(SEM III) THEORY EXAMINATION 2024-25 DIGITAL ELECTRONICS

This examination evaluates a student’s understanding of number systems, Boolean algebra, combinational and sequential circuits, logic families, memory technologies, hazards, counters, and digital system design concepts. The paper is structured to test theoretical clarity, analytical reasoning, and design-oriented problem-solving skills.

The paper is divided into three sections, covering everything from basic definitions to circuit-level design and Boolean simplification.

SECTION A – Short Answer Questions (14 Marks)

This section consists of seven compulsory short questions, each testing essential fundamentals of digital electronics.

Key Concepts Covered

• SOP and POS Forms

Understanding Sum of Products and Product of Sums representations in Boolean algebra.

• Universal Gates

NAND and NOR gates — their ability to realize any Boolean function.

• Magnitude Comparator

Purpose of comparing binary numbers to check greater, smaller, or equal conditions.

• Synchronous vs Asynchronous Counter

Clocking mechanism differences and reasons synchronous counters are faster and reliable.

• Essential Hazards

Signal transition problems in asynchronous circuits and stability issues.

• Fan-Out

Maximum number of inputs a gate can drive without performance loss.

• Lowest Power Logic Family

CMOS logic family known for minimal power consumption.

This section tests conceptual clarity and precise technical definitions.

SECTION B – Application-Based Questions (21 Marks)

(Attempt any three questions)

This section includes numerically oriented, conceptual, and small design-based questions.

Topics Include:

• Number System Conversions

Decimal ↔ Binary
Binary ↔ Decimal
Hexadecimal ↔ Decimal
Octal ↔ Binary
Ensuring accuracy in fractional and integer conversions.

• Multiplexed Display Systems

Use of time-division multiplexing in seven-segment displays, LED matrices, and embedded display modules.

• Flip-Flop Conversion

Converting JK → D flip-flop using logic equations and circuits.

• Race-Free State Assignment

Ensuring predictable behavior in asynchronous circuits by preventing race conditions.

• Programmable Logic Array (PLA)

Structure (AND-plane, OR-plane), programmability, and implementation of logic functions.

This section tests ability to explain working concepts, design procedures, and practical applications.

SECTION C – Long Analytical / Design Questions (35 Marks)

(One question from each sub-section)

C1 – Boolean Simplification & Code Converters

Option A – Design an Excess-3 to BCD Converter
Creating truth tables, deriving Boolean expressions, and designing the final logic circuit.

Option B – K-Map Simplification + Circuit Design
Using Karnaugh map to simplify the function:
f(A,B,C,D) = Σm(0,1,5,6,12,13,14) + d(2,4)
Then drawing the minimized logic circuit using basic gates.

C2 – Arithmetic Circuits

Option A – BCD Adder Working
Step-by-step addition, correction using +6, and comparison with binary adders.

Option B – Half Subtractor
Truth table, Boolean expressions, and circuit diagram for difference and borrow.

C3 – Counters

Option A – 4-bit Synchronous Counter
Clocking, flip-flop excitation, circuit diagram, and operation.

Option B – Johnson Counter
Operation, timing diagram, and practical applications.

C4 – Sequential Circuit Concepts

Option A – State Assignment Significance
Why binary codes must be assigned carefully; different state assignment techniques (one-hot, binary, Gray code).

Option B – Synchronous vs Asynchronous Circuits
Clocking differences, speed, hazards, examples of each.

C5 – Memory & Logic Families

Option A – SRAM vs DRAM
Internal cell structure, working principle, access time difference, refresh mechanism.

Option B – TTL Logic Gates
Transistor-based implementation, totem-pole output, current characteristics, and working.

Purpose of This Examination

This exam ensures that students can:

Understand the fundamentals of digital number systems and Boolean algebra

Design combinational circuits (adders, subtractors, converters, PLAs, decoders)

Implement sequential circuits including counters, flip-flops, and state machines

Perform Boolean simplification using K-maps

Understand logic families, hazards, and memory architecture

Analyze and compare synchronous and asynchronous systems

Interpret timing diagrams and circuit behavior

Apply digital electronics knowledge to real-world system design

This exam evaluates both theoretical grounding and practical digital design skills expected of a B.Tech (CSE/IT/ECE/EEE) student.