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?

A. I only
B. II only
C. III only
D. all of the above

Answer: Option A

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:

A. C + (A ⊕ B)
B. (A + B + C)(A + B + C)
C. (A + B + C)(A + B + C)(A + B + C)
D. (A + B + C)(A + B + C)(A + B + C)

Answer: Option D

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 :

A. 98.45%
B. 45.84%
C. 91.67%
D. none of these

Answer: Option C

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

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

Answer: Option C

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

A. 64 kHz
B. 32 kHz
C. 8 kHz
D. 4 kHz

Answer: Option D

Explanation:

According to Nyquist Criteria, minimum sampling frequency = 2 x 4 k = 8 kHz

But each cycle can accommodate two bits

Thus minimum frequency = 4 kHz.