Home » 2016 » July

# Monthly Archives: July 2016

## GATE : Previous questions on Feedback Amplifier

GATE 1993
1) Negative feedback in Amplifiers

a) Improves the Signal to Noise ratio at I/p
b) Improves the Signal to Noise ratio at O/p
c) Does not improvee signal to noise ratio at I/o
d) Reduces distortion

Ans : d

GATE 1995
2) To obtain very high input impedance and output impedances in a feedback Amplifier , which is the suitable configuration

a) Voltage – Series
b) Current – Series
c) Voltage – Shunt
d) Current – Shunt

Ans : b

Note: Explanation is given after 3rd question.

GATE 1998
3) In a shunt-shunt negative feedback Amplifier, as compared to the basic Amplifer.

a) Both input and output impedance increases
b) Input impedance decreases, but output impedance increases
c) Input impedance increases, but output impedance decreases
d) both input and output impedance increases

Hint : Feedback topology naming convention goes this way

Shunt(I/P) – Shunt(O/P). Remember Shunt-Shunt is also called as Voltage – Shunt. You can read that HERE
Impedance at input or output side depends on the word Series/Shunt. If Series then impedance increases
and if Shunt, then impedance decrease. In this question, the feedback topology is shunt-shunt, hence the correct answer is option d.

Ans : d

GATE 1991

4) Two non-inverting amplifiers, one having a unity gain and other having a gain of twenty, are made using identical operational amplifiers. As compared to the unity gain amplifier, the amplifier with gain twenty has

a) less negative feedback
b) More input impedance
c) Less Bandwidth
d) None of the above

Ans : c

Try to dissolve the above question.

## GATE : Transmission lines questions

1) Two very long loss-less cables of characteristic impedances of 50 $\Omega$ and 100 $\Omega$ respectively are to be joined for reflection less transmission. If the quarter wave transformer is used for matching, what should be the value of its impedance(in ohms)?

a) 70.7
b) 35.7
c) 50
d) 2

2) A transmission line with a characteristic impedance of 100 $\Omega$ is used to match a 50 $\Omega$ section to a 200 $\Omega$ section. If the matching is to be done both at 429MHz and 1GHz, the length of the transmission line can be approximately

a) 82.5cm
b) 1.05m
c) 1.58m
d) 1.75m

3) Impedance of a transmission line repeats itself every

a) $\lambda$/4
b) $\lambda$/2
c) $\lambda$
d) 2$\lambda$

4) If $Z_{open}$ is the impedance of a open circuited transmission line and $Z_{short}$ is the impedance of a short circuited transmission line whose characteristic impedance is $Z_o$. Then which of the following relationship is true?

a) $Z_o$ =$\frac{Z_{open}}{Z_{short}}$
b) $Z_o$ =$Z_{open}$ * $Z_{short}$
c) $Z_o$ =$\sqrt{\frac{Z_{open}}{Z_{short}}}$
d) $Z_o$ =$\sqrt {Z_{open}*Z_{short}}$

5) A transmission line of characteristic impedance 50 $\Omega$ is terminated by a 50 $\Omega$ 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

a) 0.8* $10^8$ m/s
b) 1.2*$10^8$ m/s
c) 1.6*$10^8$ m/s
d) 3*$10^8$ m/s

A transmission line of characteristic impedance 50 $\Omega$ is terminated in a load impedance . 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 $\lambda$/4 from the load. The value of $Z_L$ is
(A) 10 $\Omega$
(B) 250 $\Omega$
(C) (19.23 + j46.15) $\Omega$
(D) (19.23 – j46.15) $\Omega$

….

1-a, 2-c , 3-b, 4-d