This paper presents a CMOS energy detector for a non-coherent ul-tra-wideband receiver. The proposed active squarer generates differen-tial output using an NMOS and PMOS transistor pair in low supply voltage operation. Because the applied common-mode feedback circuit makes each differential output bias voltage the same, the output dc off-set can be easily removed. The proposed squarer with the integrator is designed using the 0.11-μm CMOS technology. The designed squarer with integrator operates at 3-5 GHz and only dissipate 1.01 mW at a supply voltage of 1.2 V.