Digital Electronics: Principles, Devices and Applications

90 Digital ElectronicsABCD(AB+CD+EF+GH)EFGHFigure 4.31Four-wide, two-input AND-OR-INVERT gate.varying input. Figure 4.32 shows the response of an inverter circuit when fed with a slow varyinginput both in the case of an ideal signal [Fig. 4.32(a)] and in the case of a practical signal havinga small amount of AC noise superimposed on it [Fig. 4.32(b)]. A possible solution to this problemlies in having two different threshold voltage levels, one for LOW-to-HIGH transition and the otherfor HIGH-to-LOW transition, by introducing some positive feedback in the internal gate circuitry, aphenomenon called hysteresis.There are some logic gate varieties, mainly in NAND gates and inverters, that are availablewith built-in hysteresis. These are called Schmitt gates, which interpret varying input voltagesaccording to two threshold voltages, one for LOW-to-HIGH and the other for HIGH-to-LOWoutput transition. Figures 4.33(a) and (b) respectively show circuit symbols of Schmitt NAND andSchmitt inverter. Schmitt gates are distinguished from conventional gates by the small ‘hysteresis’symbol reminiscent of the B − H loop for a ferromagnetic material. Figure 4.33(c) shows typicaltransfer characteristics for such a device. The difference between the two threshold levels isOutputVoltageInputThreshold(a)timeFigure 4.32Response of conventional inverters to slow varying input.