A level shifter includes a level shifting circuit, a variable bias voltage generator, and a bias voltage generator controller. The level shifting circuit is configured to level shift an input signal at a first voltage level to an output signal having a second voltage level. The second voltage level is higher than the first voltage level. The level shifting circuit includes a current mirror, an input circuit for receiving the differential input signals, and a coupling circuit for coupling the current mirror to the input circuit in response to a variable bias voltage. The variable bias voltage generator is configured to provide the variable bias voltage at one of a plurality of voltage levels. The bias voltage generator controller provides a select signal to select the voltage level from the plurality of voltage levels in response to measuring the duty cycle of the output signal to maintain the duty cycle of the output signal at a predetermined duty cycle.