A dual band voltage-controlled oscillator (VCO) that can cover 13- and 22-GHz bands is proposed and implemented using an InGaP/GaAs heterojunction bipolar transistor (HBT) technology. The frequency band is selected by mode switching between differential mode at low frequency and common mode at high frequency. The differential mode means that voltages appearing at two tank ports have opposite polarities and the common mode, the same polarities. Mode switching allows the VCO to operate at two resonant frequencies with a single LC tank. The measured phase-noise performances of a dual band VCO are -108 and -106 dBc/Hz at 1 MHz offset at the frequencies of 13 and 22 GHz, respectively, while drawing 22 mA and 16 mA from a 4-V supply. Switching times between two bands are less than 24 ns.