Tracking-Vgs: A Temperature Compensation Technique to Implement all-MOS Reference Voltages

A novel temperature compensation technique to implement reference voltages in CMOS circuits is described, starting from the theoretical basis up to results obtained from experimental verification of a test circuit. The proposed technique is based on the fact that, since the V _GS voltage of a MOS transistor can either increase or decrease with increasing temperature depending on the operating channel current I_D, a voltage with amplitude nV_GS produced by a stack of n transistors can feature the same rate of change with the temperature as the V_GS of a single transistor, provided the single transistor and the stack of n transistors are biased each with the appropriate channel currents. In such conditions, the difference between the two voltages is constant. An alternative to produce the mentioned nV_GS voltage, instead of a stack of n transistors, is a floating gate transistor, whose equivalent V_GS voltage can be adjusted by depositing charges in its floating gate. This is actually the preferred implementation of the proposed technique cause it adds the benefit of adjusting infield the amplitude of the reference voltage. Samples of a test circuit were fabricated in 0.35mum CMOS aiming at to validate the technique. Results have confirmed what was expected from the proposed technique: Measured from - 40 to 120degC the circuits have shown a temperature coefficient of circa 100ppm/%deg;C. While the insensitivity to the temperature obtained from the implemented reference voltage has not exceeded the current state of the art, which is about 1ppm/degC, the proposed technique contributes to the implementation of reference voltages that use only MOS transistors and that feature in-field adjustment