A voltage controlled oscillator (VCO) is provided. The VCO may include a first ring oscillation circuit that may have a plurality of delay cells and may output first differential oscillation signals, and a second ring oscillation circuit that may have a plurality of delay cells and may output second differential oscillation signals. The delay cells of the first ring oscillation circuit may be respectively cross-coupled to the corresponding delay cells of the second ring oscillation circuit. Each of the delay cells may include a differential amplification circuit that may output a first differential signal based on a first control signal, and a negative resistance circuit that may be connected in parallel to a pair of output terminals of the differential amplification circuit, may receive a second differential signal, may adjust the phase of the first differential signal based on a second control signal, and may then output the first differential signal.