Large-Signal Characterization of Coupled RF Amplifiers for Parallel Transmit

Target audience: This work is relevant to those interested in parallel transmit amplifiers. Purpose: Previous work has demonstrated the use of high efficiency switch mode power amplifier (SMPA) topologies for miniature on-coil amplifiers aimed at use in parallel transmit coil arrays. Though such amplifiers demonstrate very high efficiency and power density in isolation, there has not yet been any detailed quantification of how these amplifiers behave when the RF coils are coupled to each other, primarily due to their relatively complex behavior. Linear power amplifiers (LPA) are normally assumed to be time invariant, homogeneous, and additive, making them simpler to model using linearized parameters such as transconductance, complex impedance, and scattering parameters. SMPA topologies, such as the Current Mode Class D (CMCD) topology shown in fig 1, are different in that they are inherently periodically-timevarying systems, and they show strong nonlinear behavior. To address these issues, we have developed a method for experimentally quantifying the effective open loop output impedance and power efficiency of coupled power amplifier topologies across their full control space, and we demonstrate its application to the CMCD topology. Methods: CMCD coupling model: The coupling phenomenon observed in coil arrays is due to magnetic and electric fields from one coil element inducing an electromotive force ε in neighboring coils. This work focuses solely on the role of magnetic flux coupling, described by km between two coils with inductances L1 and L2. The ε21 inducted in coil L1 by current IL2 in coil L2 is given by 21 L2 m 1 2 ε =jωI k L L . This ε21 will alter the current in coil L1 according to