As described in the previous article in this series, the power demands of sub-6GHz 5G base stations are driving a shift from LDMOS amplifiers to GaN-based solutions. The high power density, efficiency and wider frequency support make it a compelling solution for many RF applications. As any embedded systems designer will tell you, every material comes with a tradeoff. Harnessing the full benefits of GaN RF power amplifiers often require small shifts in approach, with the results being well worth the effort.
Before exploring design best practices, it’s worthwhile to address the common misperceptions around GaN.
GaN is perceived to be cost prohibitive by many in the engineering community. From a narrow perspective that’s accurate; GaN is more expensive today to produce than a pure silicon or LDMOS solution. However, that ignores the performance gains that can offset additional system costs. Price-to-performance ratio is the key figure to evaluate. Depending on the need, GaN may bring the overall costs of the entire system down as you’re able to meet power needs in a smaller package. The smaller packages not only reduce board sizes and costs, but also the heatsinks where large savings can be found. Multiband and wideband GaN amplifiers can replace multiple separate narrowband amplifiers in systems that can further reduce total system costs. That’s not to say it’s the perfect fit for every application, but viewed through the lens of performance per dollar, GaN often translates to savings. Total cost of ownership is where GaN can show it’s technology benefits.
In addition, GaN production volumes have grown dramatically. This is very clear in the massive MIMO space with the increasing number of PAs used in a given base station system. As GaN gains market share in these various sub-markets – 5G base stations being one of the larger ones – suppliers are able to scale up bulk production driving supply chain costs down to very competitive levels. The means that GaN delivers better performance for cheaper, garnering wider adoption and additional savings from scale. The price of GaN will only get more competitive going forward.
Not All GaN Behaves The Same
There is a misconception that all GaN power amplifiers are similar enough as to be commoditized. It’s an easy assumption to make coming from engineers that have relied on LDMOS solutions. If you look at the characteristics of LDMOS devices from different vendors at a semiconductor level, they’re incredibly similar. That isn’t the case in the GaN space. Each vendor approaches development differently to solve for GaN production challenges, and that translates to different strengths and weaknesses. Consequently, GaN from each vendor behaves differently, and vendors often have varied solutions to accommodate for their unique PAs. Embedded designers should not assume that an experience they’ve had with GaN in the past will hold true across all vendors. Coordinate closely with your supplier to make sure you get the most out of each unique GaN PA.
Embedded designers see high gate current on a data sheet for a GaN PA and have concerns. They assume that high gate currents cause device failures. The truth is that high gate doesn’t always mean it’s a reliability issue. Reliability is very technology dependent, and it comes back to what was discussed before – not all GaN behaves the same. With simple bias circuits adjustments to accommodate higher currents, both system power efficiency and density improve substantially.
Design Solutions to Maximize GaN Performance
As discussed in previous articles, GaN delivers increased power density, efficiency and frequency flexibility. To harness the semiconductor’s full potential however, embedded designers should play to the materials strengths. Here are some systems level design practices to consider.
Designing for Linearization
The biggest concern for most embedded designers before moving forward with GaN is linearization. There is a perception that GaN is difficult to linearize. There are situations where that is the case, but there are also manageable ways to address linearized deficiency that mitigate nonlinear and trapping effects. It can be done with system design approaches that put the device in the ideal application space, or software algorithms that control for IQ drift and track trapping effects. The vendor ecosystem has grown to address these exact challenges.
There’s work that needs to be done, but the payoff is significantly better power efficiency. It’s a tradeoff that needs to be considered. Depending on the need, some designers will make that trade and others will fall back to a traditional LDMOS solution.
While opportunities for GaN linearity improvement still exist, the low drain-source capacitance of GaN transistors can offer better responses to wide and ultrawide instantaneous bandwidth signals, leading to better overall system linearization for those signals. Video bandwidth is also an area where GaN can outperform competing technologies.
It’s also worth noting that linear efficiency is the communication industry’s primary R&D focus. Thanks to both digital processing and device level improvements, analysts expect GaN linearization to dramatically improve over the next three to five years. Don’t be surprised when future generations of GaN deliver market-leading linearity.
Thermal Dissipation Awareness
GaN power amplifiers operate at temperatures that silicon-based technologies can’t reach, simplifying heat-sink and cooling requirements within a system. However higher heat densities can create challenges if embedded designers aren’t careful, especially if using fewer GaN PAs has reduced the size of a system’s form factor. A smaller system will put more heat pressure on unexpected components.
Engineering teams tend to focus on the junction temperature. Since GaN PAs support such high junction temperature, other parts of the system become the limiting factor. Solder joints are an often-overlooked example. System designs need to account for this. Engineers are best served by reevaluating internal reliability requirements with the knowledge that GaN PAs can work at higher temperatures and take full advantage of that during the design.
Take Advantage of Supplier Expertise
It’s not surprising that vendors know the ideal applications of their own product, but customers can be shocked by application engineers’ knowledge of embedded systems outside RF. To be as effective as possible, a GaN PA needs to work in concert with the rest of the device. That requires whole product optimization around elements like carrier and peak voltages. This is fairly standard across PA technologies, but it’s important to note that this same type of trade space exists for GaN applications.
Still, some customers don’t take full advantage of a vendor’s team of application engineers. It’s always worth consulting your GaN supplier on how to best implement a solution. They’ll know all the tricks to get the most performance out of a PA safely. One quick phone call and they’ll be in the lab next to you working to get a design challenge resolved.
In the next article in this series, we’ll examine some of potential GaN innovations that could transform base station applications in the near future.
|Roger Hall is the General Manager of High Performance Solutions at Qorvo, Inc., and leads program management and applications engineering for Wireless Infrastructure, Defense and Aerospace, and Power Management markets.|
- 5G and GaN: Understanding sub-6Ghz Massive MIMO infrastructure
- 5G and GaN: The shift from LDMOS to GaN
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- 5G’s biggest challenges for communications service providers
- Preparing to migrate cellular IoT to low-power LTE
- 10 key trends in wireless technology
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