COMPARED TO THEIR gallium-arsenide (GaAs) counterparts, gallium-nitride transistors can offer significant performance improvements. GaN HEMT transistors in particular are suitable for Class F amplifiers, as they have a high fmax compared to the fundamental operating frequency. This aspect helps in the generation of higher-order harmonics needed for waveshaping. At the University of California, Santa Barbara, David Schmelzer and Stephen I. Long successfully designed, fabricated, and tested a Class F amplifier using a GaN HEMT transistor and hybrid printed-circuitboard (PCB) packaging. The amplifier flaunts a peak power-added efficiency (PAE) of 85 percent with output power of 16.5 W at 2 GHz.
This amplifier uses GaN HEMT devices on a silicon-carbide (SiC) substrate from Cree, which has a 3.6-mm gate periphery and an estimated fmax of 40 GHz. To compare the effect of the transistor parameter of different technologies, such as Ron and Vbr, a value for Vmin has to be estimated or calculated. Because it is hard to predict the actual Vmin, one should assume an ideal transistor with a fixed Ron and ideal drain waveforms. A simplified calculation is provided that will give drain efficiency trends for the values of Pout, Ron, VDD, and RL. Comparisons can then be made between different technologies, operating conditions, and ideal Class F and Inverse F amplifiers.
If the operational frequency is considered low, the researchers assert that increasing the gate periphery to reduce Ron while increasing RL will result in an efficiency improvement. Yet that improvement may come at the expense of gain and bandwidth. Ideally, a Class F amplifier should have higher drain efficiency than an Inverse F when their peak voltages are equal. See "A GaN HEMT Class F Amplifier at 2 GHz With >80% PAE," IEEE Journal of Solid-State Circuits, Oct. 2007, p. 2130.
