RF Transmitter: Class E Transmit Array

RF Transmitter · Electronics

An array of switching amplifiers placed close to the coil, rather than one large linear amplifier placed far from it.

Schematic of a class E amplifier stage feeding a transmit coil element — A class E stage for every element. The switch closes at zero voltage, so the amplifier is efficient enough to sit next to its coil element.

Class E radiofrequency amplifier circuit board — A class E amplifier board built in our laboratory.

The question

Conventional transmit chains use a large linear power amplifier in the equipment room, feeding the coil through a long cable. That is wasteful and it fixes the drive of every coil element to the same waveform. A switching amplifier, operating in class E, is efficient enough to sit next to the element it drives, which makes per-element control of amplitude and phase an electronics problem rather than a cabling problem.

What we work on

Class E amplifier analysis and design for the operating frequency and the load the coil presents.
Impedance transformation between the amplifier and the coil element, which is where much of the efficiency is won or lost.
Array operation, where each element has its own amplifier and the elements interact.