Advancement of CMOS Schmitt Trigger Circuits

Schmitt triggers are electronic comparators that are widely used to enhance the immunity of circuits to noise and disturbances and are inherent components of various emerging applications. Conventional Schmitt triggers, composed of operational amplifiers, suffer from some inevitable drawbacks which are not prominent in CMOS Schmitt triggers. In this paper, a review on the advancement of Schmitt trigger circuits are illustrated in different literatures is discussed with their merits and demerits.

The term trigger is used because of the output, which remains in its state until the input level changes abundantly by triggering a change.This threshold's dual function is called hysteresis and shows that the Schmitt trigger has some memory as well.Actually it is a bistable multivibrator circuit which converts a varying voltage into an unvarying logical voltage signal (zero or one).Numerous design and implementation of Schmitt trigger circuits for different applications and advantages has been proposed in literatures by many researchers (Lotze & Manoli, 2012;Saini, Veeramachaneni, Kumar, & Srinivas, 2009;Kulkarni, Kim, & Roy, 2007;Wu & Chiang, 2004;Morimura et al., 2004;Kim, Kim, & Chung, 2007;Dokic, 1984;Dokic & Bundalo, 1985;Wang, 1991;Kim, Joongsik, & Kim, 1993;Dokic, 1996;Zhang, Srivastava, & Ajmera, 2003;Franco, 1988;Dokic, 1984;Azeemuddin & Sayeh, 2011;Chen & Ker, 2005;Kuang & Chuang, 2001;Stayaert & Sansen, 1986;Singhanat et al., 2011;Ahdal & Toumazou, 2012).This paper presents a detail review on the merits and demerits of each circuit from its architecture and performance point of view illustrated in literatures for last two decades.The article is organized as follows: in Section 1, basic description, history and applications of Schmitt trigger circuit is dicussed while the principle of operation of traditional Schmitt triggers is illustrated in Section 2, Section 3 discuss about the advancement of Schmitt trigger in perspective of their circuit structures and performances and the summury of the review is stated in Section 4.

Working Principle of a Traditional Schmitt Trigger
Since its invention, the Schmitt trigger circuit has relied on changing the voltage or current threshold levels by means of positive feedback in the analogue loop.In their textbook, Millman and Halkias (1972) discuss how this is done by means of a resistive voltage divider.Other voltage mode feedback circuits, which are more suitable for VLSI implementation, are discussed by Stayaert and Sansen (1986) and Dokic, Iliskovic, and Bundelo (1988).Schmitt trigger circuits with current feedback are discussed by Filanovsky (1988) and Wang and Guggenbuhl (1988).
Schmitt trigger is one kind of regenerative circuit, mainly worthwhile in digital systems (Marufuzzaman, Reaz, Rahman, & Ali, 2010;Akter, Reaz, Yasin, & Choong, 2008)    V IN goes high.End result is, a higher input voltage is required to turn on M N1 and turn off M P1 .On the other side, when the input signal goes high, the high output signal V OUT (or V DD ) is fed back to the body of M N1 and M P1 .In this scenario, the threshold voltages of M N1 and M P1 relate to the values at V DD substrate bias, creating the threshold voltages of M N1 and M P1 lower and higher than their threshold voltages, compared to the situation when V IN goes low.The end result; a low input voltage is necessary to turn off M N1 and turn on M P1 .These alterations in the changing voltages reason the hysteresis.Schmitt trigger will not react and will be resistant to the unwanted noise if the magnitude of the input signal is less than the switching threshold difference.Numerous methods have been suggested to implement Schmitt trigger.The conventional Schmitt trigger was implemented using operation amplifiers with two resistors connected to its positive feedback configuration (Franco, 1988).This sort of Schmitt trigger is not appropriate in CMOS technology, mainly because of the correctness restriction of resistors and op-amp design tasks e.g.Low offset and high DC gain necessities.Different carrying out of Schmitt triggers can adventure the probable advantages of CMOS technology has been offered (Dokic, 1984).By using only six transistors without latch can be transformed into a Schmitt trigger circuit (Dokic, 1984).The circuit operates with less than 3.3 volts supply without having gate oxide, high voltage overstress.By using a multilayer Schmitt trigger, higher voltage difference between two switching threshold voltages was succeeded as stated by Kuang and Chuang (2001).Unluckily that design needed the stack of four transistors between the power rail and ground rail therefore preventing practice in low voltage applications.Standard CMOS inverter with positive feedback was engaged by Stayaert and Sansen (1986).Only two stacked of transistor used in between ground and power rails.An additional active pull down route was brought in operation for the first inverter while the input was shifting from high to low.

Conclusion
Schmitt triggers are widely used in different circuits to increase noise immunity by using only single input threshold; the noisy input signal close to the threshold might source the output to switch swiftly back and forth from noise itself.Schmitt trigger's noisy input signal near to threshold can grounds only one switch in output value after which it would have to move away from the other threshold in order to switch another.A common disadvantage of the taking place single ended Schmitt triggers is that; hysteresis is set by process parameters, device dimensions, supply voltage and differs with process situations.Tunable hysteresis is extremely required to overcome this insufficiency.These are analytically needed in applications where disturbances and noise level combined to the triggering signals.
Figure 1.A

Figure 7
Figure 7(b) clarifies the recommended Schmitt trigger circuit of three stage CMOS inverters.DTMOS transistors are employed in the first stage CMOS inverters so that effective transconductance as well as speed of the input stage is increased.The second and third stages are linked as cross-coupled inverters.Transistors M N2 and M P2 do a similar job as transistors M N2 and M P2 in Figure 7(a) whereas the third stage inverter M N3 and M P3 are auxiliary to work for two purposes, they are: firstly, the second and third stage CMOS inverters are coupled in a positive feedback configuration thus speeding up the swapping process and the second one is control voltages of the third stage CMOS inverter control the strength of the feedback signal as a result the switching threshold voltage of the Schmitt trigger.
. Schmitt Trigger is usually composed of a comparator which has two different levels of threshold voltages.If the input voltage level is high, the output