A high gain, high CMRR two-stage fully differential amplifier using gm/Id technique for bio-medical applications

This paper presents a well-defined method for the design of a high gain, high CMRR two-stage CMOS operational amplifier using 0.18μm CMOS technology for Bio-medical applications. The Op-amp consists of a cascade of Folded-cascode differential amplifier in first stage followed by a fully differential amplifier with PMOS current source load in second stage. The g_m/I_d technique is employed in designing the Op-amp. Trade-offs among the factors such as bandwidth, gain, phase margin, bias currents, signal swing, slew rate and power dissipation are explained. Common mode feedback is employed in the design for high CMRR. The designed Op-amp has a gain of 106.31dB, CMRR of 131.02dB, phase margin of 57.53°, power consumption of 685μW and a Slew rate of 3.87 V/μs for a 3.3V power supply in the design corner TT. As the number of stages increase, stability of op-amp is a concern. So, Frequency compensation techniques are employed for the proposed design. This design provides a considerable insight into the overall operation and advantages of CMFB network. This design obtains a high gain and high CMRR which makes it suitable for Biomedical applications.