No Rest for the Weary RF Power Amplifier Designer

Designing high-performance radio frequency (RF) power amplifiers has never been easy, but it’s arguably never been more difficult because the performance of the entire transmission path depends on how well you juggle more conflicting requirements than ever before.

For example, it would be terrific if the amplifier could operate in its linear region where distortion products are minimal. But as less RF output power can be delivered in this region, more gain stages will be needed to achieve the required gain, which adds complexity, cost, size and weight. And if it is operated near or even beyond its saturation point to maximize conversion efficiency and generate as much power as possible, the inevitable result is AM/AM and AM/PM distortion that must be compensated by using digital predistortion or other techniques.

What’s more, higher-order modulation schemes such as 64/128/256 QAM employed in satellite communications are extremely sensitive to non-linear behavior. Distortion will “move” the states around the constellation, making it difficult to determine the signal’s state. The inner states have less power, so they may not be distorted, but the outer ones will likely drive the amplifier into compression, resulting in distortion. Ideally, a modulation scheme for satellite transmission would have higher spectral efficiency than QPSK but be more resistant to distortion than QAM. APSK (Amplitude Phase Shift Keying) is increasingly used because it has the best of both and lends itself well to pre-distortion.

Another consideration is peak-to-average power ratio (PAPR), the ratio of the highest power the amplifier will produce to its average power. PAPR important because the amount of data that can be sent is proportional to the average power, but the size of the amplifier needed for a given format depends on the peak power. There are other conflicting challenges as well, but these are more than enough to keep you up at night, staring at the ceiling for a revelation.

When Microchip introduced its first GaN MMIC power amplifier, the GMICP2731-10, all these issues were considered because it’s designed to serve both space and ground-based commercial and defense satellite communications, 5G networks, and other aerospace and defense systems, in which these challenges prevail.

Microchip chose Ka-band for its first GaN MMIC offering because the spectrum between 27.5 to 31 GHz is where satellites increasingly operate, and the commercial space market is experiencing its most dramatic changes in decades. When NASA opened the market to private companies, it created massive opportunities for space-launch companies as well as satellite operators that are now populating low Earth orbit with thousands of spacecraft to deliver a wide range of services, from delivery of broadband Internet access to navigation, maritime surveillance, and remote sensing. Ka-band’s 3.5 GHz of available bandwidth is more than four times that of lower frequency allocations and is sorely needed to support the massive amount of traffic being generated by video and other data-intensive applications.

GaN offers tremendous advantages over traveling-wave tubes (TWTs) that are the traditional power sources at these frequencies but have lower efficiency and require extremely high operating voltages. And gallium nitride is also inherently radiation-tolerant, which is important when the satellite is far from Earth in geosynchronous orbit.

GaN-based amplifiers are much smaller than TWT-based amplifiers, which makes them better suited for use with active phased array antennas without the need for complex and cumbersome power combiners. When compared to gallium arsenide (GaAs) they deliver more RF power in a smaller footprint and operate at higher voltages, making them more efficient, although GaAs devices remain the choice for low-power driver stages.

Microchip’s GMICP2731-10 produces up to 10 W of saturated RF output power with power-added efficiency of 22% and 22 dB of small-signal gain and 15 dB of return loss. It complements the company’s existing portfolio of GaAs MMIC RF power amplifiers, switches, low-noise amplifiers, and Wi-Fi front-end modules, as well as a GaN-on-SiC High Electron Mobility Transistor (HEMT) driver and final amplifier transistors for radar systems. You can get more information about the GMICP2731-10 here, along with design information and other resources.

To summarize, when President John F. Kennedy promoted space as the new frontier in the 1960s, it proved to be more than a slogan during the Space Race, and over a half-century later it continues to create technical challenges throughout the world, RF amplifier designers prominent among them.