# Electronics and Communication Engineering - Exam Questions Papers

16.
1. Hall coefficient RH is directly proportional to the charge density
2. RH depends upon type of the semiconductor material (n - type or p - type)
3. Hall co-efficient RH is directly proportional to hall voltage VH
From the above statements which are the false statements?
I only
II only
III only
all of the above
Explanation:

where r is charge density

and RH = + ve, then the material is p - type

= - ve, then the material is n - type

RH ∝ VH where VH is Hall voltage.

17.
The equivalent form of the logical expression (ABC + ABC + ABC + ABC + ABC) is:
C + (A ⊕ B)
(A + B + C)(A + B + C)
(A + B + C)(A + B + C)(A + B + C)
(A + B + C)(A + B + C)(A + B + C)
Explanation:

K-map for the expression:

18.
In a series operated thyristor configuration, each thyristor should support 100 V. However, the three thyristors only support voltage of 100 V, 90 V and 85 V respectively. Hence, the string efficiency in the case is :
98.45%
45.84%
91.67%
none of these
Explanation:

String efficiency =

= = 0.9167 = 91.67%.

19.
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
0
0.3
0.5
0.8
Explanation:

Thus reflection coefficient = = - 0.5

Thus magnitude is 0.5 .

20.
A speech signal, band limited to 4kHz and peak voltage varying between + 5V and - 5V is sampled at the Nyquist rate. Each sample is quantized and represented by 8 bits. If the bits 0 and 1 are transmitted using bipolar pulses, the minimum bandwidth required for distortion free transmission is
64 kHz
32 kHz
8 kHz
4 kHz