Logic Gates

Abstract

In the previous chapter the inverter was described, which is the basic building block for making logic circuits. A schematic diagram of a NOR gate is shown in figure 5.1, with transistor T₃ as the load transistor. If a high voltage is applied on input A, transistor T₁ conducts so that the output voltage is low. Thus, the output has the state of logical zero. If B is changed there is no change of logic state. As the inputs A and B are identical, the logical function of the circuit may be written $$ . One may ask how the performance of this circuit differs from that of an inverter. When either A or B is logical one the circuit functions like an inverter, only one transistor conducting. The same is true when A or B is logical zero. If both A and B are logical one the two transistors T₁ and T₂ conduct in parallel. In general, the two transistors T₁ and T₂ are identical and may be considered as equivalent to one transistor of double the width. Thus the parameter α (see page 47) is doubled.