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How GaN improves the bandwidth and power of RF power amplifiers

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Author : AmpliVsionS
Update time : 2021-03-09 17:34:50

The telecommunications industry continues to require higher data rates, and industrial systems continue to require higher resolutions, which has boosted the continuous increase in the operating frequency of electronic devices that meet these needs. Many systems can work in a wider frequency spectrum, and new designs usually require further bandwidth increases. In many such systems, people tend to use a signal chain that covers all frequency bands. The advancement of semiconductor technology makes the function of high-power broadband amplifier advance by leaps and bounds. The GaN revolution has swept the entire industry and allowed MMICs to generate more than 1 W of power at dozens of bandwidths. Therefore, this field dominated by traveling wave tubes in the past has begun to give way to semiconductor equipment. The emergence of GaAs and GaN transistors with shorter gate lengths and the upgrading of circuit design technology have spawned some new devices that can easily operate at millimeter wave frequencies, opening up new applications that were unimaginable decades ago. We will briefly describe the state of the semiconductor technology that supports these developments and the circuit design considerations for achieving optimal performance in thearticle. It also lists GaAs and GaN broadband power amplifiers (PA) that demonstrate today's technology.
Many radio subsystems can cover a wide frequency range. In the military industry, radar frequency bands can cover frequencies ranging from several hundred MHz to GHz. Some electronic warfare and electronic countermeasures systems need to work under extremely wide bandwidths. A variety of different frequencies, such as MHz to 20 GHz, and even higher frequencies, are now facing challenges. As more and more electronic devices support higher frequencies, the demand for higher frequency electronic warfare systems will blow out. In the telecommunications industry, the operating frequency of base stations is around 450 MHz to 3.5 GHz, and it continues to increase as the demand for higher bandwidth grows. The operating frequency of the satellite communication system is mainly from the C-band to the Ka-band. The instruments and meters used to measure these different electronic devices need to be able to work at all these necessary frequencies in order to be internationally recognized. Therefore, system engineers need to work hard to design some electronic devices that can cover the entire frequency range. Thinking that a single signal chain can cover the entire frequency range, most system engineers and purchasers will be very excited. Covering the entire frequency range with a single signal chain will bring many advantages, including simplified design, faster time-to-market, and reduced device inventory to be managed. The challenge of the single-signal chain solution is always the performance degradation of the broadband solution relative to the narrowband solution. The core of the challenge lies in the power amplifier, which has first-class power and efficiency performance for the narrow bandwidth.
Semiconductor technology
In the past few years, traveling wave tube (TWT) amplifiers have been using higher power electronics as the output power amplifier stage in many of these systems. TWT has some good features, including kilowatt-level power, octave bandwidth or even multi-octave bandwidth operation, efficient fallback operation, and good temperature stability. TWT also has some drawbacks, including poor long-term reliability, low efficiency, and requires a very high voltage (about 1 kV or more) to work. Regarding the long-term stability of semiconductor ICs, electronic equipment has been moving forward over the years, and GaAs is the first to bear the brunt. When possible, many system engineers have been working hard to combine multiple GaAs ICs to generate large output power. The entire company is completely built on the basis of technology portfolio and effective implementation. And then gave birth to many different types of combination technology, such as space combination, business combination and so on. These combination technologies all face the same fate—combination causes losses. Fortunately, it is not necessary to use these combination technologies. This motivates us to start designing with high-power electronics. The easiest way to increase the RF power of a power amplifier is to increase the voltage, which makes GaN transistor technology very attractive. If we compare different semiconductor process technologies, we will find how power usually increases with high working voltage IC technology. Silicon Germanium (SiGe) technology uses a relatively low operating voltage (2 V to 3 V), but its integration advantages are very attractive. GaAs has a microwave frequency and a working voltage of 5 V to 7 V, and has been widely used in power amplifiers for many years. The silicon-based LDMOS technology has a working voltage of 28 V and has been used in the telecommunication field for many years, but it mainly plays a role in frequencies below 4 GHz, so it is not widely used in broadband applications. The emerging GaN technology operates from 28 V to 50 V, and has low-loss, high thermal conductivity substrates (such as silicon carbide, SiC), which opens up a series of new possible applications. Today, GaN on silicon technology is limited to operating frequencies below 6 GHz. The RF loss associated with the silicon substrate and its lower thermal conductivity relative to SiC offset the gain, efficiency, and power advantages that increase with frequency. Figure 1 compares different semiconductor technologies and shows how they compare to each other.

Figure 1. Process technology comparison of power electronics in the microwave frequency range.
The emergence of GaN technology has allowed the industry to abandon TWT amplifiers and use GaN amplifiers as the output stage of many systems. The driver amplifiers in these systems still mainly use GaAs, because this technology has been deployed in large numbers and is always improving. In the next step, we will look for how to use circuit design to extract greater power, bandwidth and efficiency from these broadband power amplifiers. Of course, compared to a GaAs-based design, a GaN-based design can provide higher output power, and its design considerations are largely the same.
Design considerations
When choosing how to start a design to optimize power, efficiency, and bandwidth, IC designers can use different topologies and design considerations. The most common type of monolithic amplifier design is a multi-stage, common-source, transistor-based design, also known as a cascaded amplifier design. Here, the gain amplifier will increase from each stage to achieve high gain and allow us to increase the output transistor size to increase the RF power. GaN offers some advantages here, because we can greatly simplify the output synthesizer and reduce losses, thereby improving efficiency and reducing chip size, as shown in Figure 2. Therefore, we can achieve wider bandwidth and improve performance. A less obvious advantage of shifting from GaAs to GaN devices is the ability to achieve a given RF power level, which may be 4 W. Transistor size will be smaller, thereby achieving higher gain per stage. This will result in fewer design levels and ultimately higher efficiency. The challenge of these cascaded amplifier technologies is that it is difficult to achieve octave bandwidth without significantly reducing power and efficiency, or even without the help of GaN technology.

Figure 2. Comparison of multistage GaAs power amplifier and equivalent GaN power amplifier.
Lange coupler
One way to achieve a wide bandwidth design is to use Lange couplers at the RF input and output ends to achieve a balanced design, as shown in Figure 3. The return loss here ultimately depends on the coupler design, because it will be easier to optimize the gain and frequency power response, and there is no need to optimize the return loss. Even in the case of using Lange couplers, it is more difficult to achieve octave bandwidth, but it allows the design to achieve good return loss.

Figure 3. Equalization amplifier with Lange coupler.
Distributed amplifier
Another topology to consider is the distributed power amplifier, as shown in Figure 4. The advantages of distributed power amplifiers can be realized by applying the parasitic effects of transistors in the matching network between devices. The input and output capacitance of the device can be combined with the gate and drain line inductance, respectively, to make the transmission line almost transparent, except for transmission line losses. In this way, the gain of the amplifier should only be limited by the transconductance of the device, not the parasitic capacitance of the device. This happens only when the signal traveling down the gate line is in phase with the signal traveling down the drain line. Therefore, the output voltage of each transistor will be in phase with the output of the previous transistor. The signal transmitted to the output will actively interfere, so the signal will increase with the drain line. Any reverse wave will arbitrarily interfere with the signal because these signals will not be in phase. This includes the gate line terminal electrode, which can absorb any signal that is not coupled to the gate of the transistor. It also includes the drain line terminal electrode, which can absorb any reverse traveling waves that may wantonly interfere with the output signal and improve the return loss at low frequencies. Therefore, frequencies from kHz to GHz can be achieved in dozens of bandwidths. When multiple octave bandwidths are required, this topology becomes very popular, and it also brings several good advantages, such as smooth gain, good return loss, high power, etc. Figure 4 shows an example of a distributed amplifier.

Figure 4. Simplified block diagram of a distributed amplifier.
Here, a challenge that the distributed amplifier faces is that the power function is determined by the voltage used by the device. Since there is no narrow-band adjustment function, you can essentially provide a resistance of 50 Ω or close to 50 Ω to the transistor. In Equation 1, the average power, RL, or optimal load resistance of the PA will essentially become 50 Ω. Therefore, the achievable output power is set by the voltage applied to the amplifier, so if we want to increase the output power, we need to increase the voltage applied to the amplifier.

This is the role of GaN. We can quickly convert the 5 V power supply voltage with GaAs into the 28 V power supply voltage in GaN, and only need to convert GaAs to GaN technology to convert the achievable power from 0.25 W to Around 8 W. There are also other factors to consider, such as the gate length of the available processes in GaN, and whether they can achieve the desired gain at the high frequency band end. Over time, more GaN processes will appear.
The cascade amplifier needs to optimize the amplifier power through a matching network to change the resistance of the transistor. In contrast, the 50 Ω fixed RL of the distributed amplifier is different. There is an advantage when using a cascade amplifier to optimize the resistance of a transistor, that is, it can increase the RF power. In theory, we can continue to increase the size of the transistor peripherals, thereby continuing to increase the RF power, but this has some practical limitations, such as complexity, chip support, and consolidation losses. Matching networks also limit bandwidth because they are difficult to provide the best impedance in a wide frequency range. There are only transmission lines in the distributed power amplifier, and its purpose is to make the signal actively interfere with the amplifier, and there is no matching network. There are some technologies that can further increase the power of distributed amplifiers, such as the use of a cascode amplifier topology to further increase the amplifier's power supply voltage.
In conclusion
Regarding the trade-offs of providing the best power, efficiency, and bandwidth, we have explained various techniques and semiconductor technologies. Each of the different topologies and technologies may occupy a place in the semiconductor market. This is because each of them has advantages, which is why they can survive at present. Here, we will focus on several trustworthy results to show the possibilities of these current technologies in achieving high power, efficiency and bandwidth.
The emergence of new semiconductor materials such as GaN has opened up the possibility of achieving higher power levels covering a wide bandwidth. The frequency range of shorter gate length GaAs devices has been extended from 20 GHz to 40 GHz and above. The reliability of these devices has almost exceeded 1 million hours, and they are commonly used in today's electronic equipment systems. In the future, we expect to continue to develop towards higher frequencies and wider bandwidths.

AmpliVisionS engineers have developed a proprietary open architecture/common platform GaN-based power amplifier family, which relies on advanced power commbining design and unique MMIC device selection. Leveraging ”building block” combinations of these documented solutions and the extensive experience of our R&D team, we react swiftly to new requirements and to offer a variety of cost-effective, value-added solutions.