An extremal voltage detector produces an output voltage from an operational amplifier having its non-inverting input terminal connected to a first node and its inverting input terminal connected to a second node. A number of identical metal-oxide-semiconductor field-effect transistors (MOSFETs) controlled by respective input voltages are connected in parallel between the first node and a first power supply terminal. Another identical MOSFET, controlled by the output voltage, is connected between the second node and the first power supply terminal. Alternatively, a plurality of identical MOSFET detection circuits, controlled by the input and output voltages, are connected in parallel between the first power supply node and the first and second nodes. A pair of constant-current circuits conduct equal currents from the first and second nodes to a second power-supply terminal.