EC GATE 2011 - Online Test
Q1. The channel resistance of an N-channel JFET shown in the figure below is 600Ω when the full channel thickness (tch) of 10μm is available for conduction. The built-in voltage of the gate P+ N junction (Vbi) is -1 V. When the gate to source voltage (VGS) is 0 V, the channel is depleted by 1 μm on each side due to the built in voltage and hence the thickness available for conduction is only 8 μm
The channel resistance when VGS = 0 V is
Answer : Option C
Explaination / Solution:
Q2. 
The bias current IDC through the diodes is
Answer : Option A
Explaination / Solution:
The current flows in the circuit if all the diodes are forward biased. In forward biased there will be 0 7. V drop across each diode.
Q3. 
The ac output voltage Vac is
Answer : Option B
Explaination / Solution:
The forward resistance of each diode is
Q4. A transmission line of characteristic impedance 50Ω is terminated by a 50 W load. When excited by a sinusoidal voltage source at 10 GHz, the phase difference between two points spaced 2 mm apart on the line is found to be π/4 radians. The phase velocity of the wave along the line is
Answer : Option C
Explaination / Solution:
We have d = 2 mm and f = 10 GHz
Q5. The modes in a rectangular waveguide are denoted by 
where
m n and are the eigen numbers along the larger and smaller dimensions of the
waveguide respectively. Which one of the following statements is TRUE?
where
m n and are the eigen numbers along the larger and smaller dimensions of the
waveguide respectively. Which one of the following statements is TRUE?
Answer : Option A
Explaination / Solution:
TM11 is the lowest order mode of all the TMmn modes
Q6. The Column-I lists the attributes and the Column-II lists the modulation systems. Match the attribute to the modulation system that best meets it
Column-I
P. Power efficient transmission of signals
Q. Most bandwidth efficient transmission of voice signals
R. Simplest receiver structure
S. Bandwidth efficient transmission of signals with Significant dc component
Column-II
1. Conventional AM
2. FM
3. VSB
4. SSB-SC
Answer : Option B
Explaination / Solution:
In FM the amplitude is constant and power is efficient transmitted. No variation in power.
There is most bandwidth efficient transmission in SSB- SC. because we transmit only one side band.
Simple Diode in Non linear region ( Square law ) is used in conventional AM that is simplest receiver structure.
In VSB dc. component exists.
Q7. A transmission line of characteristic impedance 50 W is terminated in a load
impedance ZL. The VSWR of the line is measured as 5 and the first of the
voltage maxima in the line is observed at a distance of λ/4 from the load. The
value of ZL is
Answer : Option A
Explaination / Solution:
Since voltage maxima is observed at a distance of λ/4 from the load and we know
that the separation between one maxima and minima equals to λ/4 so voltage
minima will be observed at the load, Therefore load can not be complex it must
be pure resistive.
also RL = R0/s (since voltage maxima is formed at the load)
RL = (50/5)Ω
Q8. x(t) is a stationary random process with auto-correlation function. 
This process is passed through the system shown below. The
power spectral density of the output process y(t) is
This process is passed through the system shown below. The
power spectral density of the output process y(t) is
Answer : Option A
Explaination / Solution:
The given circuit can be simplified as
Power spectral density of output is
Q9. A message signal m(t) = cos 200πt + 4cosπt modulates the carrier
c(t) = coe 2πfct where fc = 1MHz to produce an AM signal. For demodulating
the generated AM signal using an envelope detector, the time constant RC of
the detector circuit should satisfy
Answer : Option B
Explaination / Solution:
Highest frequency component in m(t) is 
Carrier frequency fc = 1 MHz
For Envelope detector condition
Q10. In the circuit shown below, the network N is described by the following Y matrix: 
the voltage gain (V2/V1)is
the voltage gain (V2/V1)is
Answer : Option D
Explaination / Solution:
From given admittance matrix we get
