A back gate bias voltage generator circuit consists of three MOS transistors (Q4, Q5, Q6) with a separate load element (Q1, Q2, Q3) coupled to the drain of each and a voltage clamp (Q7) connected to an output terminal (16). A terminal at the potential of a power supply (VCC) serves as one input and a terminal at the substrate potential (VSub) serves as another input. When the power supply (VCC) potential and the substrate potential are within normal operating ranges, the output terminal (16) assumes a reference potential (VSS). The potential of the output terminal increases in magnitude if either of the two input potentials (VSS, VSub) goes outside preselected operating ranges.