# Electronics and Communication Engineering - Exam Questions Papers

6.
The impulse response of an initially relaxed linear system is e-2tu(t) . To produce a response te-2tU(t), the input must be equal to __________ .
e-2t U(t)
e-lt U(t)
e-1u(t)
Explanation:

Given, h(t) = e-2tu(t) H(s) =

y(t) = te-2t, u(t) y(s) =

Using Laplace Transform

x(t) = e-2tu(t)

Input must be equal to e-2tu(t).

7.
The continuous time system with impulse response h(t) u(t) is stable, if __________ (n is even).
a = 0
a > 0
a < 0
none of these
Explanation:

h(t) =

consider

ax = x

ax = x2

a = x

For h(t) to be bounded, exponential term should have a negative power.

i.e. e-t will have response as shown.

Hence should be < 0

a < 0.

8.
Consider the following diagrams showing vector fields :
The correct statements are
For figure (a), ∇ . = 0, ∇ x = 0
For figure (a), ∇ . = 0, ∇ x ≠ 0
For figure (b), ∇ . = 0, ∇ x ≠ 0
For figure (c), ∇ . = 0, ∇ x ≠ 0
Explanation:

In figure

Fx = 0 hence div F = ∇ . = 0, but curl F ≠ 0.

In figure

magnitude of F remains constant and hence ∇ . = 0 and also it is possible that ∇ . = 0, as there can be an electrostatic field in which and circulation around any closed path in it may be equal to zero.

In Figure

∇ x = 0, ∇ ≠ 0.

9.
In the signal flow graph of figure, y/x equals.
3
5/2
2
None of the above
Explanation:

P1 = 5 x 2 x 1 = 10

L1 = - 4

.

10.
Find resistance RB to bring transistor to threshold of saturation (VCB = 0), VBE = 0.7 V, a = 0.96
1.5 KΩ
15 KΩ
10 KΩ
1 KΩ