This paper presents a systematic procedure for the design of amplifiers using weak inversion (WI) region MOS transistors. The optimization of the design constraints takes into consideration parameters such as Power Supply, Gate-Source Voltage (V _GS ), Aspect Ratio, Biasing Voltages and Temperature. The work is supported by a case study on Common Source (CS) amplifiers. The case study provides the results of WI operation constraint - temperature range - and compares it to the operation in Strong Inversion (SI). It also proves that WI can be used for Ultra Low Power application if the temperature range is feasible.