A frequency mixer includes a first N channel MOS transistor, second and third N channel MOS transistors constituting a local oscillator signal differential pair, and having substantially identical properties, a first load, and a second load. The first N channel MOS transistor receives an RF signal at its gate. A local oscillator signal is applied to the gates of the second and third N channel MOS transistors. The drain current of the second and third N channel MOS transistors is output to the drain of the first N channel MOS transistor. An amplitude-current conversion circuit receives the RF signal and provides an output current to the drain of the first N channel MOS transistor to decrease monotonously the output current with respect to the amplitude of the RF signal.