A Power Efficient Fully Differential Super Class-AB OTA M. Pilar Garde and Antonio Lopez-Martin Institute of Smart Cities Public University of Navarra Pamplona, Spain antonio.lopez@unavarra.es Jaime Ramírez-Angulo Klipsch School of Electrical and computer Engineering New Mexico State University Box 30001/Dept.3-0 Las Cruces, NM 88003-0001 jairamir@nmsu.edu Abstract— A fully differential Super Class AB Operational Transconductance Amplifier (OTA) is presented. Quasi-Floating Gate (QFG), adaptive biasing and Local Common Feedback (LCMFB) techniques have been applied to achieve improved dynamic current boosting, slew rate (SR) and gain-bandwidth product (GBW). Simulation results in a 0.5μm CMOS process show an increase in slew rate and GBW by a factor of 86 and 16, respectively, versus the class A version using the same supply voltage and bias currents. The overhead in terms of area, noise, and static power consumption, is minimal. Index Terms— Adaptive biasing, Local Common Feedback, Quasi Floating Gate, Analog CMOS ICs, Class AB circuits, OTA. I. INTRODUCTION Nowadays, the use low-power circuits is more and more frequent in order to employ smaller and lighter batteries and extend their lifetime in wireless and portable electronic systems. The operational transconductance amplifier (OTA) is one of the most used analog building block due to its several applications. Besides low power consumption, these OTAs should have good performance, especially in terms of slew-rate (SR) and gain-bandwidth product (GBW) in order to achieve fast settling response. Class A amplifiers do not fulfill these requirements, since the bias current limits the maximum output current. Hence class A circuits must increase their static power consumption in order to get large output currents. This drawback can be overcome using class AB amplifiers, as dynamic currents are not limited by the quiescent current [1]- [2]. These amplifiers often adaptively bias the differential pair to get the required current boosting, which provide low and well-controlled quiescent currents, thus achieving very low static power dissipation. Besides adaptive biasing, Local Common-Mode Feedback (LCMFB) techniques can be applied in order to increase the current in the active load of the differential input pair. The resulting topologies were coined as Super Class AB [3] as their output current is boosted proportionally to Vid 4 instead of Vid 2 (Vid is the differential input voltage) as in most Class AB topologies. However, the fully differential version of Super Class AB OTAs does not fully exploit the dynamic improvement achievable as the output branches are biased by conventional current mirrors. This paper presents a new fully differential Super Class AB OTA that overcomes this issue. It is based on applying Quasi Floating Gate (QFG), adaptive biasing and LCMFB techniques to improve the conventional class AB OTA. A modification in the conventional Common Mode Feedback circuit of the OTA is also introduced, increasing the gain-bandwidth product (GBW). II. PRINCIPLE OF OPERATION Figure 1 shows the conventional class A fully differential current mirror OTA (Fig. 1(a)), a Super Class AB OTA (Fig. 1(b)) [3] and the proposed Super Class AB QFG OTA (Fig. 1(c)). The constant differential pair bias current source 2IBIAS of Fig. 1(a) is replaced in Figs. 1(b) and 1(c) by an adaptive biasing circuit, which provides very small quiescent currents to M1 and M2. When such adaptive circuit senses a large differential input, it automatically boosts the bias current provided. In addition, in Figs. 1(b) and 1(c), LCMFB [3] has been used. Finally, in Fig. 1(c), the QFG technique [4] is applied to dynamically bias the output current mirrors. These three techniques employed are described in the following paragraphs. 2.A. Adaptive Biasing of the Input Pair Fig. 2(a) shows the adaptive biasing scheme chosen for the differential input pair [3],[5]. It is made of two matched transistors M1 and M2 cross-coupled by two dc level shifters. In quiescent conditions VSG1 Q =VSG2 Q =VB, therefore transistors M1 and M2 carry equal quiescent currents controlled by the dc voltage VB. If VB is slightly larger than the MOS threshold voltage |VTH|, very low static currents are obtained. However, if for example VIN+ decreases, voltage at the source of M1 decreases by the same amount while the source voltage of M2 is kept constant. Hence, current through M2 increases and current through M1 decreases. These currents can achieve values much larger than the quiescent current. Moreover, the full differential input signal is applied to each differential pair transistor, doubling dc gain compared to Fig. 1(a). The implementation of the dc level shifters VB in the scheme of Fig. 2(a) is critical to the performance of the circuit. These level shifters should be simple to avoid a penalty in noise, speed or supply voltage requirements. Simultaneously, they should be able to source large currents when the OTA is driving a large This work has been supported by the Spanish Ministerio de Economía y Competitividad, grant TEC2013-47286-C3-2.