10. Metamodels for the stability analysis of RF amplifiers

RF amplifiers form the foundation of every modern wireless communication system.They provide the much needed amplification for the RF signals that we would like to transmit and receive. Unfortunately, due to their inherent nonlinear nature, these amplifiers have the tendency to become unstable and consequently produce unwanted oscillations. This unstable behaviour should be avoided at all cost.

One method to assess the unstable behaviour of these RF amplifiers involves the so-called Harmonic Transfer Functions (HTFs) [1]. These HTFs model the amplifier as a periodically time-varying system and can take into account the up- and down-conversion of your RF signal. Unfortunately, predicting the influence of system parameters on the HTFs is not possible with state-of-the-art modeling techniques. However, for design purposes, it is crucial to derive this influence to be able to propose the optimal solution that satisfies the specifications.

For example, consider the low-noise RF amplifier in the figure below. As one can see, this amplifier consists of many different components (resistors, capacitors, coils, etc.). How to choose the component values such that we have a stable amplifier? Or in other words, what would happen with the behaviour of the system if one wants to change these component values? One way to deduce the influence of these components is to perform a high number of different simulations and extract for each one of these simulations the HTFs for stability assessment purposes. This is a very time-consuming task and can even become impossible whenever computationally expensive simulations are involved. An effective way to mitigate the high number of simulations is to build a metamodel of your system.

Low noise amplifier

Figure copied from Infineon Application Note No. 51 - Sieget 45: Low Noise Amplifier with BFP520 at 1.9 GHz.

The goal of this thesis is to build accurate metamodels to speed up the stability analysis of RF amplifiers. During this thesis, you will

  •  study how parameter variations can be included within the modeling of the HTFs,
  •  build accurate metamodels that can be time-efficiently constructed for high-dimensional parameter spaces,
  •  apply your developed techniques for the stability analysis of RF amplifiers.

Interested or curious to know more? Feel free to contact dries.peumans [at] vub.be!


[1] Mori, Libe, Ibone Lizarraga, Aitziber Anakabe, Juan-Mari Collantes, Vincent Armengaud, and Geoffroy Soubercaze-Pun. "Efficient Calculation of Stabilization Parameters in RF Power Amplifiers." IEEE Transactions on Microwave Theory and Techniques (2020).

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