Physics · Semiconductor Electronics : Materials, Devices And Simple Circuits · NEET
An AND gate gives Y = 1 only when every input is 1. If even one input is 0, the output is 0. Rule: Y = A.B, and the dot acts like multiplication (0.anything = 0, 1.1 = 1). Truth table for inputs (A B): 00 to 0, 01 to 0, 10 to 0, 11 to 1. So out of 4 rows, only the last is 1.
An OR gate gives Y = 1 when at least one input is 1. It is 0 only when all inputs are 0. Rule: Y = A+B, but this plus is logical OR, not normal addition: 1+1 = 1 (not 2). Truth table (A B): 00 to 0, 01 to 1, 10 to 1, 11 to 1. Only the first row is 0.
A NOT gate (inverter) has only ONE input. It simply flips the value: input 0 gives output 1, input 1 gives output 0. It is written Y = A-bar (A with a bar on top). The small bubble (circle) on a symbol always means inversion.
Almost. AND behaves exactly like multiplication of 0s and 1s: 0.0=0, 0.1=0, 1.1=1. OR behaves like addition EXCEPT 1+1 = 1 (there is no 2 in Boolean logic, outputs are only 0 or 1). This is the one place beginners slip: 1 OR 1 stays 1.
List all input combinations (2 inputs give 4 rows: 00, 01, 10, 11). Then apply the gate rule to each row. For AND look for the single 1 (bottom row). For OR look for the single 0 (top row). If the output column is exactly the opposite of a known gate, a NOT (bubble) is involved.
A logic circuit gives the output Y as per the truth table: for inputs (A B) = 00 Y=1, 01 Y=0, 10 Y=1, 11 Y=0. The expression for the output Y is:
An output Y (an LED) is driven so that it glows (logic 1) when either input A or input B is 1, and stays off (logic 0) only when both A and B are 0. The Boolean operation represented is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Four. With n inputs there are 2^n rows because each input can be 0 or 1. Two inputs give 2^2 = 4 rows: 00, 01, 10, 11. Three inputs would give 8 rows.
AND is a flat-back D shape. OR is a curved-back shield shape with a pointed nose. NOT is a triangle with a small circle (bubble) at its tip. The bubble always means inversion.
AND: Y = A.B. OR: Y = A + B. NOT: Y = A-bar. Here the dot means AND, the plus means OR, and the bar on top means NOT (inversion).
The NOT gate. AND and OR need at least two inputs to combine, but NOT works on a single input and just reverses it (0 becomes 1, 1 becomes 0).
NEET asks 1 question almost every year from Semiconductor Electronics on gates: identifying a gate from a circuit, completing a truth table, or writing the Boolean output. Knowing the three basic gates plus NAND and NOR covers all of them.
