Page 1076 - Basic Electrical Engineering
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Figure 16.1 (a) Circuit for a positive logic NOT gate; (b) symbol of a NOT gate; (c) truth table of a
NOT circuit
Figure 16.2 (a) Logic OR gate circuit using diodes; (b) symbol of an OR gate; (c) truth table
16.5.2 OR Gate
The OR gate provides an output 1 when one of its inputs is in 1 state. That is,
its output is 1 whenever at least one of its inputs is 1. The output of an OR
gate is 0 only when all its inputs are 0. The circuit using diodes for such a
logic has been shown in Fig. 16.2 (a). The standard symbol for an OR gate
and its truth table have also been shown along with the figure.
Output will occur when there is input in any one of the inputs A or B or at
both. If A = B = 0, then the two diodes D and D will be reverse biased, and
1
2
hence X output, X will be zero. If A = 1 and B = 0 or if A = 0 and B = 1, i.e.,
when any one of the inputs is high, the output, X will be 1. If A = 1 and B =
