Amplifier designers will often refer to their experience with and knowledge of a particular transistor device when they acknowledge success in their work. Achieving any kind of success with a linear or nonlinear computer-aided-engineering (CAE) program requires not only accurate, but comprehensive device models (that is to say, models that account for as much of the transistor's behavior as possible, under a wide range of operating conditions). Fortunately, a seven-page application note from Maury Microwave Corp., "Compact Transistor Models: The Roadmap to First-Pass Amplifier Design Success," details the steps needed to construct the best possible compact-type transistor models. In the process, it helps to improve the chances of achieving success when designing an RF/microwave amplifier.
The application note, which is based on a technical article originally appearing in the March 2012 issue of Microwave Journal magazine, compares the different types of transistor models: physical, compact, and behavioral. The first model type is based on the physics of a device technology. It can require extremely complex equations related to the simulation. Yet it also can provide the best results over a wide range of operating conditions. Compact transistor models are based on current-voltage (I-V) and scattering-parameter (S-parameter) measurements of a device of interest. These models are focused less on the device itself and more on the device and its interactions with the surrounding circuits.
Compact transistor models can be extracted for pulsed I-V and S-parameter data and contain a reduced set of parameters compared to physical device models. They can also take into account complex phenomena, such as electro-thermal and trapping effects. These types of models have also been found to accurately predict a device's responses to complex modulated signalsfor example, in terms of error-vector-modulation (EVM) performance.
The last type of transistor modelbehavioralis less flexible than the other types. It is based on frequency-domain measurements of the device. Behavioral models are often considered as "black box" -type models, where only the behavior of the modeled component is known in response to carefully controlled stimuli. As a result, predictions made for that model are only valid under precisely known operating conditions.
The note explores the approaches for constructing linear and nonlinear models for high-power transistors, including gallium-nitride (GaN) power devices, using pulsed I-V and RF measurements. It offers block diagrams for an automated, microwave, vector-network-analyzer load-pull measurement system, which is capable of making continuous-wave (CW) or pulsed measurements. Those measurements are useful for validating the accuracy of a compact transistor model (for impedances other than 50 O) by varying the impedances of the input and output tuners connected to a device under test (DUT). The application note is available for free download at http://www.maurymw.com/pdf/datasheets/5A-052.pdf.