating voltages of 5 V to 7 V. Silicon LDMOS technology operating at 28 V has been used for many years in telecommunications, but it is mainly useful below 4 GHz, so it’s not as widely used in broadband applications.
The emergence of GaN technology operating at 28 V to 50 V on a low loss, high thermal conductivity substrate like silicon carbide (SiC) has opened up a range of new possibilities. Today, GaN on silicon technology is limited to operation below 6 GHz. The RF losses associated with the silicon substrate and its lower thermal conductivity compared to SiC compromises the gain, efficiency, and power as the frequency increases. Figure 1 (web only) shows a comparison of various semiconductor technologies and how they compare to each other.
The emergence of GaN technology has created an industry shift away from TWT amplifiers and a move toward GaN amplifiers as the output stage of many of these systems. The driver amplifier in many of these systems is still commonly GaAs, as much of this technology already exists and continues to be improved. Next we will look at how to use circuit design to extract as much power, bandwidth, and efficiency out of these wideband power amplifiers. Certainly GaN-based designs are capable of higher output powers than GaAs-based designs, and the design considerations are largely the same.