Electronics and Communication Engineering - Exam Questions Papers

Exercise : Exam Questions Papers - Exam Paper 7

21.

A Hartley oscillator uses FET with g_m of 2 ms and r_d = 2 KΩ. The total coil inductance is 10 μH with turns ratio of input to output side of 1 : 10. It is tuned with a 50 pf capacitor. Find frequency of oscillations, amplifier gain margin in dB.

7.11 MHz, 13.97 dB

7.3 MHz, 14.1 dB

6.93 MHz, 12.9 dB

7.4 MHz, 13 dB

Answer: Option

Explanation:

= 7.11 MHz

μ = g_m x r_d

22.

The recursion relation to solve x = e^-x using Newton Raphson method is

X_n+1 = e^-x_n

X_n+1 = X_n - e^-x_n

Answer: Option

Explanation:

f(x_n) = x_ne^-xn

f(x_n) = 1 + e^-x_n f'(x_n) = 1 + e^-x_n

23.

In the circuit shown in the given figure the diode states at the extremely large negative values of the input voltage v_i are :

D₁ off, D₂ off

D₁ on, D₂ off

D₁ off, D₂ on

D₁ on, D₁ on, D₂ on

Answer: Option

Explanation:

Input V₁ is extremely large negative value, so D₁ is ON.

Diode D₂ is shunted by R₃ and the junction of R₂R₃ is more negative than the lower end.

Diode D₂'s cathode is at negative potential.

But the anode of D₂ is less negative than E as the input voltage is extremely negative and D₂ is OFF.

24.

Consider a baseband binary PAM receiver shown below. The additive channel noise n(t) is whit with power spectral density S_N(f) = N₀/2 = 10^-20 W/Hz. The low-pass filter is ideal with unity gain and cutoff frequency 1 MHz. Let Y_k represent the random variable y(t_k).
Y_k = nN_k if transmitted bit b_k = 0
Y_k = a + N_k if transmitted bit b_k = 1
Where N_k represents the noise sample value. The noise sample has a probability density function, P_Nk(n) = 0.5ae^-a|n| (This has mean zero and variance 2/a²). Assume transmitted bits to be equiprobable and threshold z is set to a/2 = 10^-6 V.