Compact CMOS constant-gm rail-to-rail input stages with gm-control by an electronic zener diode

A family of compact rail-to-rail input stages are presented. To obtain a constant-gm over the common-mode input range, an electronic zener diode is inserted between the tails of the complementary input pairs. The zener keeps the sum of the gate-source voltage of the input pairs, and therefore the gm of the input stage, constant. Two possible implementations of the zener have been realized. The input stages have been inserted in a two stage compact amplifier. Both amplifiers have been realized in a 1 ¿m BiCMOS process. The unity-gain frequency is 2-MHz, for a load of 20 pF