The demand for building ultra-low power circuit emerges as a challenge in recent years. This demand is due to the fast growth in battery operated wearable computing systems and implantable medical instruments. The design challenges in such systems are; low sensitivity to supply voltage variation and independence in performance with respect to power supply. Furthermore, these challenges are to be mitigated with the objective of ultra-low power consumption. Subthreshold regime of operation of the devices is preferred these days which satisfies the need of ultra-low power operation. The devices are operated at subthreshold regime when the supply voltages are scaled below the threshold voltage of the devices.