A Sub-1-V Bulk-Driven Opamp With an Effective Transconductance-Stabilizing Technique

The effective transconductance (G_m) of a bulkdriven 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-μm n-well CMOS process. The opamp consumes 261 μW 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 pFII1 MΩ.