# Online Electronics and Communication Engineering Test - Previous Exam Papers Test 7

Instruction:

• This is a FREE online test. DO NOT pay money to anyone to attend this test.
• Total number of questions : 20.
• Time alloted : 30 minutes.
• Each question carry 1 mark, no negative marks.
• DO NOT refresh the page.
• All the best :-).

1.

The amplifier circuit shown below uses a silicon transistor. The capacitors Cc and CE can be assumed to be short at signal frequency and the effect of output resistance r0 can be ignored. If CE is disconnected from the circuit, which one of the following statements is TRUE?

A.
 The input resistance Ri increases and the magnitude of voltage gain Av decreases
B.
 The input resistance Ri decreases and the magnitude of voltage gain Av decreases
C.
 Both input resistance Ri and the magnitude of voltage gain Av decrease
D.
 Both input resistance Ri and the magnitude of voltage gain Av increase

Explanation:

Given circuit after removing CE will behave as current-series feedback.

Overall voltage gain will decrease as feedback signal comes into picture and since it is current-series feedback, input impedance increases.

2.

With a line charge along z-axis the field intensity is E = ; which of the following figure shows an exact field distribution?

A.
B.
C.
D.

Explanation:

The given figure fails to show the symmetry with respect to Φ.

The figure shows symmetry with respect to f, also the length of arrow is decreasing away from the charge shows that magnitude E is decreasing away from line charge.

But problem with this figure is longest lines must be shown in most crowded region.

Here we use lines of fixed segments but different thickness. But this attempt also makes the region crowded near origin.

This figure shows compromise. A symmetrical distribution of lines (at every 45°) shows azimuthal symmetry and arrowheads show direction.

3.

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.
 a is positive and β is positive
B.
 a is negative and β is negative
C.
 a is positive and β is negative
D.
 a is negative and β is positive

Explanation:

h(t) = e+atu(t) + eβtu(- t)

For h(t) to be stableh(t) dt < ∞

It will happen when a is negative and β is positive.

4.

The dual transform of the given network as

A.
B.
C.
D.
 none of the above

Explanation:

Dual of R → G

5.

The following statements are true with respect to "Cascode Amplifier".

1. The reduction in h12 and Miller capacitance result in negligible reverse internal feedback.
2. A's is large loads cannot be connected.
3. As a narrow band amplifier, it provides small range of cμ is reduced.
4. As a narrow band amplifier it provides small range of frequencies parameters the effect of cμ reduces.

A.
 only 1
B.
 only 1 and 4
C.
 only 2 and 4
D.
 only 1 and 3

Explanation:

Cascode is a wide band amplifier and it provides wide range of frequencies than CE stage and as is large, large loads can be connected.

6.

For a given transfer function: Damping ratio is:

A.
 0.707
B.
 0.695
C.
 1.414
D.
 0.798

Explanation:

1 + G(s)H(s) = s2 + 2s + 2

ωn = 2 , 2εeωn = 2, εe = = 0.707.

7.

If the open-loop transfer function is a ratio of a numerator polynomial of degree 'm' and a denominator polynomial of degree 'n', then the integer (n - m) represents the number of:

A.
 breakaway points
B.
 unstable poles
C.
 separate root loci
D.
 asymptotes

Explanation:

Asymptotes.

8.

The expression given is F(A, B , C, D, E, F, G, H) = (33, 48, 102, 158, 222, 229, 233, 243, 255); then the __________ and __________ minterms are grouped together into dual and variable __________ is reduced using the dual

A.
 243, 233, D
B.
 222, 158, B
C.
 243, 255, A
D.
 229, 243, E

Explanation:

We use Quine - McCluskey method of reduction :

We can observe that minterms 158 and 222 can be grouped together to reduce the variable 'B'.

9.

A uniform plane wave in the free space is normally incident on an infinitely thick dielectric slab (dielectric constant εr = 9). The magnitude of the reflection coefficient is

A.
 0
B.
 0.3
C.
 0.5
D.
 0.8

Explanation:

Thus reflection coefficient = = - 0.5

Thus magnitude is 0.5 .

10.

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.

11.

A white noise X(t) with two-sided power spectral density 1 x 10-10 W/Hz is input to a filter whose magnitude squared response is shown below.

The power of the output process Y(t) is given by

A.
 5 x 10-7W
B.
 1 x 10-6W
C.
 2 x 10-6W
D.
 5 x 10-6W

Explanation:

Since bandwidth is 10 kHz, thus output power is 10 x 10-11 x 10 x 103 = 1 x l0-6 W.

12.

A germanium diode has a reverse saturation current of 30 μA at 125°C and forward current of 1 mA at a forward potential of 0.8 V and the reverse potential is 0.6 V What are its dynamic forward and reverse resistance at this temperature.

A.
 10.7 Ω, 70 kΩ
B.
 34 Ω, 20 kΩ
C.
 80 Ω, 20 kΩ
D.
 800 Ω, 40 kΩ

Explanation:

We have,

I = I0•(eV/nVT -1)

For forward voltage greater than few teeth's of volts, I >> I0

Forward dynamic resistance,

= 34.3 Ω

Reverse dynamic resistance = = 20 kΩ.

13.

In the circuit below, the diode is ideal. The voltage V is given by

A.
 min(Vi, 1)
B.
 max(Vi, 1)
C.
 min (-Vi, 1)
D.
 max (-Vi, 1)

14.

The correct sequence of subsystems in an FM receiver is

A.
 Mixer, RF amplifier, Limiter, IF amplifier. Discriminator, Audio amplifier
B.
 RF amplifier, Mixer, IF amplifier, Limiter, Discriminator, Audio amplifier
C.
 RF amplifier, Mixer, Limiter, Discriminator IF amplifier, Audio amplifier
D.
 Mixer, IF amplifier, RF amplifier, Audio amplifier, Discriminator

15.

A fully charged mobile phone with a 12 V battery is good for a 20 minute talktime. Assume that, during the talk time, the battery delivers a constant current of 2 A and its voltage drops linearly from 12 V to 10 V as shown in the figure. How much energy does the battery deliver during this talk time?

A.
 220 J
B.
 12 kJ
C.
 13.2 kJ
D.
 14.4 kJ

16.

An audio signal is to be transmitted digitally. Which is the system best suited for good fidelity?

A.
 8 bit PCM
B.
 13 bit PCM
C.
 32 bit PCM
D.
 PCM system with non-uniform quantizer

17.

When transistor is used as a switch __________ is defined as the interval which elapses between the transition of input waveform and the time when to transition to 90% of Ic(sat).

A.
 storage time
B.
 fall time
C.
 turn-on time
D.
 turn-off time

18.

For the circuit of the following figure, with an ideal op-amp, the maximum phase shift of the output with reference to the input is :

A.
 0°
B.
 90°
C.
 -90°
D.
 ± 180°

19.

The junction capacitance of a linearly graded pn junction (with applied voltage = VB) is proportional to

A.
B.
C.
D.

20.

Assertion (A): An unbiased p-n junction develops a built-in potential at the junction with the n-side positive and the p-side negative.

Reason (R): The p-n junction behaves as a battery and supplies current to a resistance connected across its terminals.

A.
 Both A and R are individually true and R is the correct explanation of A
B.
 Both A and R are individually true but R is not the correct explanation of A
C.
 A is true but R is false
D.
 A is false but R is true