Digital Electronics: Principles, Devices and Applications

Logic Gates and Related Devices 91VoltageOutputInputThresholdtime(b)Figure 4.32(continued).the hysteresis. These characteristics have been reproduced from the data sheet of IC 74LS132,which is a quad two-input Schmitt NAND belonging to the low-power Schottky TTL family.Figure 4.33(d) shows the response of a Schmitt inverter to a slow varying noisy input signal.We will learn more about different logic families in Chapter 5. It may be mentioned here thathysteresis increases noise immunity and is used in applications where noise is expected on inputsignal lines.4.9 Special Output GatesThere are many applications where it is desirable to have both noninverted and inverted outputs.Examples include a single-input gate that is both an inverter and a noninverting buffer, or a two-inputlogic gate that is both an AND and a NAND. Such gates are called complementary output gates andare particularly useful in circuits where PCB space is at a premium. These are also useful in circuitswhere the addition of an inverter to obtain the inverted output introduces an undesirable time delaybetween inverted and noninverted outputs. Figure 4.34 shows the circuit symbols of complementarybuffer, AND, OR and EX-OR gates.Example 4.11Draw the circuit symbols for (a) a two-wide, four-input OR-AND-INVERT gate and (b) a four-wide,two-input OR-AND-INVERT gate.Solution(a) Refer to Fig. 4.35(a).(b) Refer to Fig. 4.35(b).