Broadband Characterization of GaN Transistors for Software Defined Radio Power Amplifier Applications

Abstract

A technique for applying a series of narrowband load pull measurements of a Gallium Nitride (GaN) transistor to the design of a broadband power amplifier (PA) is presented with potential application to software defined radio (SDR). Load pull uses computer controlled tuners to apply a series of impedances at discrete frequencies to the device under test (DUT) and then calculates performance contours such as gain, efficiency, output power, etc. The basics of load pull testing and its application to device characterization for PA design are reviewed. The operation of a load pull system is presented, along with test fixture design to accommodate broad band measurements. A GaN device is measured and contours of output power and efficiency are determined for each frequency. These results are interconnected to obtain an impedance locus vs. frequency from which an output matching network is designed. The technique is applied to the design of a 500 MHz to 2500 MHz single-stage GaN PA with a saturated output power of +41 dBm. It achieves a minimum efficiency of 35% at nominal drain voltage and achieves a minimum efficiency of 46% when subjected to drain voltage optimization over frequency.