MIXED SIGNAL LETTER A high-speed differential CMOS Schmitt trigger with regenerative current feedback and adjustable hysteresis Fei Yuan Received: 20 February 2009 / Revised: 14 July 2009 / Accepted: 7 August 2009 / Published online: 22 August 2009 Ó Springer Science+Business Media, LLC 2009 Abstract This paper presents a brief overview of Schmitt triggers and proposes a new differential current-feedback Schmitt trigger. The hysteresis of the proposed Schmitt trigger is generated using regenerative current feedback and can be adjusted by varying the current of the regen- erative feedback network. The center of the hysteresis can also be adjusted by varying the common-mode input voltage. The proposed Schmitt trigger has the characteris- tics of current-mode circuits, making it particularly attractive for low-voltage high-speed applications. The proposed Schmitt trigger has been designed in TSMC- 0.18 lm 1.8 V CMOS technology and analyzed using Spectre from Cadence Design Systems with BSIM3V3 device models. Simulation results are presented. Keywords Schmitt trigger  Current-mode feedback  Variable hysteresis 1 Introduction Schmitt triggers are bistable networks that have been widely used to enhance the immunity of circuits to external noise and disturbances [1]. They play a critical role in a number of emerging applications including frequency doublers [2], retinal focal-plane sensors [3], sub-threshold SRAM [4], image sensors [5], and wireless transponders and sensors [6, 7]. Schmitt triggers are traditionally implemented using operational amplifiers with a resistive positive feedback [8]. These Schmitt triggers suffer from the drawbacks of high power consumption. Perhaps the most widely cited single-ended Schmitt trigger was ini- tially proposed by Dokic [9] with a detailed analysis of it appeared 10 years later in [10]. Dokic’s design stacked four transistors between power and ground rails and is therefore not particularly attractive for applications where supply voltages are low. Steyaert and Sansen proposed a low-voltage Schmitt trigger with only two transistors piled between the power and ground rails [11]. Although only one triggering voltage was implemented in their published work, their design can readily be extended to have two distinct triggering voltages, as demonstrated by Wang in [12]. The Schmitt trigger proposed by Kim et al. [13] uses ten transistors to form the needed regenerative feedback. A low-power CMOS Schmitt trigger was also proposed by Al-Sarawi [14]. Zhang et al. [15] introduced an ultra low- voltage Schmitt trigger by using a body biasing technique. The Schmitt trigger proposed by Pedroni [16] employs two static inverters that have distinct threshold voltages such that different triggering voltages can be obtained. Pedroni’s design is an open-loop approach with no regenerative mechanism. Sapawi et al. replaced the two cascaded pMOS transistors in Dokic’s design with two pMOS tran- sistors connected in parallel so that it can operate at a low supply voltage. A common drawback of the proceeding single-ended Schmitt triggers is that the hysteresis is set by device dimensions, process parameters, supply voltage, and varies with process conditions. Schmitt triggers with tunable hysteresis are highly desirable to overcome this deficiency. They are also critically needed in applications where the level of noise and disturbances coupled to the triggering signals is not known a prior. Wang [12] modified Steyaert– Sansen’s design by varying the current of the regenerative networks subsequently the intensity of the positive F. Yuan (&) Department of Electrical and Computer Engineering, Ryerson University, Toronto, ON M5B 2K3, Canada e-mail: fyuan@ee.ryerson.ca 123 Analog Integr Circ Sig Process (2010) 63:121–127 DOI 10.1007/s10470-009-9374-y