A $g_m$-control technique using a new electronic zener diode (EZD) for CMOS rail-torail input stages is presented. A regulated CMOS inverter is used as an EZD to obtain a constant-$g_m$ input stage. The turn-off characteristic of the proposed EZD is better than that of the existing EZD using two complementarey diodes, and thus, better $g_m$-control can be achieved. With this input stage, a 3V constant-$g_m$ rail-to-rail CMOS op-amp has been designed and fabricated using a $0.8muextrm{m}$single-poly, double-metal CMOS process. Measurements results show that the $g_m$ variation is about 6% over the entire input common-mode range, and the op-amp has a dc gain of 88dB and a unity-gain frequency of 4MHz for $C_L=20pF, R_L=10kOmega$