Name: NOT Gate
Uploaded: 2019-07-19
Duration: 4 min 12 s
Description: A NOT gate gives us an inverted output of what we provide as input. Here in this video, let us discuss the analogy and expression for this gate.

Question

Identify which basic logic gates are represented by the circuits in dotted-line rectangles $1, 2$ and $3$ in the following complicated digital circuit. Give the truth table for the entire circuit.

Accepted Answer

The basic logic gates represented by dotted-line rectangles $1, 2$ and $3$ are NOT, OR and AND respectively. Let us now redraw the given circuit, replacing rectangles $1, 2$ and $3$ by the basic logic gates NOT, OR and AND respectively.

The single input of the NOT gate is $A$ and its output is $\bar{A}$ (negation of $A$ ). The inputs of the OR gate are $\bar{A}$ and $B$ , and its output is $\bar{A} + B$ , that is, $\bar{A}$ OR $B$ . The inputs of the AND gate are $\bar{A} + B$ and $B$ , and the final output $Y$ is $(\bar{A} + B) .$ , that is, $\bar{A}$ OR $B$ AND $B$ .

The truth table for the output $Y$ can be obtained, Using the truth tables of NOT, OR and AND gates as shown. Columns $A, B$ and $Y$ form the truth table of the given circuit.

$A$ $B$ $\bar{A}$ $\bar{A} + B$ $Y = (\bar{A} + B) . B$

$0$ $0$ $1$ $1$ $0$

$0$ $1$ $1$ $1$ $1$

$1$ $0$ $0$ $0$ $0$

$1$ $1$ $0$ $1$ $1$

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