In this paper, a broadband balun using an active circuit with negative group delay (NGD) is proposed. The unit cell of the active NGD circuit is based on a Field Effect Transistor (FET) in cascade with an RLC series network. First, a comparison between measurements of a two-stage prototype of this active topology and simulations validate the synthesis method of this innovative device. Then, thanks to the NGD circuit, a constant phase can be generated if this circuit is associated with a classical transmission line. By implementing such phase shifters into the two branches of a resistive splitter, we obtain a new balun topology. The NGD balun simulation results show a rather constant differential output phase (180°±9°), insertion losses above -2.4 dB and an excellent isolation below -59 dB for all three ports, for a bandwidth from 3.5 GHz to 5.5 GHz.