Linearization of frequency tuning in voltage controlled oscillator by using pMOS varactors

In this work, we have demonstrated the linearization of frequency tuning in VCO (Voltage-Controlled Oscillators). Typical varactors made from monolithic CMOS (Complementary Metal Oxide Semiconductor) technology have a narrow tuning range and highly nonlinear tuning characteristics. We propose an improved varactor with linear (analog) tuning by using back-to-back connection of MOS transistors. Increased reverse bias to the bulk terminal widens the depletion region beneath the channel and the gate-to-bulk capacitance shows greatly improved linearity. A wide tuning range is achieved by digitally tuned pMOS varactors connected in parallel with the analog tuning pMOS varactor. Digital input controls three pMOS transistor pairs. When 3-bit digital and analog inputs are applied to the designed circuits, the voltage-controlled oscillator shows the tuning feature in a frequency range between 2.3 GHz and 2.64 GHz. At the power supply voltage of 2.5 V, phase noise is -128 dBc/Hz at 3MHz offset from the carrier. Total power dissipation is 7.5 mW.