Constant gm, rail-to-rail input transconductance stage with output common mode current compensation

This paper presents a compensation structure to reduce the output common mode current of a rail-to-rail constant g _m transconductance stage. The transconductance stage is a differential structure with the input transistors biased in weak inversion. This results in low power consumption and a simple relation between g_m and tail currents. The constant gm is achieved by doubling the tail current in the active differential couple when the other one goes to cut off. Together with constant bias current in the output stage this can result in common mode bias current flowing from the output depending on the common mode input voltage. The proposed solution eliminates this problem by variation of the bias current in the output stage in relation to the tail currents in the differential couples. The transconductance stage was manufactured in a 0.35 mum CMOS process. Measurement results are presented together with simulation data. The compensation structure cancels the common mode output currents for common mode input signals up to a frequency of 7 MHz.