ELECTRONIC
CIRCUITS II
2-Marks Questions
and Answers
UNIT - I
1. Define feedback factor
of a feedback amplifier. [May
2012]
The ratio of the output voltage of
feedback circuit (Vf) to its input voltage (Vo) is called feedback
ratio or feedback factor.
Feedback
Factor β = Vf / Vo
Since Vf is very
small compared to Vo, β is always between 0 and 1.
2. Mention three networks
that are connected around the basic amplifier to implement feedback concept.
[Nov 2012]
Mixer or Comparator, Sampler and Feedback networks are connected around
the basic amplifier to implement feedback concept.
3. List the
characteristics of an amplifier which are modified by negative feedback. [Nov
2013]
The following characteristics of an
amplifier are modified by negative feedback.
Gain is reduced
Bandwidth is increased
Distortion and Noise generated due to
change in gain are reduced
Input resistance may be increased and
output resistance may be lowered which is desirable in a practical amplifier.
Gain is stabilized against variation
in temperature and variation in frequency
4. State the effect on
output resistance and on input resistance of amplifier when current shunt
feedback is employed. [ May 2011, May 2012, Nov 2012]
In a current shunt feedback amplifier, since current is to be sampled in
series from the output, the output resistance increases. Since the feedback is
shunt i.e. feedback is given in parallel to the input, the input resistance
decreases.
5. Draw the block diagram
of voltage shunt feedback amplifier and write the expressions for its input and
output resistances. [Nov 2011]
Since the sampled parameter (output)
is voltage, it is to be taken in parallel and hence the output resistance is
reduced by a factor (1+βA). If the output resistance without feedback is Ro,
the output resistance with feedback will become [Ro/(1+βA)].
Similarly the input resistance also decreases by a factor (1+βA) because the
feedback is given in parallel to the input circuit. If the output resistance
without feedback is Ri, the output resistance with feedback will
become [Ri/(1+βA)].
6. What is the effect of
input and output impedance of an amplifier if it employs voltage series
negative feedback? [May 2013]
Output resistance decreases and input resistance increases in a voltage
series negative amplifier.
7. What is return ratio of
feedback amplifier? [May 2011, Nov 2011]
The basic amplifier and the feedback circuit form a loop. The product of
feedback factor β and the gain A in this loop is called loop gain or return
ratio. Return Ratio = β x A
8. What is the impact of
negative feedback on noise in circuits? [May 2010]
Negative feedback
reduces the noise.
9. Calculate the closed
loop gain of a negative feedback amplifier if its open loop gain is 100,000 and
feedback factor is 0.01 [May 2013]
10. If a negative feedback
amplifier has A=100, β = 0.04 and signal voltage Vs = 50 mV
find (a) gain with feedback (b) output voltage (c) feedback factor
(d) feedback voltage [Nov 2013]
Solution:
Given data:
A=100
β = 0.04
Vs = 50 mV = 50 x 10-3 Volts
b) Af = Vo /
Vs = Vo / (50 x 10-3)
Hence
Vo = Af x (50 x 10-3) = 20 x 50 x 10-3 =
1 Volt
c) Feedback factor = β = 0.04
d) Feedback Voltage Vf =
Vo x β = 1 x 0.04 = 0.04 V or 40 mV
11. Define sensitivity and desensitivity
of gain in feedback amplifiers. [May 2010]
The transfer gain of the amplifier is sensitive to
temperature i.e. it will change when the temperature increases or decreases.
The sensitivity reduces by a factor of 1/
(1+ βA) when feedback is given.
If the amplifier’s gain is more
sensitive, then its gain is less stable. Therefore, the stability of an
amplifier’s gain may be defined as
12. Justify that negative
feedback amplifier increases the bandwidth [Nov 2010]
The higher cutoff frequency of an amplifier is
increased by a factor 1 + βA, and the lower cutoff frequency is decreased by a
factor 1 + βA when feedback is introduced. Therefore if fH and
fL are higher and lower cutoff frequencies of a basic amplifier
without feedback and fHf and fLf are higher and
lower cutoff frequencies of an amplifier with feedback, then
fHf = fH x
(1 + βA) and fLf = fL / (1 + βA)
Bandwidth of an amplifier without
feedback = BW = fH - fL ≈ fH (... fL <<
fH)
Bandwidth of an amplifier with feedback
= BWf = fHf - fLf = fHf (... fLf <<
fHf)
Hence BWf = fHf =
fH x (1 + βA) = BW x (1 +
βA) (... BW
≈ fH )
i.e. Bandwidth with Feedback = (1 +
βA) x Bandwidth without feedback
13. The distortion in an
amplifier is found to be 5%, when the feedback ratio of negative feedback
amplifier is 1%. When the feedback is removed, the distortion becomes 10%. Find
the open loop gain.
Solution:
5 + [5 x (0.01 x A)] = 10
5 x (0.01 x A) = 10-5 = 5
A = 5 / (5 x 0.01)
= 100
14. An amplifier has a
mid-frequency open loop gain of 100 and a bandwidth of 200 KHz. What should be
the amount of feedback, if the bandwidth is to be 1 MHz?
Bandwidth with feedback BWf =
Bandwidth without feedback x (1 + βA)
1
x 106 = (200 x 1000) x [1 + (β x 100)]
5 = 1 + (β x 100)
4 = β x 100
β = 4/100 = 0.04
or 4%
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