A compact realization of hybrid class-B/C quadrature voltage-controlled oscillator (QVCO) with output buffering is proposed. It employs a four-phase amplitude detector feedback loop to shift the operation mode from hybrid class-B/class-C to class-C after power up. The power-efficient class-C core allows quadrature-locking via passive capacitive coupling, whereas the auxiliary class-B core ensures a robust startup. For higher output drivability, PMOS-based source followers are stacked atop the QVCO for reusing its bias current and avoiding any AC-coupling networks. The buffered QVCO can directly drive up the 50-Ω port of testers, or to withstand a large capacitive load in on-chip signal distribution. The fabricated 9 GHz QVCO in 65 nm CMOS draws 10.5 mW and exhibits -99.37-dBc/Hz phase noise at 1 MHz offset. The active area is just 0.137 mm by using two compact spiral inductors with patterned ground shields.