Disclosed herein is a multiplexer circuit. The multiplexer circuit includes a first differential output unit, a second differential output unit, and a selection unit. The first differential output unit receives NRZ input signals (D1 and D1) and a clock signal (CLK), and generates differential RZ-mode outputs (R1 and R1). The second differential output unit receives NRZ input signals (D2 and D2) and an inverted clock signal ( CLK), and generates differential RZ-mode outputs (R2 and R2). The selection unit receives the RZ-mode output signals (R1, R1, R2, and R2) generated at the first differential output unit and the second differential output unit, and generates NRZ mode outputs in each half cycle of the clock signal (CLK).