A 0.65V rail-to-rail constant gm opamp for biomedical applications

A low power, low voltage rail-to-rail operational amplifier (opamp) was designed in a conventional 0.18 mum CMOS process for implantable biomedical applications. The input stage of the opamp utilizes both a native NMOS differential pair and a normal NMOS differential pair for rail-to-rail operations. However, an undesired conduction path is occurred within the native pair and degrades the overall transconductance when the tail current of the native pair is turned off. A technique is proposed to eliminate this shunting effect even for very low supply voltage. The overall g_m variations of the input stage are less than 18% and 8% for a supply voltage of 0.6 V and 0.8 V, respectively. The entire opamp consumes 3.3 muA at a supply voltage of 0.65 V.