A 27-nV/√Hz 0.015-mm2 three-stage operational amplifier with split active-feedback compensation

This paper presents a low-noise small-area three-stage operational amplifier using split active-feedback compensation (SAFC) which is suitable for biomedical arrayed sensors. The proposed SAFC amplifier only requires a small capacitance for phase compensation, while it can achieve a low input-referred noise by increasing the first-stage transconductance without sacrificing the phase margin. The proposed SAFC amplifier has been implemented using a standard 0.18-μm CMOS process. The measurement results show that the proposed SAFC amplifier achieves >120-dB DC gain, 6.2-MHz gain bandwidth product, and phase margin of 60°. The measured input-referred noise is 27 nV/√Hz. The current dissipation is measured as 177 μA at a power supply of 1.5 V and it achieves a noise efficiency factor of 14.