Digital Electronics and Logic Circuits
Digital Electronics and Logic Circuits tests Boolean algebra, logic gates, truth tables, simplification, and the behaviour of combinational and sequential blocks. Expect questions on gate equivalents, Karnaugh maps, flip-flops, counters and multiplexers.
Core rules
- AND: output 1 only when all inputs are 1. OR: output 1 when any input is 1. XOR: output 1 when inputs differ.
- NAND and NOR are universal gates; any logic function can be built from either.
- De Morgan: (A·B)' = A' + B'; (A+B)' = A'·B'.
- Combinational logic has no memory; sequential logic uses flip-flops and a clock.
- A D flip-flop copies D to Q on the active clock edge; a JK flip-flop toggles when J = K = 1.
- An n-bit counter has 2^n states; a multiplexer with n select lines routes 2^n inputs.
Worked example
Simplify F = A·B + A·B' + A'·B. First pair: A·B + A·B' = A(B + B') = A. Then F = A + A'·B. Apply A + A'B = A + B. So F = A + B, an OR gate. Check with A=0, B=1: original gives 0 + 0 + 1 = 1; simplified gives 1. Correct.
Traps
- Applying De Morgan without changing the operator: (A+B)' is not A' + B'.
- Forgetting that NAND is NOT-AND, so its truth table is inverted at the output.
- Mixing active-low signals: a bar over a name means the function is true when the signal is 0.
- Leaving CMOS inputs floating; they can oscillate and draw excess current.