Creating Highly Efficient and Reliable mmWave Power Amplifiers

Optimizing High-Frequency RF Power Amplifiers

mmTron, a Silicon Catalyst portfolio company based in Redwood City, California, specializes in designing high power, high linearity, yet efficient and reliable millimeter wave (mmWave) integrated circuits for advanced wireless and satellite communication systems. mmTron creates RF components in high-speed III-V processes, including GaAs, GaN, InP, SiGe, and RFSOI.

Dr. Seyed Tabatabaei founded mmTron in 2020 from his experience in RF test and measurement equipment circuit designs at Hewlett Packard, the birthplace of Keysight technology, and more recently, time at two other mmWave component design startups. mmTron’s seven engineers use Keysight PathWave Advanced Design System (ADS) in a workflow with tools from Altium, Ansys, Cadence, and others.

The TMC211 is mmTron’s latest 27-31 GHz power amplifier (PA). Its single-die GaN design delivers 50W power with 28% power-added efficiency (PAE), strong linearity, and a lowered operating junction temperature, which contributes to a higher mean time between failures (MTBF). Multi-domain electromagnetic (EM) and electro-thermal simulation capability in ADS and PathWave RFPro drives similar outcomes across mmTron’s products.

Challenge: Deliver more power without compromises

Much of the history of engineering has been a collection of practical tradeoffs during a project. Designers choose a set of parameters critical to their application and incorporate those in a design using physically implementable components and techniques to the best of their abilities. Attention then turns to sub-optimizing, maximizing a few desirable parameters while keeping others within acceptable limits. Classical hand calculations or single-domain simulation perform sub-optimization well.

Most manufacturers optimize RF PAs for the obvious parameter: maximizing the power a PA can deliver, with a secondary emphasis on linear performance across the operating bandwidth. More output power means more input power from the DC supply. Adding in power losses due to inefficiency and dissipated power in the PA translates to elevated junction temperature, lower reliability, and a shortened lifespan.

With more power amplifiers heading into long life cycle wireless communication applications – like defense and SATCOM – manufacturers have turned to multi-die PA solutions to spread power density across the package. Interconnect losses between dies cut power efficiency, linearity can be harder to control, and mmWave frequencies make effects more pronounced. Package size can also be a concern in aggressive SWaP-C (size, weight, power, and cost) environments.

Tabatabaei, shown in the lab in Figure 1, and his engineers asked: what would mmTron need to do to deliver more power with a single-die PA while still achieving linearity and SATCOM levels of reliability?

III-V processes like GaN provide the power density customers want. Arriving at a fully optimized design, unlocking the power potential while maintaining excellent linearity on a single die at a lower operating temperature, would require much more sophisticated behavioral models and simulation to account for cross-domain effects. “We knew going into the TMC211 design that thick metalization and close coupling between PA stages would create resonances making accurate EM simulation difficult,” says Tabatabaei. A coordinated electro-thermal simulation would also be crucial for optimizing the design.

Solution: Authentic waveforms and enhanced models in multi-domain simulations

Pushing the performance of RF components and system design calls for a suite of EDA tools for various tasks. Tabatabaei was deeply familiar with ADS, but some of mmTron’s engineers were less familiar with the Keysight RF EDA environment. mmTron adopted Keysight PathWave ADS as the linchpin of its workflow, and its teams now see the advantages. “ADS enormously speeds up our product development because we can copy templates and set-ups quickly and start a new design,” says Tabatabaei.

Templates are essential, but many EDA vendors offer templates. Another reason brought Tabatabaei to select ADS. “I definitely place a high value on Keysight’s people,” Tabatabaei continues. “The knowledge and skills they bring around ADS help us catch more issues earlier and work through situations like scripting, integration, and more that are extremely valuable in our mmWave RF component design efforts.”

The next big decision for mmTron was a commitment to using authentic waveforms in ADS simulations. The alternative uses sinusoidal stimulation at several points across the bandwidth, then designing in excess margin, but the outcome is far less certain to predict as conditions change. “There are many PA subtleties that only appear under modulated signals,” states Tabatabaei. “How the signal phase changes between stages, memory effects, interactions with capacitor values and placements, and power consumption all depend on modulation.”

Customers are approaching mmTron with questions such as how crest factor and peak-to-average power ratio (PAPR) look in specific modulation scenarios. “Our core competence is using design and layout steps for optimizing the PA structure to provide the best performance in a modulated environment, and we have a lot of scripts written to help do that,” says Tabatabaei. For example, Keysight assisted mmTron in developing ADS scripts to help keep PA power delivery constant as modulation varies.

Figure 2 is an example of the mmWave die structures mmTron creates using ADS, showing the thick metalization between some of the circuit elements.