Electronic Circuits - Feed Back Amplifiers - Important Short Questions and Answers: Feed Back Amplifiers

__FEEDBACK AMPLIFIERS__

**1. State
the nyquist criterion to maintain the stability of negative feedback amplifier**

The nyquist criterion forms the basis of a
steady state method of determining whether an amplifier is stable or not.

__Nyquist Criterion__

The Aβ is a function of frequency. Points in
the complex plane are obtained for the values of Aβ corresponding to all values
of ‘f’ from - to . The locus of all these points
forms a closed curve.

The criterion of nyquist is that amplifier is unstable if this curve encloses the point (-1+j0), and
the amplifier is stable if the curve does not enclose this point.

**2.** **Define sensitivity and desensitivity of
gain in feedback amplifiers.**

** Sensitivity :**The fractional change im amplification with feedback divided by the
fractional change

** Desensitivity**: Desensitivity is defined as the reciprocal of sensitivity. It
indicates the factor by which

Desensitivity factor (D) = 1+A β.

Where A = Amplifier gain.

. β =
Feedback factor.

**3.
****What is the effect on input and output
impendence of an amplifier if it employs voltage series negative feedback?**

When voltage series feedback is employed in an
amplifier, its input resistance increases and output resistance decreases.

**4.
****Define ‘feedback factor’ of a feedback
amplifier.**

It is the ratio between the feedback voltages
to the output voltage of the amplifier.

β = V_{f}/ V_{o}

Where β is a feedback factor (or) feedback
ratio. V_{f} is the feedback voltage. V_{o} is the output
voltage.

**5. What
is the impact of negativefeedback on noise in circuits? **

When negative feedback is employed in an
amlifier ,the noise is reduced.

Let N= noise without feedback

N_{f} = noise with feedback

The noise with feedback is given by the
following relation

From above equation it is clear that when the
feedback is applied the noise is reduced by a factor (1+Aβ)

**6. ****What is
the effect on input and output impendence of an amplifier if it employs current
shunt negative feedback?**

When **current
shunt** feedback is employed in an amplifier, its input resistance decreases
and output resistance increases.

**7. ****What is
return ratio of feedback amplifier?**

A path of a signal from input terminals through
basic amplifier, through the feedback network and back to the input terminals
forms a loop. The gain of this loop is the product of –Aβ. This gain is known
as loop gain or return ratio. Here the minus sign indicates the negative
fedback.

**8.
Justify that negative feedback amplifier increases bandwidth.**

When negative feedback is employed in an
amlifier ,the bandwidth is increased. Let BW= bandwidth without feedback

BW_{f} = bandwidth with feedback

The bandwidth with feedback is given by the
following relation,

BW_{f}=BW(1+Aβ)

From above equation it is clear that when the
feedback is applied the bandwidth is increased is by a factor (1+Aβ)

**9.
Distinguish between series and shunt feedback amplifiers Series feedback:**

(i). In series feedback amplifier the feedback
signal is connected in series with the input signal.

(ii). It increases the input resistance.

**Shunt
feedback:**

(i). In shunt feedback amplifier the feedback
signal is connected in shunt with the input signal.

(ii). It decreases the input resistance.

**10.** **What** **is current-series feedback amplifier. (or)
What is transconductance amplifier?**

In a current series feedback amplifier the
sampled signal is a current and the feedback signal (Which is fed in series)is
a voltage.

Gm =I_{o} / V_{i}

Where G_{m} = Amplifier gain.

I_{o} = Output current.

V_{i} =
Input voltage.

**11.
****List the four basic feedback topologies.**

ü Current series feedback.

ü Current shunt feedback.

ü Voltage series feedback

ü Voltage shunt feedback

**12.
****List the characteristics of an amplifier which
are modified by negative feedback**.

·
It
reduces the gain of an amplifier

·
It
increases the stability of an amplifier.

·
It
increases the bandwidth

·
It
decreases noise and distortion

**13. What
is Feedback Amplifier? & draw the diagram.**

An amplifier with feedback network is known as
dfeedback amplifier. With the help of feedback network, ‘a portion of the
output signal is fedback to the input & combined with the input signal to
produce the desired outputs’

**14.
****Mention the three networks that are connected
around the basic amplifier to implement the feedback concept.**

The three networks that are connected around
the basic amplifier to implement the feedback concept are

ü Mixing Network

ü Sampling Network

ü Feedback Network

**15.
****What happens to the input resistance based on
the type of feedback in an amplifier?**

·
If the
feedback signal is added to the input in **series**
with the applied voltage, it **increases**
the input resistance.

·
If the
feedback signal is added to the input in shunt with the applied voltage, it **decreases** the input resistance.

**16. What
are the steps to be carried out for complete analysis of a feedback amplifier? **

Step 1 :
Identify the topology

Step 2,3: Find input and output circuit

Step 4 :
Replace transistor by its h-parameter equivalent circuit

Step 5 :
Find open loop voltage gain

Step 6 :
Indicate V_{0} and V_{f} and calculate β

Step 7 :
Calculate D, A_{vf}, R_{if}, R_{of} and R_{of.}

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