Design of Fully Differential Telescopic Op-amp with Common Mode Feedback in 0.25um CMOS Technology

Abstract

This paper presents an enhanced two stage fully differential op-amp with common mode feedback that achieves improved dc gain, common mode rejection ratio and speed of the op-amp. Two stage op-amp implemented using the telescopic cascode amplifier and common source amplifier. Telescopic operational Op-amp has higher gain, higher pole frequency and lower power dissipation than other configuration and common source amplifier used as transconductance amplifier that help to achieve specification with lower tail current. A high gain, high speed and wide output swing CMOS fully differential operational amplifier is designed using 0.25µm CMOS technology and its various parameters are simulated by using T-Spice. In addition nulling resistor compensation is used to cancel the second pole which is implemented by a MOSFET operating in deep triode region. After the simulation we achieve the Open loop gain =80dB, Common mode gain is =85dB, Phase margin =50o, Gain bandwidth frequency 200MHz and Output Swing =1.2V peak to peak. Finally we conclude that the fully differential Op-amp has many advantages over the single ended counterparts thus it can used in many application like analog to digital convertor, rail to rail design, and other analog IC design.

Keywords: Op-amp, CMOS, Telesccopic Cascode Op-amp, Open Loop Gain, CMRR, Compensation

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