Design of Hybrid SiC Varactor Driver Circuit using SiC MESFET

Abstract: The main challenge for mobile service providers is the exponential growth of the wireless data traffic and the increasing number of users. This challenge increases the complexity of radio base stations and power amplifiers. A lot of effort is done to increase the power amplifier efficiency, and dynamic load modulation is one of the successful techniques. A tunable matching network is used at the output to provide the optimum load for a specific output power. A varactor, voltage controlled capacitor, is used for the fast tuning of the matching network. In this thesis work a hybrid varactor driver circuit is designed and implemented to drive anti-series high power SiC Schottky varactors. These high power SiC varactors can provide a very high tuning ratio of 6:1 at a voltage swing of 60 V. Two types of varactor driver circuit configurations are simulated, implemented and compared in terms of important figure of merits like driver DC power consumption, driver bandwidth, anti-series varactor RF impedance, and driver linear static transfer function. In one configuration a high resistor is used for biasing the switching transistor and in the other configuration a gate to source connected MESFET is used for biasing the switching transistor. The designed driver circuit has average DC power consumption less than 1 W, isolation of driver and anti-series varactors less than -18 dB, driver speed above 3 MHz and driver output voltage from 2 to 50 V. The driver circuit configuration with two SiC MESFETs is faster (≥ 10 MHz), but it has a steeper static transfer function as compared to the configuration with one SiC MESFET and a high resistor.