# 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 __________ .

 A. e-2t U(t) B. e-lt U(t) C. e-1u(t) D. 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. a = 0 B. a > 0 C. a < 0 D. 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 A. For figure (a), ∇ . = 0, ∇ x = 0 B. For figure (a), ∇ . = 0, ∇ x ≠ 0 C. For figure (b), ∇ . = 0, ∇ x ≠ 0 D. 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 has a central field (radial).

∇ x = 0, ∇ ≠ 0.

9.

In the signal flow graph of figure, y/x equals. A. 3 B. 5/2 C. 2 D. 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 A. 1.5 KΩ B. 15 KΩ C. 10 KΩ D. 1 KΩ

Explanation: = 0.07 A = 24 = 3 mA = 1433.33 Ω ≅ 1.5 KΩ.

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