A new operational amplifier working with supply voltages 1.8V has been designed in SMIC 0.18- ?m CMOS technology. The p-channel differential input pair and a level-shift p-channel differential input pair operate in weak inversion. A constant trans-conductance (within 0.5% of maximum fluctuation) based on current mode is ensured for the entire input common mode range by a simple and novel small signal control circuit that offsets second-order effects under constant tail current, such as variation of the slope factors of weak inversion transistors and finite output resistance. The class AB output stage biased by a new cross-coupling voltage level shifter has a full voltage swing and better high frequency performance. This circuit provides a 46MHz gain bandwidth for a power consumption of 0.6mW with 5pF load.