High-Power, High-Efficiency, GaN HEMT Power Amplifiers for 4G Applications

Today’s wireless system requirements demand increasing performance from power amplifiers. The higher gain and output power available from today’s transistors reduce the number of amplifier stages, and improved efficiency decreases system DC power requirements and generated heat. But at these higher power and efficiency levels, power amplifier linearity needs to meet or exceed the requirements of past systems. Gallium nitride (GaN) HEMT based power transistors offer an ideal technology solution for these amplifiers due to their exceptionally high operating power density. Cree’s GaN HEMT devices deliver RF power densities as high as 8 W/mm [1] of gate periphery due to superior thermal properties provided by the silicon carbide substrates on which they are fabricated. This is of considerable advantage when compared with silicon substrates. This article details three amplifier designs that cover a variety of 4G frequency bands. The first two designs are 2 GHz Doherty amplifiers with capable peak output power of 480 W employing the new CGH21240F or CGH25240F pre-matched GaN HEMT transistors. These transistors offer twice the peak power of previously released transistors, CGH21120F and CGH25120F, from Cree [2]. Doherty amplifiers offer a unique solution for linear amplifiers since the efficiency is maintained at powers backed off from saturation. The third design is a 900 MHz 120 W saturated power design for lower frequency operation using the new CGH09120F broadband (unmatched) transistor. Linearity of all three designs is maintained through the use of digital pre-distortion (DPD).