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This paper presents an efficient and robust circuital implementation of a rail-to-rail input stage with transconductance control, tailored for low-noise operational amplifiers based on differential pairs biased in the sub-threshold region. The proposed g m -control circuit design is indeed based on transistors ratio only, and allows efficient control with scaled currents and dimensions. The architecture guarantees nearly constant performance in terms of bandwidth and power consumption over the whole common mode input range in a power- and area-efficient way. The achievable precision of the transconductance control is also analyzed with transistor equations and its deviation predicted with a simple analytical model, which suggests also a strategy for the minimization of the error by proper tuning. A prototype of the input---output rail-to-rail operational amplifier has been fabricated in a standard 0.35 μm CMOS technology, confirming the validity of the g m -control loop. The amplifier consumes 597 μA from a 3.3 V supply, with an open-loop gain of 107 dB and a gain-bandwidth product of 42.6 MHz.