2-18Up to now you have seen inverters used alone or on the output of AND and OR gates. Inverters mayalso be used on one or more of the inputs to the logic gates. Take a look at the examples as discussed inthe following paragraphs.AND/NAND GATE VARIATIONSIf we place an inverter on one input of a two-input AND gate, the output will be quite different fromthat of the standard AND gate.In figure 2-17, we have placed an inverter on the A input. When A is HIGH, the inverter makes it aLOW going into the AND gate. In order for the output to be HIGH, A would have to be LOW while B isHIGH, as shown in the Truth Table. If the inverter were on the B input, the output expression would thenbe f = A B .Figure 2-17. —AND gate with one inverted input.Now let’s compare a NAND gate to an AND gate with an inverter on each input. Figure 2-18 showsthese gates and the associated Truth Tables. With the NAND gate (view A), the output is HIGH wheneither or both inputs is/are LOW. The AND gate with inverters on each input (view B), produces a HIGHoutput only when both inputs are LOW. This comparison also points out the differences between theexpressions f = A B (A AND B quantity NOT) and f = A B (NOT A AND NOT B).Now, look over the Truth Tables for figures 2-17, 2-18, and 2-19; look at how the outputs vary withinverters in different positions.
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