Differentially tunable varactor with built-in common-mode rejection

A differentially tunable MOS varactor with enhanced intrinsic common-mode noise rejection is proposed and analyzed. Using a complementary topology the C-V characteristics of two MOSFETs are arranged to sum under differential excitation and to roughly cancel under common-mode excitation. The benefits of this topology are two-fold: (a) by nature, differential tuning resists common-mode noise and (b) the new structure acts to further reduce common-mode disturbances. Simulations show the effectiveness of this approach against standard techniques. Differential tuning circuits are proposed and alternate varactor structures discussed.