Class-D RF Amplifier with Amplitude Modulation for 17 MHz NMR MOUSE Sensor

Class D amplifiers are known for their excellent power efficiency1,2,3,4 and are now used extensively for audio applications5,6,7. Due to the switching speed limitations of power MOSFET devices, Class D amplifiers typically operate under 1MHz. With the recent availability of some new high speed and high power MOSFET devices in the market, it is now possible to develop Class D RF amplifiers that operate up to 20MHz thus making them viable for mobile low frequency NMR applications.
Higher frequency Class D amplifiers are normally operated in a resonant mode with a constant amplitude output. Here we demonstrate a Class D amplifier operating at 17MHz and working with an NMR MOUSE Sensor8 . With some intelligent switching control, amplitude modulation is also possible. A prototype has been built on a 4-layer PCB with an area of 40cm 2, output power is 10W as required by the MOUSE sensor. Test results show the class D amplifier is operating well at 17MHz with fast turn on and turn off times, producing constant amplitude pulses as short as 2µs, and is able to run long CPMG experiments with tens of thousands of echoes