A CMOS voltage-controlled oscillator (VCO) which generates quadrature triangular waveforms is presented. The proposed VCO consists of two-stage differential relaxation oscillators with sequential common-mode comparators. Post-layout simulation results show that the 2.25 MHz VCO designed in 0.18 m CMOS generates triangular quadrature outputs with less than 1% phase mismatch. With digitally programmable frequency and amplitude control blocks, the VCO achieves a tuning range of nearly 40%. The concept of the proposed method can be extended to enable multi-phase triangular and rectangular waveform generation by setting multi-level thresholds.