I. Introduction
Low power Operational Transconductance Amplifiers - OTA are widely used in different applications including amplifiers for biomedical circuits in which it is important to guarantee low power consumption under low power supply voltages. When small input signals are applied, under both low supply and low biasing voltages, the amplifiers become more susceptible to parasitic signals, which can affect the their output. The parasite signals can be coupled to the amplifier inputs in common mode, thus it is necessary to increase the amplifier CMRR to attenuate them. The CMRR is defined as the ratio of the output voltage produced by a difference voltage compared to the output produced by a common mode signal of equal amplitude [1] . Some techniques are used in MOS circuits to increase the CMRR. The most used ones are the Common Mode Feedback – CMFB and the Common Mode Feedforward – CMFF [2] .