2-6
THE AND GATE
The AND gate is a logic circuit that requires all inputs to be TRUE at the same time in order for the
output to be TRUE.
LOGIC SYMBOL
The standard symbol for the AND gate is shown in figure 2-2. Variations of this standard symbol
may be encountered. These variations become necessary to illustrate that an AND gate may have more
than one input.
Figure 2-2. —AND gate.
If we apply two variables, A and B, to the inputs of the AND gate, then both A and B would have to
be TRUE at the same time to produce the desired TRUE output. . The symbol f designates the output
function. The Boolean expression for this operation is f = A·B or f = AB. The expression is spoken, "f =
A AND B." The dot, or lack of, indicates the AND function.
AND GATE OPERATION
We can demonstrate the operation of the AND gate with a simple circuit that has two switches in
series as shown in figure 2-3. You can see that both switches would have to be closed at the same time to
light the lamp (view A). Any other combination of switch positions (view B) would result in an open
circuit and the lamp would not light (logic 0).
Figure 2-3.—AND gate equivalent circuit: A. Logic 1 state; B. Logic 0 state.
Now look at figure 2-4. Signal A is applied to one input of the AND gate and signal B to the other.
At time T0, both inputs are LOW (logic 0) and f is LOW. At T1, A goes HIGH (logic 1); B remains LOW;
and as a result, f remains LOW. At T2, A goes LOW and B goes HIGH; f, however, is still LOW, because
the proper input conditions have not been satisfied (A and B both HIGH at the same time). At T4, both A
