Compact CMOS constant-gm rail-to-rail input stage with g m-control by an electronic zener diode

A family of compact CMOS rail-to-rail input stages with constant-g _m is presented. To attain a constant-g_m over the whole common-mode input range, an electronic zener diode is inserted between the tails of the complementary input pairs. This zener keeps the sum of the gate-source voltages of the input pairs, and therefore the g _m of the rail-to-rail input stage, constant. Two possible implementations of the zener have been realized and inserted in a rail-to-rail input stage. These input stages are implemented in two two-stage compact amplifiers. Both amplifiers have been realized in a 1 μm BiCMOS process. They have a unity-gain frequency of 2-MHz, for a capacitive load of 20 pF