A Study of GaN HEMTs Current Collapse Impacts on Doherty Multistage PA Linearity

The effect of current collapse with regard to operating class of GaN HEMTs is studied. It is shown that AM-AM deviation due to current collapse is maximized in class AB, while it is reduced under both class A and class C region. A Doherty multi-stage amplifier is designed considering these effects to achieve both high efficiency and linearity. A 2W-class 1.8GHz multistage GaN Doherty PA shows PA chain efficiency of 38% with the power gain of 50.5dB. ACLR of -51dBc is obtained for 20MHz LTE carrier using a commercially available RF predistorter IC.