Realization of a broadband hybrid X-band power amplifier based on fT-doubler technique

Abstract

This paper presents the design steps for the fabrication of a hybrid microwave power amplifier (PA) based on fT-doubler technique. The PA is implemented using two 6 W discrete GaN-HEMTs on SiC substrate. The fT-doubler technique is used to enhance the frequency response and bandwidth of the PA as compared to the common source (CS) configuration. Accurate simulation results show about 90% improvement in the unity current gain frequency (fT) for the fT-doubler structure in comparison with a common-source parallel structure. However, the hybrid realization of a high-frequency PA based on fT-doubler technique is challenging mainly due to severe thermal management failures and large parasitic effects of the interconnections such as bond wires and long microstrip lines. The proposed hybrid integrated strategy relieves the thermal problem while the parasitic effects are well controlled. Thermal analysis and accurate 3D thermal simulation of the proposed hybrid integrated structure show that the channel temperature of transistors does not exceed its maximum tolerable value. Implementing the optimal source/load pulls for the transistors and well-designing the input/output matching networks by considering the parasitic effects of the interconnections result in a fabricated PA with a broad bandwidth of 6.5–10.4 GHz, a sufficient small-signal gain of 10 dB, a considerably high output power (Pout) of around 10 W and a power added efficiency (PAE) of 43% at 8 GHz.