A Push-Pull Class-C CMOS VCO

A CMOS oscillator employing differential transistor pairs working in Class-C in push-pull configuration is presented. The oscillator exhibits the same advantages enjoyed by complementary topologies on oscillators based on a single differential pair, while yielding a substantial power consumption reduction thanks to the Class-C operation. The phase-noise performance and the fundamental conditions required to keep the transistors working in Class-C are analyzed in detail. It is shown that, for an optimal performance, both nMOS and pMOS transistors should not be pushed into the deep triode region by the instantaneous resonator voltage, and a simple circuit solution is proposed to accommodate a large oscillation swing. A 0.18-mu m CMOS prototype of the (voltage-controlled) oscillator displays an oscillation frequency from 6.09 to 7.50 GHz. The phase noise at 2-MHz offset is below -120 dBc/Hz with a power dissipation of 2.2 mW, for a state-of-the-art figure-of-merit ranging from 189 to 191 dBc/Hz.