IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS—I: REGULAR PAPERS, VOL. 54, NO. 7, JULY 2007 1459 Advances in Reversed Nested Miller Compensation Alﬁo Dario Grasso, Member, IEEE, Gaetano Palumbo, Fellow, IEEE, and Salvatore Pennisi, Senior Member, IEEE Abstract—The use of two frequency compensation schemes for three-stage operational transconductance ampliﬁers, namely the reversed nested Miller compensation with nulling resistor (RN- MCNR) and reversed active feedback frequency compensation (RAFFC), is presented in this paper. The techniques are based on the basic RNMC and show an inherent advantage over traditional compensation strategies, especially for heavy capacitive loads. Moreover, they are implemented without entailing extra tran- sistors, thus saving circuit complexity and power consumption. A well-deﬁned design procedure, introducing phase margin as main design parameter, is also developed for each solution. To verify the effectiveness of the techniques, two ampliﬁers have been fabricated in a standard 0.5- m CMOS process. Experimental measurements are found in good agreement with theoretical anal- ysis and show an improvement in small-signal and large-signal ampliﬁer performances. Finally, an analytical comparison with the nonreversed counterparts topologies, which shows the superiority of the proposed solutions, is also included. Index Terms—Analog integrated circuits, CMOS, feedback am- pliﬁer, frequency compensation, multistage ampliﬁer. I. INTRODUCTION T HE operational transconductance ampliﬁer (OTA) is a basic building block in most analog and mixed-signal elec- tronic systems. An increasing number of applications require high-gain high-bandwidth ampliﬁers able to drive capacitive loads under low-voltage supply conditions. As the supply voltage continues to scale down, traditional cascode topologies are no longer suitable for achieving high dc gains, since they cause a reduction of the voltage swings. To avoid cascoding, dc gains in excess of 100 dB are achieved by cascading three transconductance gain stages. However, this approach causes bandwidth reduction, since each stage inevitably introduces low-frequency poles which require additional compensation capacitors to provide adequate closed-loop stability. For this purpose, compensation of three-stage ampliﬁers, where the second stage is noninverting and the last is inverting, is obtained through the nested Miller compensation (NMC) technique [1]–[5]. This approach employs two compensa- tion capacitors and exploits the Miller effect to split the low frequency poles and achieve the desired phase margin and tran- sient response. However, this solution results in bandwidth and slew rate reduction (the gain-bandwidth product is one-quarter as that achievable by a single-stage ampliﬁer, [6]) and in a high power consumption. Recently, different compensation topolo- gies have been proposed in order to overcome the inherent Manuscript received September 5, 2006; revised February 16, 2007. This paper was recommended by Associate Editor P. Carbone. The authors are with Dipartimento di Ingegneria Elettrica Elettronica e dei Sistemi (DIEES), Universita’ Di Catania, I-95125-Catania (Italy) (e-mail: ad- grasso@diees.unict.it; gpalumbo@diees.unict.it; spennisi@diees.unict.it). Digital Object Identiﬁer 10.1109/TCSI.2007.900170 Fig. 1. Block diagram of the basic RNMC. limits of NMC [6]–[13], especially for heavy capacitive loads [14]–[16], [23]. Indeed, many applications require high-gain OTAs driving loads in the order of hundreds of picofarads in battery-powered equipments, such as high-accuracy modulators, ﬂash and pipeline analog-to-digital converters, linear regulators, and active matrix display drivers. When the inner OTA stage is the only inverting one, another kind of compensation scheme, termed the reversed NMC (RNMC) is the most suitable option [4], [5], [17]–[20]. This technique exploits the same operating principle of the NMC but provides an inherent bandwidth improvement since, as shown in Fig. 1, the inner compensation capacitor does not load the output node [4], [19]. In this paper, we shall discuss two simple and high-perfor- mance compensation strategies, namely the RNMC feedforward with nulling resistor (RNMCFNR) and reversed active feedback frequency compensation (RAFFC). It is shown that the tech- niques signiﬁcantly improve small-signal and large-signal per- formance, while maintaining low the circuit complexity, since they can be implemented using only passive components. The paper is organized as follows. The analysis and design equations of the two techniques are presented in Section II. Circuits implementation along with some simulation results are discussed in Section III. Experimental measurements, and some considerations regarding the peculiar features of the proposed approaches, including an analytical comparison between the proposed compensation topologies and the nonreversed coun- terparts, are reported in Section IV. The authors’ conclusion and additional remarks are given in Section V. Appendix A, providing useful and general results related to three-stage ampliﬁers, is also included. II. COMPENSATION TECHNIQUES The block diagrams of a three-stage ampliﬁer exploiting the two proposed techniques are illustrated in Figs. 2 and 3. In par- ticular, Fig. 2 shows the topology named RNMCFNR, where the compensation is achieved by means of the Miller capacitors 1549-8328/$25.00 © 2007 IEEE Authorized licensed use limited to: IEEE Xplore. Downloaded on November 21, 2008 at 11:35 from IEEE Xplore. Restrictions apply.