A reference voltage circuit 2 comprises: a bandgap circuit portion comprising first and second reference transistors (Q1, Q2) and a current source arranged to drive the first and second reference transistor at different current densities, wherein the first and second reference transistors are connected to first and second nodes (N1, N2) respectively; an operational transconductance amplifier (M4, M5, M10, M11, M12) arranged to produce an output current that is proportional to a difference between a voltage at the first node and a voltage at the second node; an output current mirror circuit portion (M3) arranged to generate a mirror current that is a scaled version of the output current and drive said mirror current through a load (R3) so as to produce a reference voltage (Vref); and a reference monitoring circuit portion (6) arranged to monitor the operational transconductance amplifier and generate a flag (Vready) if a current flowing through the operational transconductance amplifier exceeds a threshold.