Match List I with List II and select the correct answer using the codes given below the lists :List IList II(a) Oscillator(i) Reverse biased input junction(b) Amplifier(ii) Noise rejection(c) Active filter(iii) Enhanced output(d) JFET(iv) Positive feedback

The correct matching is a-iv, b-iii, c-ii, d-i.An oscillator relies on positive feedback to sustain oscillation (iv).An amplifier produces an enhanced output (iii).An active filter passes wanted frequencies and rejects unwanted ones, such as noise (ii).A JFET operates with its gate-channel junction reverse biased, which gives it its high input impedance (i).The other codes give positive feedback to the amplifier or the reverse-biased junction to the oscillator, which mixes up the defining features.

Feedback Amplifiers Questions and Answers

111.

Match List I with List II and select the correct answer using the codes given below the lists :

List I	List II
(a) Oscillator	(i) Reverse biased input junction
(b) Amplifier	(ii) Noise rejection
(c) Active filter	(iii) Enhanced output
(d) JFET	(iv) Positive feedback

A.a-i, b-ii, c-iii, d-iv
B.a-ii, b-iv, c-i, d-iii
C.a-iii, b-i, c-iv, d-ii
D.a-iv, b-iii, c-ii, d-i

Exam Year(s)

MH SET - 2023, SET Exam - 2023,

Answer: D

The correct matching is a-iv, b-iii, c-ii, d-i.

An oscillator relies on positive feedback to sustain oscillation (iv).
An amplifier produces an enhanced output (iii).
An active filter passes wanted frequencies and rejects unwanted ones, such as noise (ii).
A JFET operates with its gate-channel junction reverse biased, which gives it its high input impedance (i).

The other codes give positive feedback to the amplifier or the reverse-biased junction to the oscillator, which mixes up the defining features.

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112.

Consider the following :

1. Oscillator
2. Emitter follower
3. Cascaded amplifier
4. Power amplifier Which of these use feedback amplifiers ?

A.1 & 2
B.1 & 3
C.2 & 4
D.3 & 4

Exam Year(s)

UGC NET - 2013,

Answer: A

An oscillator needs positive feedback to sustain oscillation, and an emitter follower has 100 per cent negative feedback (its whole output is fed back to the input), so both use feedback. A simple cascaded amplifier is only a series of stages and a power amplifier is just an output stage, so neither uses feedback in general. A power amplifier may use feedback too, but it does not have to, and the cascaded amplifier is just several stages in a row, so the question goes only by the circuits that always use feedback, namely the oscillator with its positive and the emitter follower with its complete negative feedback. The answer is 1 and 2.

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113.

Consider the following statements, regarding an Op-Amp :

1. All types of negative feedback reduce nonlinear distortion.
2. All types of negative feedback reduce the output offset voltage.
3. Non-inverting (current and voltage) feedback increases the input impedance.
4. Inverting (current and voltage) feedback decreases input impedance.
Codes :

A.1 only
B.2 & 3 only
C.2 & 4 only
D.1, 2, 3 & 4 only

Exam Year(s)

UGC NET - 2013,

Answer: D

Negative feedback reduces nonlinear distortion, since the feedback signal corrects the error, and also reduces the output offset voltage by the factor \(1 + A\beta\), so statements 1 and 2 are right. Series (non-inverting) feedback increases the input impedance and shunt (inverting) feedback reduces it, so statements 3 and 4 are correct. All four are correct. The answer is 1, 2, 3 and 4 only.

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114.

Match the following :

List – I	List – II
a. Voltage series feedback connection	i. increases input impedance
b. Voltage shunt feedback connection	ii. decreases the input impedance
c. Current series feedback connection	iii. increases the output impedance
d. Voltage series feedback connection	iv. decreases the output impedance

The correct matching order is
a b c d

A.iv ii iii i
B.iv ii i iii
C.i iii iv ii
D.i ii iii iv

Exam Year(s)

UGC NET - 2013,

Answer: D

Voltage series feedback increases the input impedance (i) and decreases the output impedance (iv). Voltage shunt feedback decreases the input impedance (ii) and current series feedback increases the output impedance (iii). The printed lists give voltage series feedback twice (a and d), and the code that fits the pairs a-i, b-ii, c-iii and d-iv is the last option. The other codes pair the impedance effects with the wrong connection.

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115.

If the gain of an ideal amplifier decreases, the bandwidth

A.increases
B.decreases
C.remains zero
D.remains unaltered

Exam Year(s)

UGC NET - 2013,

Answer: A

When the gain of an amplifier decreases, its bandwidth increases, so the answer is option (A). The gain-bandwidth product of an amplifier stays practically constant, so trading gain away (as negative feedback does) gives a wider bandwidth.

The bandwidth does not decrease (option B), since that would break the constant product. It does not remain zero (option C), which is meaningless for a working amplifier, and it does not remain unaltered (option D), because the product is fixed, not the bandwidth.

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116.

An amplifier has an open loop gain of 100 and its lower and upper hand frequency of 100 Hz and 100 KHz, respectively. A feedback network with a feedback factor of 0.99 is connected to the amplifier, the new lower and upper cut off frequencies are at

A.\(f_{H} = 10\) MHz and \(f_{L} = 1\) Hz
B.\(f_{H} = 25\) MHz and \(f_{L} = 10\) Hz
C.\(f_{H} = 100\) MHz and \(f_{L} = 100\) Hz
D.\(f_{H} = 10\) MHz and \(f_{L} = 10\) Hz

Exam Year(s)

UGC NET - 2013,

Answer: A

With negative feedback the bandwidth is extended by the factor \(1 + A\beta\), where \(A = 100\) and \(\beta = 0.99\):

\[ 1 + A\beta = 1 + 100 \times 0.99 = 100 \]

The upper cut-off frequency rises and the lower one falls by that factor:

\[ f_{H}' = 100\ \mathrm{kHz} \times 100 = 10\ \mathrm{MHz} \]\[ f_{L}' = \frac{100\ \mathrm{Hz}}{100} = 1\ \mathrm{Hz} \]

This is option (A). Options B, C and D use factors other than 100, so their cut-off frequencies do not follow from the feedback relations.

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117.

Negative feedback in amplifier results in

(i) Reduced voltage gain
(ii) Reduced sensitivity
(iii) Decreased bandwidth
(iv) Reduced distortion Of these statements :

A.(i) and (ii) are correct
B.(i), (ii) and (iii) are correct
C.(i), (ii) and (iv) are correct
D.(i) and (iv) are correct

Exam Year(s)

UGC NET - 2013,

Answer: C

Negative feedback reduces the voltage gain, reduces the sensitivity of the gain to changes in the components, and reduces the distortion, so statements (i), (ii) and (iv) are correct, option (C).

Statement (iii) is wrong because negative feedback increases the bandwidth, by the factor \(1 + A\beta\), since the gain-bandwidth product stays constant. Options B and D include (iii), and option A leaves out the reduction of distortion that feedback gives.

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118.

Match the following lists :

List - I	List - II
a. Voltage series feedback	i. Trans resistance amplifier
b. Current series feedback	ii. Current shunt feed back
c. Current amplifier	iii. Trans-conductance
d. Voltage shunt feedback	iv. Voltage amplifier

Codes : a b c d

A.iv i iii ii
B.iv ii i iii
C.iv iii ii i
D.iii ii i iv

Exam Year(s)

UGC NET - 2014,

Answer: C

The correct match is a-iv, b-iii, c-ii, d-i. Voltage series feedback is the feedback of a voltage amplifier (input voltage, output voltage). Current series feedback senses output current and returns a voltage in series, which is the transconductance amplifier. A current amplifier uses current shunt feedback. Voltage shunt feedback senses voltage and mixes current at the input, which is the trans-resistance amplifier.

The options that give voltage series feedback the trans-resistance or transconductance connection mix up the series and shunt ways of connecting the feedback.

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