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.