Appendix G: Structure of Logic Gates

It was mentioned in Chapter 22 that all the digital electronics components are made out of analog electronic circuits, such as transistors. Construction of some of the logic gates are shown in Figures G.1 and G.2. These are the simplest designs, which are based on transistor-transistor logic. Other designs are also possible. Furthermore, the basic designs can be enhanced by amplification.

Figure G.3a depicts a simple transistor working as a NOT circuit. An enhancement of the same circuit with another transistor to amplify the input is shown in Figure G.3b. Also, another version of the NOR gate is illustrated in Figure G.3c. However, the previous one in Figure G.2b is preferable to this in the sense that the two inputs cannot have interaction with each other, so that one influences the other (when one is high and the other is low). A XOR gate is more involved and can be synthesized from other gates.

In practice, logic gates come as packages in the form of integrated circuit (IC). For example, the family of 74xx IC’s is a series of logic gates and other digital devices all defined by the number 74 and some other characters and numbers. Figure G.4 shows the inside structure and the pin-out of a sample called 7486, which contains four two-input XOR gates.

Figure G.1 Structure of (a) AND and (b) OR gates from transistors.

Figure G.2 Structure of (a) NAND and (b) NOR gates from transistors.

Figure G.3 (a) NOT gate, (b) NOT gate with input amplification, and (c) alternative NOR gate.

Figure G.4 Layout of four XOR gates in 7486 IC.