Linearity of bulk-controlled inverter ring VCO in weak and strong inversion

Frequency modulation in ring VCOs is investigated. Primarily, the linearity of conversion from input voltage to output frequency is considered. Bulk-voltage control of the threshold voltage of the VCO transistors is found to be a very promising approach for applications in frequency /spl Delta//spl Sigma/ converters. Different approaches apply in presence of high supply voltages, when transistors work in strong inversion, compared to low supply voltages, when transistors are in weak inversion. In strong inversion, second-order effects controlled by the supply voltage linearize the VCO modulation, while in weak inversion an improved linearity can be obtained using soft rails, at the expense of a reduced sensitivity.