In this work, the design concept of a highly efficient and ultra-broadband GaN power amplifier is presented. The amplifier uses a novel concept for realizing the impedance matching networks with multi octave bandwidth. These matching networks are based on planar, broadside coupled micro-strip lines that are interconnected to a 4:1 Guanella impedance transformer. To achieve an even mode impedance as high as possible, the ground metallization underneath the coupled lines is removed.
A high even mode impedance is the key for achieving wide bandwidth. The designed amplifier, deploying a commercial 25 W GaN HEMT from CREE™ achieved a CW output power of more than 42 dBm over a frequency range from 0.4 to 2.8 GHz with a minimum PAE of 45% over the whole bandwidth and peak PAE-values of more than 65%.