The need for extending low power circuits increased with the advent of use of large number of portable devices like cell phones, calculators, miniature computers, etc. In all these devices, a long battery life is desired. An increase in battery life can be achieved by reducing power consumption of individual circuits. One of the methods to reduce the power consumption is by operating the devices at low current and low voltages. Operating the devices below threshold voltages is called as sub-threshold operation and the region of operation is called sub-threshold region.