UNIT
III 2-Mark Questions and Answers
1. Why
quality factor is kept as high as possible in tuned circuit?
What
is the effect of Q on the resonant circuit?
When the quality factor is high, inductor
losses are less. When Q is high,
bandwidth is low and hence we get better selectivity. Therefore, Q is kept as
high as possible in tuned circuits.
2. What
is the need for neutralization in tuned amplifiers? Nov.
/ Dec. 2008, 09,10, May 2013
What
is narrow band neutralization? June
2008, May 2011, Nov 2012
Since tuned amplifiers are operated at
radio frequencies the junction capacitance between base and emitter will have
low reactance at these high frequencies. Hence the output signal at the
collector is fed back to the base as input through this small capacitance. If
the feedback is positive and the signal has sufficient energy to generate
oscillations then the amplifier is unstable. The effect of collector to base
capacitance i.e. the feedback signal is cancelled by introducing a signal at
the base of the transistor that has same magnitude but opposite in polarity to
the feedback signal and it is called as neutralization. Neutralization is
implemented using coils and capacitors.
3. What
is the effect of cascading n stages of identical single tuned amplifiers on
overall bandwidth? May 2011
The
bandwidth of n stages of identical amplifier is given as,
4. What
is stagger tuned amplifier? Dec 2011
Mention
two important features of stagger tuned amplifiers. June 2013
If the resonant frequencies of single
tuned cascaded amplifiers are separated/ displaced by an amount equal to the
bandwidth of each stage, then such a circuit is called as stagger tuned
amplifier. Hence the bandwidth is
increased and selectivity is decreased for stagger tuned amplifiers compared to
single stage amplifier
5. Mention
two applications of tuned amplifiers? Nov 2008
a) Tuned
amplifiers are used in radio receivers to select a particular band of
frequencies and amplify it
b) Tuned
class B and class C amplifiers are used as output RF amplifiers in radio
transmitters to increase the output efficiency and to reduce the harmonics.
c) Tuned
amplifiers are used as active band pass filters and provide amplification to
the desired narrow band.
6. Mention the applications of class C tuned amplifiers?
Class C amplifiers are used in radio
frequency transmitters to provide high power and higher efficiency at high
frequencies.
7. A
tuned circuit has resonant frequency of 1600 kHz and Bandwidth of 10 kHz. What
is the value of its Q-factor? May
2012
Given: fr= 1600 kHz and BW = 10 kHz
fr 1600
Q = ------- =
-------- = 160
BW 10
8. A
tuned amplifier is designed to receive AM broadcast of speech signal at 650
kHz. What is needed Q for
amplifier? Nov 2009
Given:
fc = 650 kHz
The
frequency range required for speech signal = 3 kHz = fm
The
bandwidth required for speech signals = 2 x fm = 2 x 3 = 6 kHz
Q
= fr / BW = 650 kHz / 6 kHz = 108.33
UNIT
IV 2-Mark Questions and Answers
9. What
is ‘tilt’ applicable to RC circuits? Give an expression for tilt. Nov
2011
The
tilt is the drop in the amplitude of the output voltage when the input voltage
maintains its level constant.
P
= (Vi - Vo) / Vi
10. A
RC low pass circuit has R= 1.5 KΩ and C = 0.2 µF. What is the rise time of the
output when excited by a step input. May
2013
In
a low pass circuit tr = 0.35 /f2
f2
= 1/2
RC
RC
11. In
a low pass RC circuit, rise time is 35 ns. What is the bandwidth that can be obtained
using the circuit? Nov 2012
In
a low pass circuit tr = 0.35 /f2 , where f2 is
the higher cut off frequency which is
equal to the bandwidth of the circuit. Hence
BW
= 0.35 / tr
= 0.35 / (35 x 10-9)
= 350 x 106 / 35
= 10 x 106 Hz = 10 MHz.
12. If
the rise time of a BJT is 70 ns, what is the bandwidth that can be obtained
using this BJT?
Given
tr = 70 ns
tr
= 0.35 / BW
BW
= 0.35 / tr
= 0.35 / (70 x 10-9)
= 350 x 106 / 70
= 5 x 106 Hz = 5 MHz.
13. What
is the use of diode comparators?
How
does a diode act as a comparator? Nov 2010
Diode comparators are used to compare
the input continuously and indentify the instant at which the input reaches the
particular reference level.
Diode comparators can be used as zero
crossing detectors.
These comparators can be used to
generate square wave from a sinusoidal wave.
These are used in analog to digital
converters
14. What
is meant by clipper circuit? May 2011
Clipper is a circuit which used to clip
off (remove) a portion of the waveform. For example a negative clipper removes
the lower part of the waveform and provides a portion of the positive waveform
in the output.
15. What
is meant by clamper circuit? May 2012
A clamper adds dc level to the input and
it does not affect the shape of the input waveform.
16. Why
do we call astable multivibrator as free running multivibrator? Nov
2012
Astable multivibrator does not require
any external pulse either to start oscillations or to change its states when it
is oscillating. Therefore it is called free running multivibrator.
17. What
type of distortion is observed in astable multivibrator? Nov 2011
The vertical edges of the output
waveform of astable multivibrator will be little bit rounded and this is called
rounding distortion.
18. State
the role of commutating capacitors in bistable multivibrator.
Commutating capacitors
allow fast rise time and fall time to avoid distortion in the output and make
instantaneous transitions between states.
19. A
20 KHz, 75% duty cycle square wave is used to trigger continuously a monostable
multivibrator with triggered pulse duration of 5 µs. What will be the duty cycle of the waveform
at the output of the monostable multivibrator?
May 2010
20. Determine
the value of capacitors to be used in an astable multivibrator to provide a
train of pulse 2 µs wide at a repetition rate of 75 kHz with R1=R2=
10 KΩ Nov 2010
Pulse width T1
= 2 µs
T = 1/f = 1/ (75 x 103)
= 13.33 µs
T = T1 + T2
T2 = T – T1
= 13.33 – 2 = 11.33 µs
T1 = 0.69R1C1
2 x 10-6 =
0.69R1C1 = 0.69 x 10 x 103 x C1
C1 = 289.85
pF
T2 = 0.69R2C2
11.33 x 10-6
= 0.69R2C2 = 0.69 x 10 x 103 x C2
C2 = 1.642
nF
21. Determine
the value of capacitors to be used in an astable multivibrator to provide a
train of pulse of 4 µs wide at a repetition rate of 80 kHz if R1=R2=10
KΩ. Nov 2013
22. Give
two applications of Schmitt Trigger circuit.
May 2012
Schmitt trigger is used
to convert a continuously varying waveform (Sine wave) to a square wave.
It is used as Flip-flop
It is used as
comparator
23. Define
threshold points in a Schmitt trigger circuit.
Nov 2013
When the input
increases continuously, at a particular point the output state changes and this
point is called Upper Triggering Point (UTP). When the input decreases, at a
particular point which is less than UTP, the output again changes its state and
this point is called as Lower Triggering Point (LTP)
24. What
is regenerative comparator? Give an example circuit. May 2013
A Schmitt trigger uses positive feedback
to obtain infinite forward gain and hence it is called as regenerative
comparator.
UNIT
IV 2-Mark Questions and Answers
25. What
are the applications of blocking
oscillators? Nov 2010, May 2011
Mention
any two applications of blocking oscillator
May 2010, May 2013
a) Blocking
oscillators are used to provide narrow pulses for triggering purposes.
b) Blocking
oscillators are used to provide high peak power pulses.
c) Output
can be isolated from the ground using tertiary winding pulse transformer.
d) Output
pulses of either polarity can be obtained by using a tertiary winding pulse
transformer
26. Draw
and explain practical equivalent circuit of pulse transformer. Nov
2009
Draw
the equivalent circuit of a pulse transformer. Name the various elements in
it. Nov. 2011
R1 is the combined resistance of primary winding and the source
R1 is the combined resistance of primary winding and the source
C
is the lumped capacitance of the transformer’s interwinding capacitance and
reflected external shunt loading capacitance
L
Magnetizing inductance of the pulse transformer
σ Total leakage inductance
R2
Secondary winding resistance and the load resistance reflected in the primary
side
n
is Turns ratio
27. What
is a blocking oscillator? Give its one application. Nov
2009
A
blocking oscillator is special type of wave form generator that produces a
single or train of narrow pulses using transistor and pulse transformer. This
type of blocking oscillator is used to trigger a system that requires pulses.
28. What
are
the advantages and disadvantages of the diode controlled blocking oscillator?
The
waveforms are better and smooth
The
overshoots after the pulse are absent
Low
duty cycle operation is impossible due to unstable behaviour
The
frequency cannot be varied easily and it is temperature sensitive because of
the diode network used.
29.
State the characteristics of pulse
transformer.
a) Uses
small core with high permeability and high magnetizing inductance
b) Leakage
inductance is minimum
c) Low
inter winding capacitance
30.
State any two applications of pulse
transformer. May 2012
Pulse transformers are
used to generate pulses in blocking oscillators.
They are used to
provide isolation between source and load.
They are used to couple
stages in multistage pulse amplifiers.
They are used in
digital signal transmission lines
They are used to change
the amplitude of a pulse
They are used to change
/ match the output impedance of the circuit with the load
31.
Sketch and define the ‘displacement
error’ of a voltage sweep waveform. Nov 2011
Define
slope error and displacement error
May 2011
Displacement
error is the maximum difference between the actual sweep voltage and linear
sweep voltage from the beginning to end of the actual sweep
The
slope error or sweep speed error is the ratio of difference in slope at
beginning and end of the sweep to the initial value of the slope.
32.
What is the advantage of core saturation
method of frequency control in a blocking oscillator? Nov
2012
The pulse duration depends on the supply
voltage and characteristics of the core and not on the transistor parameters.
Therefore the frequency is not affected by any variation in the transistor
parameters.
33.
What is the function of time base
circuit May 2010
A time base circuit generates a voltage
or current waveform which varies linearly with time.
34. List
the applications of time base generators.
Nov 2013
Time base generators are used in Cathode
Ray Oscilloscopes to sweep the electron beam horizontally.
They are also used in Television and
Radar
35. State
any two methods of achieving sweep linearity of a time-base waveform.
Nov 2012
a) Linearity
in sweep is achieved by using constant current source
b) Miller
sawtooth generators provide better linearity
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