This set of Digital Circuits Multiple Choice Questions & Answers (MCQs) focuses on “Fast Adder & Serial Adder – 1″.

1. One positive pulse with tw = 75 µs is applied to one of the inputs of an exclusive-OR circuit. A second positive pulse with tw = 15 µs is applied to the other input beginning 20 µs after the leading edge of the first pulse. Which statement describes the output’s relation with the inputs?
a) The exclusive-OR output is a 20 s pulse followed by a 40 s pulse, with a separation of 15 s between the pulses
b) The exclusive-OR output is a 20 s pulse followed by a 15 s pulse, with a separation of 40 s between the pulses
c) The exclusive-OR output is a 15 s pulse followed by a 40 s pulse
d) The exclusive-OR output is a 20 s pulse followed by a 15 s pulse, followed by a 40 s pulse

2. How many NOT gates are required to implement the Boolean expression: X = AB’C + A’BC?
a) 2
b) 3
c) 4
d) 5

3. The inverter can be produced with how many NAND gates?
a) 2
b) 1
c) 3
d) 4

4. The carry look ahead adder is based on the principle of looking at the lower order bits of ________ and ________ if a high order carry is generated.
b) Minuend, subtrahend

a) 3 bit per second
b) Byte by byte
c) Bit by bit
d) All bits at the same time

6. A D flip-flop is used in a 4-bit serial adder, why?
a) It is used to invert the input of the full adder
b) It is used to store the output of the full adder
c) It is used to store the carry output of the full adder
d) It is used to store the sum output of the full adder

7. What are carry generate combinations?
a) If all the input are same then a carry is generated
b) If all of the output are independent of the inputs
c) If all of the input are dependent on the output
d) If all of the output are dependent on the input

8. How many shift registers are used in a 4 bit serial adder?
a) 4
b) 3
c) 2
d) 5

9. What is ripple carry adder?
a) The carry output of the lower order stage is connected to the carry input of the next higher order stage
b) The carry input of the lower order stage is connected to the carry output of the next higher order stage
c) The carry output of the higher order stage is connected to the carry input of the next lower order stage
d) The carry input of the higher order stage is connected to the carry output of the lower order stage

10. If minuend = 0, subtrahend = 1 and borrow input = 1 in a full subtractor then the borrow output will be __________
a) 0
b) 1
c) Floating
d) High Impedance