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UPPSC-LECTURER’21 EE: Daily Practices Quiz 06-Oct-2021

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UPPSC-LECTURER’21 EE:  Daily Practices Quiz

UPPSC-LECTURER’21 EE: Daily Practices Quiz 06-Oct-2021

Each question carries 3 marks.
Negative marking: 1 mark each or 1/3rd
Total Questions: 06
Time: 08 min.

Q1. For a JFET IDSS = 8 mA and peak voltage Vp = –8V, what will be the drain current for gate to source voltage of –2V?
(a) 4.5 mA
(b) 8 mA
(c) 16 mA
(d) 12 mA

Q2. The plot of modulation index versus carrier amplitude for an amplitude modulated signal yields
(a) Horizontal line
(b) Vertical line
(c) Parabola
(d) Hyperbola

Q3. Gain cross-over frequency is defined as one at which |G(jω) H(jω)| equals
(a) 0
(b) 1
(c) √2
(d) 1/√2

Q4. A MOSFET has………terminals.
(a) Two
(b) Five
(c) Four
(d) Three

Q5. Which of the following is not Maxwell’s equation?
(a) ∆×E= -∂B/∂t
(b) ∆.E= -B
(c) ∆.D=ρ
(d) ∆.B= 0

Q6. A transformer has a core loss of 64 W and copper loss of 144 W, when it is carrying 20 % overload current. The load at which this transformer will operate at the maximum efficiency……
(a) 120 %
(b) 80 %
(c) 44 %
(d) 66 %

SOLUTIONS

S1. Ans.(a)
Sol. ID=IDSS (1-VGS/VP )^2=8×10^(-3) (1-(-2)/(-8))^2=4.5 mA

S2. Ans.(d)
Sol. For an amplitude modulated signal, modulation index(μ)=Am/Ac
So, plot of μ v/s Ac will be hyperbola. (Graph similar to y=1/x).

S3. Ans.(b)
Sol. At gain cross-over frequency, the magnitude of G(jω) is 1.
Or at ω=ω_gc; |G(jω) H(jω)|=1
Phase margin is calculated at gain cross-over frequency.

S4. Ans.(d)
Sol.

UPPSC POLYTECHNIC LECTURER RECRUITMENT 2021, UPPSC LECTURER ELECTRICAL QUIZ, |_30.1

Schematic symbol of n-channel MOSFET

UPPSC POLYTECHNIC LECTURER RECRUITMENT 2021, UPPSC LECTURER ELECTRICAL QUIZ, |_40.1
S6. Ans.(b)
Sol. At 20% overload current (i.e.,I=1.2I_fl) & copper loss=144 W
As copper loss(P_cu ) α I^2
∴Copper loss at full-load, Pcu(fl) =144/(1.2)^2 =100 W
Core loss or constant loss=64 W
Load at which efficiency is maximum,
∴η_max=rated KVA×√(Pi/Pcu(fl) )=rated KVA×√(64/100)=0.8×rated KVA
∴At 80% load, efficiency is maximum.

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