Logic Gates
AND, OR, NOT, NAND, NOR, XOR — toggle A/B and watch output LED + truth table.
Key Notes
Logic gates are circuits whose output depends on Boolean inputs (HIGH/1 or LOW/0). Building blocks of all digital systems.
Basic gates: AND, OR, NOT. Universal gates: NAND, NOR (any logic can be built from either alone).
Truth tables define each gate's behaviour. AND: output 1 only if all inputs 1. OR: output 1 if any input 1. NOT: inverts input.
XOR (exclusive OR): output 1 when inputs differ. XNOR: complement of XOR.
DeMorgan's laws: (A·B)' = A' + B' and (A+B)' = A'·B'. Lets you convert AND into OR (with inversions) and vice versa.
CMOS technology builds all logic from complementary p-MOS and n-MOS pairs — very low static power consumption.
Standard CMOS voltages: V_DD = 3.3 V or 5 V; logic 0 < ~0.8 V, logic 1 > ~2.0 V (TTL levels).
Combinational logic: output depends only on current inputs (adders, multiplexers). Sequential logic: output also depends on past (flip-flops, registers, memory).
Formulas
AND
1 only when both inputs are 1.
OR
1 when at least one input is 1.
NOT (inverter)
Output is the inverse of input.
NAND
Universal — any Boolean function can be built from NANDs.
NOR
Universal — any Boolean function can be built from NORs.
XOR
1 when exactly one input is 1.
Important Points
TRUTH TABLE is the unambiguous definition of a gate. Memorise the basic four.
NAND and NOR are UNIVERSAL: any digital circuit can be built using only NANDs (or only NORs).
AND: output 0 unless all 1 — like a 'series of switches'. OR: output 1 unless all 0 — 'parallel'.
XOR is a sum mod 2 — used in binary adders, parity checks, encryption.
Sequential logic uses gates + feedback to STORE state — flip-flops are the basic memory cell.
Real gates have propagation delay (~ns), so 'instantaneous' transitions are an idealisation.
Logic Gates notes from sciphylab (also known as SciPhy, SciPhy Lab, SciPhy Labs, Physics Lab). Class 12 physics revision for JEE Mains, JEE Advanced, NEET UG, AP Physics 1/2/C, SAT, and CUET-UG.