Brush up on the theory before designing a high power Class‐E amplifier: Page 9 of 15

June 29, 2016 // By Alfio Scuto, Roberto Cammarata, ST Microelectronics
The most promising switched-mode PA for RF applications, among the several types available, is the Class-E for several reasons: high efficiency, simplicity of the load network, and a satisfying performance even with a non-optimal drive signal.

At switch-ON, the Class-E conditions were met with R_load_factor = 0.03 and RDS(on) = 82 mΩ. In Figure 7 and Figure 8 are shown voltage and current with the value of the components optimized.

Figure 7: Current waveform of the MOSFET.

Figure 8: Drain voltage of the MOSFET.

For the project, we selected the SD4933 from STMicroelectronics. The device is a 50-V N-channel MOS field-effect RF power transistor specially designed for ISM applications up to 100 MHz (7). Since the SD4933 has RDS(on) = 170 mΩ at ID = 20 A, using two devices in parallel gave us RDS(on) = 85 mΩ, which is similar to the value obtained before.

In simulation, we found a drain peak voltage of 173 V; this value is compatible with the breakdown voltage reported in the datasheet of SD4933.

In order to get an amplifier with high efficiency we estimated the losses of each component. To build the amplifier, we used ATC 100B capacitors with high Q. To minimize the total losses, we used multiple capacitors in parallel (C1 and C2), and for them we estimated an equivalent ESR = 5 mΩ. The wire wound inductors used in the amplifier were handmade; we did preliminary trials to obtain an ESR = 100 mΩ for each of them.

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