Electronics and Communication Engineering - Exam Questions Papers

Exercise : Exam Questions Papers - Exam Paper 2

31.

If a signal f(t) has energy E, the energy of the signal f(2t) is equal to __________ .

E/2

E/4

Answer: Option

Explanation:

Energy content of a signal x(t), E = |f(t)|² dt

Now, E^' = |f(2t)|² dz for signal f(2t)

Putting 2t = z, we get

E^' = |f(t)|² dz = .

32.

For a npn BJT transistor f_β is 1.64 x 10⁸ Hz. C_μ = 10^-14 F; C_p = 4 x 10^-13 F and DC current gain is 90. Find f_T and g_m (f_β = cut off frequency, C_μ = capacitance, C_p = parasitic capacitance, g_m = transconductance, f_T = gain BW product)

f_T = 1.47 x 10¹⁰ Hz ; g_m = 38 milli mho

f_T = 1.64 x 10⁸ Hz ; g_m = 30 milli mho

f_T = 1.47 x 10⁹ Hz ; g_m = 38 mho

f_T = 1.33 x 10¹² Hz; g_m = 0.37 m-mho

Answer: Option

Explanation:

∴ f_T = 90 x 1.64 x 10⁸ = 1.47 x 10¹⁰ Hz

g_m = 2pf_T(C_μ + C_p) = 2 x p x 1.47 x 10¹⁰

= (10^-14 + 4 x 10^-13)

g_m= 38 mμ.

33.

The impulse response h(t) of a linear time-invariant continuous time system is described by h(t) = exp (at) u(t) + exp (βt) u (- t), where u(t) denotes the unit step function, and a and β are constants. This system is stable if

a is positive and β is positive

a is negative and β is negative

a is positive and β is negative

a is negative and β is positive

Answer: Option

Explanation:

h(t) = e^+atu(t) + e^βtu(- t)

For h(t) to be stableh(t) dt < ∞