Electronics and Communication questions reward anyone who is comfortable moving between the time domain and the frequency domain. This set covers network theory, analog and digital circuits, signals and systems, control systems, communication systems, and electromagnetics. Numerical solutions keep the units visible at every step, since a dropped factor is the most common reason a correct method still produces a wrong option.
What is the fundamental difference between combinational and sequential circuits?
Answer: B
Combinational circuits: output = f(present inputs only). Sequential circuits: output depends on present inputs AND past states (memory elements like flip-flops store history)
Q.142Medium
In a synchronous counter with n flip-flops, what is the maximum frequency at which the counter can operate?
Answer: B
In synchronous counters, all flip-flops are clocked simultaneously, so the maximum frequency is limited only by the propagation delay of a single flip-flop, not the cumulative delay.
Q.143Easy
Which of the following is a characteristic of asynchronous counters?
Answer: B
Asynchronous (ripple) counters work by cascading flip-flops where the output of one flip-flop triggers the next, creating a ripple effect.
Q.144Easy
A BCD counter counts from 0 to 9 and then resets. How many flip-flops are minimum required?
Answer: C
BCD (Binary Coded Decimal) requires 4 bits to represent decimal digits 0-9 (2^4 = 16 combinations, but only 10 are used).
Q.145Medium
In a Johnson counter with 4 flip-flops, what is the maximum count sequence length?
Answer: B
A Johnson counter (twisted ring counter) has a sequence length of 2n, where n is the number of flip-flops. For 4 flip-flops: 2×4 = 8 states.
Q.146Medium
Which logic gate is used in a full adder to generate the carry output?
Answer: C
The carry output of a full adder is: C_out = (A·B) + (B·C_in) + (A·C_in), which requires both AND and OR gates.
Q.147Medium
What is the setup time of a flip-flop?
Answer: B
Setup time (t_su) is the minimum duration that the input signal must be stable and valid before the active clock edge for reliable operation.
Q.148Easy
In a multiplexer (MUX) with 3 select lines, how many input lines can it handle?
Answer: C
A multiplexer with n select lines can handle 2^n input lines. For 3 select lines: 2^3 = 8 input lines.
Q.149Easy
A demultiplexer (DEMUX) with 4 select lines has how many output lines?
Answer: C
A demultiplexer with n select lines has 2^n output lines. For 4 select lines: 2^4 = 16 output lines.
Q.150Medium
In a 3-to-8 decoder, if the enable pin is active low and all select lines are 0, what will be the output?
Answer: C
In a 3-to-8 decoder with active low enable and select lines = 000, only output 0 is activated (becomes 0), while all others remain 1.
Q.151Easy
Which type of flip-flop has the simplest truth table and is used for frequency division?
Answer: B
A T flip-flop toggles its output when T=1 and holds when T=0, making it ideal for frequency division and counter design.
Q.152Medium
In Gray code, how many bits change when transitioning from 5 (0101) to 6 (0110)?
Answer: A
Gray code for 5 is 0111 and for 6 is 0101. Only 1 bit changes between consecutive numbers, which is the main advantage of Gray code.
Q.153Medium
A 32×8 ROM has how many address lines and data lines respectively?
Answer: A
For 32 memory locations, we need log2(32) = 5 address lines. The data width is 8 bits, hence 8 data lines.
Q.154Easy
Which of the following is a volatile memory?
Answer: C
RAM (Random Access Memory) is volatile and loses all data when power is removed. ROM, EPROM, and Flash memory are non-volatile.
Q.155Medium
A 4-bit Ripple Carry Adder has a total propagation delay of 12 ns. If each full adder stage has a propagation delay of 3 ns, what is the delay for carry propagation?
Answer: B
In a 4-bit ripple carry adder, the carry must propagate through 3 stages (from FA1 to FA4), giving delay = 3 × 3 ns = 9 ns.
Q.156Easy
Which of the following is a sequential circuit?
Answer: C
Sequential circuits have memory elements (flip-flops) and their output depends on past inputs. Flip-flops are the basic sequential elements; multiplexers, encoders, and decoders are combinational circuits.
Q.157Hard
In Hamming code for single error correction, if the total number of data bits is 16, how many parity bits are required?
Answer: C
For Hamming code, if 'p' is the number of parity bits, then 2^p ≥ (p + data bits). For 16 data bits: 2^p ≥ p + 16. Testing: 2^5 = 32 ≥ 5 + 16 = 21. ✓
Q.158Medium
What is the main advantage of a Carry Lookahead Adder (CLA) over a Ripple Carry Adder (RCA)?
Answer: B
CLA reduces propagation delay from O(n) to O(log n) by calculating carry signals in parallel, making it much faster than RCA.
Q.159Hard
A SRAM cell requires how many transistors for a 1-bit storage?
Answer: C
A typical SRAM cell uses 6 transistors: 4 for the cross-coupled latch (2 NMOS + 2 PMOS) and 2 access transistors for read/write operations.
Q.160Easy
In state machine design, if a machine has 15 different states, the minimum number of flip-flops needed is?
Answer: B
To represent 15 states, we need n flip-flops such that 2^n ≥ 15. Since 2^4 = 16 ≥ 15, minimum 4 flip-flops are required.
