Source-drain follower has been designed and implemented for monolithically integrated biosensor array. The circuit acts as a voltage follower, in which a sensing transistor is operated at fixed gate-source and gate-drain voltages. It operates at 10 nW power dissipation. The wide-swing cascode configurations are investigated in constant and non-constant biasing methods. The constant biased cascode source-drain follower has the merit of small cell size. The chip was fabricated using 1.2 µm standard CMOS technology, and a wide range of operation between 1 nW and 100 µW was demonstrated. The accuracy of the voltage follower was 30 mV using minimum sized transistors, due to the variation of threshold voltage. The error in the output except for the threshold voltage mismatch was less than 10 mV. The temperature dependence of the output was 0.11 mV/. To improve the input voltage range and accuracy, non-constant biased cascode source-drain follower is examined. The sensor cell is designed for 10 mV accuracy and the cell size is 105.3µm 81.4 µm in 1.2 µm CMOS design rules. The sensor cell was fabricated and showed that the error in the output except for the threshold voltage mismatch was less than 2 mV in a range of total current between 3 nA and 10 µA and in a temperature range between 30 and 100.