IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS—I: REGULAR PAPERS, VOL. 52, NO. 2, FEBRUARY 2005 283 A Low-Power Low-Voltage OTA-C Sinusoidal Oscillator With a Large Tuning Range J. Galan, R. G. Carvajal, Senior Member, IEEE, A. Torralba, Senior Member, IEEE, F. Muñoz, and J. Ramirez-Angulo, Fellow, IEEE Abstract—A new operational transconductance ampliﬁer and capacitor based sinusoidal voltage controlled oscillator is pre- sented. The transconductor uses two cross-coupled class-AB pseudo-differential pairs biased by a ﬂipped voltage follower, and it exhibits a wide transconductance range with low power consumption and high linearity. The oscillator has been fabricated in a standard 0.8- m CMOS process. Experimental results show a frequency tuning range from 1 to 25 MHz. The amplitude is controlled by the transconductor nonlinear characteristic. The circuit is operated at 2-V supply voltage with only 1.58 mW of maximum quiescent power consumption. Index Terms—Analog CMOS circuits, class-AB linear transcon- ductor, ﬂipped voltage follower (FVF), low power, low voltage, operational transconductance ampliﬁer and capacitor (OTA-C) oscillators. I. INTRODUCTION T HE RAPID growth of digital mobile communications and wireless portable electronic equipment has been possible thanks to a steady reduction in cost, size, and power consump- tion. The design of voltage-controlled sinusoidal oscillators is a common topic in communication systems, instrumentation and measurement, as oscillators play an essential role in clocks, fre- quency synthesizers, phase-locked loops (PLL) and sensors. Ring oscillators are preferred for clock signal generation due to their simplicity and low power consumption. However, their harmonic distortion is high and their amplitude is ﬁxed; both characteristics are undesirable for the target applications of this voltage controoled oscillator (VCO). LC-based oscillators have a superior linearity and phase noise performance, but their required silicon area can become prohibitively large for frequencies below 1 GHz. For signal generation in the range of hundreds of kilohertz to hundreds of megahertz, an operational Manuscript received March 31, 2004; revised June 23, 2004 and August 21, 2004. This work was supported by the Spanish Ministry of Science and Tech- nology under Projects TIC-2002-04323-C03-01 and TIC2003-07307-C02-01. This paper was recommended by Associate Editor A. Wang. J. Galan is with the Department Ingeniería Electrónica, Sistemes Infor- máticos y Automática, Universidad de Huelva, E-21071 Huelva, Spain (e-mail: jgalan@diesia.uhu.es). R. G. Carvajal and F. Muñoz are with the Departamento de Ingeniería Electrónica, Escuela Superior de Ingenieros, Universidad de Sevilla, E- 41092 Sevilla, Spain (e-mail: carvajal@gte.esi.us.es; fmunoz@gte.esi.us.es). A. Torralba is with the Departamento de Ingeniería Electrónica, Escuela Su- perior de Ingenieros, Universidad de Sevilla, E- 41092 Sevilla, Spain, and also with the Department of Electrical Engineering, Texas A&M University, College Station, TX 77843 USA (e-mail: torralba@gte.esi.us.es) J. Ramirez-Angulo is with the Klipsch School of Electrical and Computer En- gineering, New Mexico State University, Las Cruces, NM 88003 USA (e-mail: jramirez@nmsu.edu). Digital Object Identiﬁer 10.1109/TCSI.2004.841599 transconductance ampliﬁer and capacitor (OTA-C) oscillator structure offers amplitude control and low distortion in a com- pact implementation. Circuits composed by OTA-C (OTA-C circuits) have been shown to be potentially advantageous for the design of high-fre- quency continuous-time analog ﬁlters [1]–[7]. OTA-C circuits take advantage of the large OTA bandwidth, while transcon- ductance tuning is used to make the circuit programmable to compensate for parasitics, fabrication tolerances and changes caused by temperature drifts, aging and power supply varia- tions. Therefore, OTA-C continuous-time ﬁlters require some kind of on-chip automatic frequency tuning circuit, typically implemented by means of a PLL with a VCO. There are other ﬁelds of application where VCOsin the low megahertz range are required, like biomedical, radio, and portable industrial applications. Recently, in an attempt to minimize production costs, integrated circuit (IC) manufacturers are introducing built-in-self-test (BIST) schemes into new analog circuits. These methods require the design of analog oscillators with a large tuning range, high linearity, and reduced power consump- tion [8], [9]. In this paper, an OTA-C oscillator based on a new transconductor that fulﬁlls these requirements is presented. One approach to design low-power circuits is by means of class-AB cells. Although these cells are inherently nonlinear, they can be combined to obtain linear circuits [10], [11]. A pow- erful class-AB cell for low-power/low-voltage analog signal processing is the so-called ﬂipped voltage follower (FVF) [12]. Different versions of this cell have been used in the past for various applications [13]–[21]. In this paper, a class-AB linear OTA based on the FVF is used to implement an OTA-C VCO with a large tuning range (from 1 to 25 MHz). The paper is organized as follows. In Section II, the design of the OTA is discussed and the versatility and usefulness of the FVF for low-power/low-voltage analog design is reviewed. An efﬁcient and simple scheme for common-mode feedback (CMFB) and common-mode feedforward (CMFF) circuits that guarantees common-mode stability in a wide transconductance range is shown. Section III reviews basic concepts used in the design of OTA-C sinusoidal oscillators, and describes the ar- chitecture selected for this application and the inﬂuence of par- asitics. Experimental results are given in Section IV. Finally, some conclusions are drawn in Section V. II. TRANSCONDUCTOR DESCRIPTION The designed OTA is based on the well-known transcon- ductance multiplier of Fig. 1(a) which uses two cross-coupled differential pairs with their common sources connected to 1057-7122/$20.00 © 2005 IEEE