# RTU Previous Question Papers BE EX 4th Semester

# Digital Electronics July-2011

**UNIT – I**

1. (a) Find the radix of the given number system.

(i) (23)_{x} +(12)_{8} =(1B)_{16}

(ii) (110)_{x} + (15)_{x} = (29)_{x}

(b) List the major advantages of digital electronics over analog electronics.

(c) What is the advantages of using Gray code ? Explain any one area of its application with the help of a block diagram.,

**OR**

1. (a) Realize the following expressions :

(i) Y = (abc + bc)c using NOR gates only

(ii) Y = (AB + BC)C using NAND gates only

(b) What is reflective code? With the help of an example explain the difference between reflective and sequential code.

(c) A seven bit hamming code is received as 1101011 is the received code correct? If not then find the correct code which was transmitted.

**UNIT – II**

2. (a) Why CMOS IC’s are preferred for battery operated function?

(b) Explain the role of diode D in the output circuit of totem – pole output gate. What will happen if it is removed from the circuit?

(c) Compare different parameters of TTL and CMOS logic families.

**OR**

2 (a) What parameters have to be taken care while driving a CMOS gate by a TTL gate ?

(b) Why ECL family is the fastest logic family? Explain.

(c) Explain the interfacing concept of TTL gate driving a CMOS gate with equal power supply voltage.

**UNIT-III**

3. (a) Minimize the following equation using K – map. Realize the output equation using NAND gates only.

Y = M _{0},M _{2},M _{4},M _{6},M _{7}

(b) Find the minimal expression using Quine-Mc Cluskey method. f (A,B,C,D) = ]jjT w (1,5,6,12,13,14) + ti(2,4)

(c) Design a digital circuit which accepts inputs between 0 to 9 and generates logic T if the input is an odd number and ‘O’ if the input is even number.

(d) If W(.4,S,C,Z)) w(0,1,2,3,4,6,7,10,11,12,15) X(A, B,C, m (0,1,2,5,6,9,10,11,12,13,14)

Find Y = W and realize the output equation using NAND gates only.

**UNIT – IV**

4 (a) Explain in easy steps how a full subtractor can be implemented using 4:1 multiplexers.

(b) Implement 3 – input OR and Ex OR gate using 2 : 1 multiplexers.

**OR**

4. (a) With the help of neat circuit diagram explain the experimental setup of BCD to seven segment decoder process using 7447 IC. Explain how leading zero’s are eliminated in multi-digit display.

(b) Explain in easy steps how a full adder can be implemented using 1:4 demultiplexers. _

**UNIT – V**

5. (a) With the help of an excitation table explain how a D – flip – flop can be converted into T flip – flop.

(b) What is a ring counter ? Where it is used ? If in a 4 – bit ring counter due to false triggering an extra bit is initiated in the ring, say 1001. Explain how this extra bit will be eliminated from the ring counter.

**OR**

5. (a) With the help of an excitation table, explain how a 3 – bit synchronous up counter can be designed using T – flip – flops.

(b) With the help of simple circuit diagram explain the difference in working of asynchronous and synchronous counter.