> REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 1 Abstract— This paper presents two topologies of low voltage fully differential operational amplifier with class AB output stage. The main objective of this article is to achieve high common mode rejection ratio (CMRR) and to minimize the trade of between power consumption and slew rate. A common mode feed forward (CMFF) circuit has been used in the design which removes the input common mode current from the output currents in order to achieve high CMRR. Adaptive biasing circuits are used to achieve high slew rate which does to by increasing the dynamic current when a differential input is applied at inputs of amplifier. The Op-Amp circuit has been designed using TSMC 180nm technology with supply voltage of 0.8 volts. This article presents comparison of two Op-amp circuits, one with CMFF circuit and other using compensating circuit. The compensating circuit was used in order to achieve better phase margin by using dominant pole concept. The Op-amp using topology-1 gives 60.1 dB gain, 84.3 dB CMRR, and unity gain bandwidth of 1.34MHz, Phase margin of 59.2° for capacitive load of 10pf and power supply of 0.8 volts. The proposed Op-amp consumes 8.54μW of power. while topology-2 gives 82.06dB of gain, 111.75KHz unity gain frequency, Phase Margin of 65.6°, 96.34dB CMRR for capacitive load of 10pF and power supply of 0.8 volts. Index Terms— Operational amplifier, Class AB, Common mode feed forward (CMFF) I. INTRODUCTION he trend towards low power equipments is increasing day by day due to the huge demand of battery powered portable equipments. In portable electronic devices as well as in biomedical equipments the low power Op-amp plays a major role. The supply voltage and the threshold voltage of Op-amp need to be reduced while maintaining high output voltage swings for obtaining low power Op-amp. Now, as the supply voltage reduces, threshold voltage scales down but it is limited up to a minimum value, hence to overcome that many techniques have been proposed. One of the technique uses a bulk driven approach [1]. Proposed Op-Amp uses a rail to rail input stage such that at a certain power supply voltage signal can be processed with their maximum signal voltage. Therefore, by using rail to rail stage at input side it is ensured circuit will work at any common mode input voltage [2]. To ensure that NMOS and PMOS transistor pairs are used at the core circuit of Op-amp. PMOS transistor pair is ON when a low common mode voltage is applied and NMOS pair is ON when a high common mode voltage is applied. Hence, placing these NMOS and PMOS pairs in parallel, rail to rail common mode input extension is obtained. Adaptive biasing circuits have been used so as to obtain power efficient operation at low voltages taking in consideration high slew rate since conventional class A operational amplifiers the bias currents limits the output current to attain maximum value. So, adaptive biasing circuits hence in enhancing slew rate by providing increase in bias current without changing quiescent current of Op-amp which also provide the benefit of low power consumption[3]. Adaptive biasing circuits provide high dynamic current when a differential voltage is applied at the input and a low quiescent current when a common mode voltage is applied. To obtain high common mode rejection ratio in an Op-amp it is needed to reject signals common to both the input terminals and to achieve this common mode feedback (CMFB) or CMFF circuits are used. Since, circuits with transistors operating in subthreshold region are limited in speed hence use of CMFF circuits is preferred over CMFB circuits for low voltage applications [4]. Class AB output stages are used in order to achieve low power consumption and also maximum output range. Using class AB stages at the output side also removes crossover distortion by keeping both transistors on at output stage [5]. For having a power efficient operation Op-amp must consume less power and also for obtaining efficient operation it must preserve fast settling response and high CMRR. II. SUB-THRESOLD OPERATION IN MOSFET’S When the gate to source voltage of MOS transistor is less than that of threshold voltage, it is assumed that MOSFET is off and no current is flowing from drain to source but practically some current flows from drain to source. Using MOSFET’s to operate under this condition is termed as subthreshold operation of MOSFET’s. Unlike the normal conditions of operation, under subthreshold operation the drain current exponentially varies Sanchit Agrawal, Anu Gupta Department of Elec. And Electronics Engg,, Birla Institute of Tech. and Science Pilani sanchit.agrawal4@gmail.com, anug@pilani.bits-pilani.ac.in, Low Power Low Voltage Fully Differential Operational Amplifier using Adaptive Biasing (May 2017) T