The effective transconductance (G(m)) of a bulk-driven operational amplifier opamp) can significantly vary with the input common-mode voltage. This variation of G(m) complicates frequency compensation and creates harmonic distortion. Thus, this brief presents a G(m)-stabilizing technique to reduce the variation of G(m) across the input common-mode range (ICMR). The idea is to use a variable positive feedback structure to adaptively control G(m) to the input common-mode voltage. A low-voltage bulk-driven opamp with the proposed G(m)-stabilizing technique has been implemented in a 0.18-mu m n-well CMOS process. The opamp consumes 261 mu W from a 900-mV supply voltage. The variation of G(m) is reduced from 132% to 25% across the rail-to-rail ICMR. The measured dc gain is 76.8 dB and the unity-gain bandwidth is 7.11 MHz when the opamp is loaded with 17 pF parallel to 1 M Omega.