Stability investigations of GaN truly-differential feedback PAs

Differential power amplifiers are well suited to boost the spurious-free dynamic range of an amplifier output signal by their inherent even-order harmonic cancellation. This paper examines for the first time the stability of truly-differential (TD) amplifiers in GaN technology and derives a theoretical relationship between common-mode stability and intrinsic HEMT parameters. Since the intrinsic feedback capacitance CGD is observed to be much smaller than CDS in GaN power HEMTs, unmatched TD-amplifiers reveal to be already unstable at low frequencies. A DC-6 GHz TD-feedback power amplifier (TD-FBPA) designed in a 0.25µm AlGaN/GaN process is taken to investigate the influences of feedback and matching components on the common-mode stability. The design shows that GaN TD-FBPAs are prone to oscillations close to the band edge. Furthermore, stabilizing design measures such as omission of the gate-source field-plates and inductive compensation of the tail-current source HEMT´s output capacitance are disclosed and assessed.