Download Feedback in Amplifiers

 Chapter 4 : Feedback in Amplifiers
Contents
Types of feedback, effect of negative feedback on gain, bandwidth, stability,
distortion and frequency response etc. voftage series, current series, voltage
shunt, current shunt feedback circuits and their analysis.
POINTS TO PONDER
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Feedback amplifiers are those in which a fraction of the output energy
is fed back to the input of the same circuit.
If the feedback signa[ is in phase with the input signal then it is a
positive feedback or direct feedback. In this case the signal is additive.
If the feedback signal is 180° out of phase with the input signal then it
is a negative feedback. In this case the signal opposes the input signal.
Amplifier without feedback gives gain:
Where

Is the output voltage and
is the nput voltage.
Amplifier with feedback i.e. negative feedback give gain:
Where, A is the amplifier gain without feedback and f is the feedback
fraction.
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Negative feedback amplifiers as used because:
It improves gain stability.
Reduce the distortion or noise.
Decreases the output impedance.
Increases the input impedance.
Increases bandwidth.
Due to reduced noise and distortion and increase bandwidth, the
negative voltage feedback is used in radio receivers, public address
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system etc.
For perfect impedance matching negative current feedback is used at
the output stage.
An emitter follower circuit is widely used in electronic instruments
because of its input.
impedance is high and output impedance is low.
Series feedback connections generally increases the input impedance
where as shunt feedback connection decreases the input impedance
Voltage feedback generally decreases the output impedance and the
current feedback generally increases the output impedance.
The series-voltage feedback are used in negative feedback amplifiers.
Q 1. Why is negative feedback employed in high gain amplifiers?
Ans
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If A is the gain of an amplifier, then the gain bandwidth product is
given by
A x BW
Where BW= f2—f1
Now when negative feebback is applied the amplifier gain is reduced
But at the same time bandwidth increases
If A is gain of amplifier and A be the gain of an amphfier with negative
feedback then
where, is the feedback fraction. Gain reduces.
Thus, high gain amplifiers are used in case of negative feedback so as
to maintain the jain to a specific level.
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Q 2.Why is —ye feedback provided in Wein- bridge oscillator ?
Ans.
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A negative feedback ensures stability and constant output In case of
Wein bridge oscillator the negative feedback is provided through the
voltage divider circuit in the output amplifier through resistances (say
R3 and R4) in Wein bridge.
Basically, a temperature sensitive tungsten lamp is used in place of R4
Its resistance increases with current As soon as, the amplitude of
output tend to increase, more current would provide more negative
feedback.
This brings back the output to its original value A reverse action would
take place if the output tends to decrease.
Q 3. How does. negative feedback increase bandwidth of an amplifier
?
Ans.
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The bandwidth of an amplifier circuit without feedback is equal to the
separation between the 3 db frequencies f1 and f2
When f1 = lower 3 dB frequency
f2= upper 3-dB frequency
BW = f2 — f1
Let A be its gain, then the gain bandwidth product is A x BW.
Now, when negative feedback is applied, the amplifier gain reduced.
Since the gain bandwidth product has to remain the same in both
cases.
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With negative feedback the lower upper 3 dB frequencies of ai
amplifier becomes
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It is as shown in fig. f’1 has decreased whereas t’2 has increased thus
gives a wider separation or bandwidth.
Since gain bandwidth product is the same in both cases
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Q 4. The voltage gain of an amplifier without feed back is 3000.
calculate the voltage gain of the amplifier is negative feedback is
introduced in the circuit. Given that feedback fraction m = 0.01
Ans.
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Voltage gain with negative feedback
is given by
Q 5. How that negative teed back changes the impedances? (Explain
with mathematical experssion)
Ans.
Following fig.. shows the typical feedback (negative) connection:
Effect of negative feedback on. input impedance
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• Thus, series voltage negative feedback increases the input impedance of an
amplifier
by a factor
• Effect of negative feedback on output impedance:
The output impedance is determined by applying a voltage V resulting in a
current I with
Thus, series voltage negative feedback reduces the output impedance of an
amplifier
by a factor
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Q6. What do you understand by bandwidth of an amplifier?
Ans.
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For each amplifier system, there is a band of frequencies in which the
magnitude of the gain is either equal or relatively close to the midband
value.
To fix the frequency boundaries of relatively high gain, O.7O7Av mid
choosen to be the ‘gain at the cut 6ff levels.
The corresponding frequencies f1 and f2 are generally called the
corner, cut off, break or half- power frequencies.
The bandwidth (or passband) -of this system is determined by f1 and
2,
i.e.
Bandwidth (BW) = f2 — f1
Q 7. Explain in brief the effect of changing Q of the coil used in tank
circuit, on its bandwidth.
Ans
Higher the value of Q, the smaller is the bandwidth.
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It also concluded that Q is a measure of ability of a tank circuit to select a
band of frequency. The higher the Q of a circuit, the greater is its ability as a
frequency select or as its bandwidth is inversely proportional to the quality
factor.
Q 8. Compare the negative feedback and positive feedback.
Ans.
Q 9. Define: Phase Margin.
Ans.
Phase margin: It is designated by PM. It is used in feedback amplifiers and
is defined as the angle of 1800 minus the magnitude of the angle at which th
value l3Al is unity 1(0db).
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Q 10. Name the feedback amplifier topologies.
Ans.
There are basic four types of feedback amplifier topologies:
(a) Voltage — series feedback amplifier
(b) Voltage — shunt feedback amplifier
(c) Current series feedback amplifier
(d) Current — shunt feedback amplifier
Q 11. The gain and distortion of an amplifier are 150 and 5%
respectively without feedback If the stage has 10% of its output
voltage applied as negative feedback, find the distortion of the
amplifier with feedback
Ans.
Given:
Gain without feedback, Av = 150
Distortion without feedback, D 5%
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Thus, distortion reduces from 5% to 0.313% by using —ye feedback.
Q 12. Explain a current shunt feedbackcircuit and perform a suitable
analysis.
OR
Write short note on Current shunt feedback ?
Ans.
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Current shunt feedback circuit It is also called as series derived shunt
fed feedback.
It is a parallel-series (PS) prototype It is as shown in fig (a).
Here, feedback network shunts the input but is in series with the
output Hence the output resistance of the ampIifer is increased
whereas its input resistance is decreased by a factor of loop gain
The two stage amplifier using a current shunt negative feedback is as
shown in fig (b)
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The feedback circuit consisting ‘of CF and RF samples the. output current and
develops
a feedback voltage is parallel with the input voltage. The unbypassed emitter
resistor of Q2 provides current sensing.
Q.13. Calculate the gain of a —ye feedback amplifier with an internal
gain Av = 75 and feedback fraction
Av doubles?
. What will be the gain if
Ans.
Gain of a —ye feedback amplifier is given by:
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Thus, it is clear from the above calculation that if 0% doubles, the gain of the
amplifiOr increases.
Q 14. With circuit diagram explain current-series feedback.
OR
Explain the current series teedback circuit in detail with a neat
diagram and perform a”suitable analysis.
Ans.
Following figure shows the block diagram current-series feed back:
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It is also called a series-derived series-fed feedback connection.
A block diagram is as shown in fig 1.
In this connection a fraction of the output current is converted into a
proportional voltage by the feedback network and then applied in
series with the input
Fig. 2 shows the circuit diagram:
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Following fig shows the series derived series fed feedback amplifier
circuit Since the emitter resistance is unbypassed, it effectively
provides current series feedback.
When 1E passes through RE, the feedback voltage drop V = 1E RE is
developed which is applied in phase opposition to the input voltage V,
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This negative feedback reduces the output voltage V0.
This feedback can, however, be eliminated by either removing or
bypassing the emitter resistor.
In case an input signal is applied, the resulting collector current IC
develops output voltage across the collector load and emitter current
1E develops ac voltage equal to ‘E RE (1E Ic) across the emitter.
The input voltage to amplifier, which is equal to VSE, is given by
Q 15. With negative feedback voltage gain reduces, explain why?
Ans.
The overall gain with negative feedback is
ie. Voltage gain reduces by the factor of
Let take a care of voltage series feedback as shown below.
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From this eq. it is clear that with feedback the amplifier gain reduced as the
factor
Q 16. Explain the effect of negative feedback on the gain, bandwidth,
frequency, response of an amplifier.
Ans.
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Effect of negative feedback on:
(i) Gain :The gain is stable by using negative feedback. As voltage gain with
negative
feedback is given by
So it is independent of internal gain i e stable
ii) Bandwidth:With negative feedback the gain reduces but gain bandwidth
product is remain constant So the bandwidth increases to compensate the
reduction in gain
(iii) Frequency: Response With negative feedback bandwidth increases and
due to which the frequency response is improved.
Q 17. Draw and design a series transistor voltage regulator circuit to
provide output of 9 V for a maximum load current of 1 0 A, when the
input voltage variation is 15 to 18 V and transistor gain is 50
Ans.
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It is known as emitter follower because voltage at the emitter follows
the base voltage In this the transistor behaves like a variable resistor
whose resistance is determined by the base current.
It is also called pass transistor because all the load current must pass
throughit.
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Q 18. Explain the concept of feedback.
Ans.
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Feedback concept is widely used in any system either electrical,
electronics or mechanical.
In any system, main aim is to maintain output parameters stable with
time inspite variation of environment change or system parameter
variations.
Feedback concept is used to maintain system stable.
Feedback is used in daily life process. Feedback is essential to maintain
performances Of the system.
The electronics, feedback is used in amplifiers, oscillators by taking a
part of or fraction of output mixed with input.
The gain of amplifiers may change due to shift in operating point. ft
can be due to change in temperature and change in components of
amplifier.
The feedback in amplifier keep gain constant inspite of above said
variations.
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Q 19. What are the different types of negative feedback? Explain
each with block and diagram.
Ans.
Different types of negative feedback are
1. Voltage feedback
2. Current feedback
The combination of both may also be present in a circuit. At the same, time,
both voltage current can be feedback to the input either in series or parallel,
then they named as:
1. Series — Voltage feedback
2. Series — Current feedback
3. Shunt — Voltage feedback
4. Shunt — Current feedback
In this circuit voltage is feedback to the input in series This employed high
input impedance and low output impedance and invanably employed in
negative feedback amplifies.
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In this circuit current is feedback in series to the input.It provide high input
impedance and output
In this circuit voltage is feedbackin shunt to the input. It provides low output
impedance input and high.
In this circuit current is feedback in shunt t6 the input. It provicies high input
and low output impedance.
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Q 20. Find
(a) feedback ratio
(b) feedback factor
(C) voltage gain without feedback
(d) voltage gain with feedback for a circuit given below.
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(d) Voltage gain with feedback:
Q 21. An amIifier has a midband gain of 125 and a bandwidth of 250
KHz.If 4% negative feedback is introduced, find the new bandwidth
and gain
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Q 22. Draw a voltage-series feedback circuit How are its input and
output resistances calculated?
OR
Draw the circuit diagram of voltage series feedback amplifier and
derive the expressions for input and output impedances .
Ans.
Voltage series feedback circuit
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Q 23. Describe with necessary derivations, the effect of negative
feedback on the bandwidth and distortion in an amplifier.
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Ans.
1. Effect of negative feedback on the distortion in an amplifier : Distortion
reduces when a negative feedback is used in amplifiers. The following
diagram clears the reduction of distortion
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In the figure let amphfier ‘A’ gives distortion ‘d’ without any feedback.
When feedback path is provided the gain ‘A’ becomes ‘Af’ and the
distortion becomes df at the output.
It is clear from figure that if ‘d’ is the distortion at the input then, ‘di’ is
the distortion at output due to negative feedback.
It is because a part of distortion ‘di’ i.e. ‘df’ is fed back to the input.
This is added in reverse direction because of negative feedback
including gain ‘A’.
1.From this expression distortion is reduced by
because of negative feedback.
2. Effect of negative feedback on bandwidth : By using negative
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feedback the bandwidth increases As the gain of an amplifier using negative
feedback decreases because
where ‘A’ is gain of amplifier without feedback Thus, a fraction of
the
is
effect in gain Due to this upper cut oft frequency i e f2 or IJCF and lower cut
off frequency i
or LCF will be affected Due to negative feedback ‘f1’ i e lower cut off
frequency decreases by factor
increases by factor
SW =(f2 — f1).
and ‘f2’ i e upper cut off frequency
Hence bandwidth increases. Because, bandwidth
Q 24. Derive an expression for the gain of negative voltage feedback
amplifier
Ans. It is as shown in fig.
The gain of amplifier without feedback is Av This then by feeding a
fraction
of the output voltage
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back to amplifier input. Thus, the
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actual input to the amplifier is signal voltage
minus feedback voltage
ie
The output V0 must be equal to the input voltage
gain Av of the amplifier.
But
multiplied by
is the voltage gain of the amplifier with feedback Thus it is given by
Thus using negative voltage feedback the gain reduced by a factor
But it does not affect the current gain of the circuit
Q 25. What are the different types of feedback? With help of diagram
explain a RC coupled amplifier. Discuss its gain response.
0R
Explain transistor RC coupled amplifier with special reference to
frequency, response, advantages, disadvantages and applications.
OR
Explain the operation and frequency analysis of RC coupled amplifier.
Ans. There are two types of feedback
1. Positive feedback
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2. Negative feedback
1. In positive feedback if the feedback voltage (or current) is so applied as to
increase the input voltage (i.e. it is in phase with it),then
it is called
positive feedback. Other names for it are regenerative or direct feedback.
2. In negative feedback if the feedback voltage (or current) is so applied as
to reduce
the amplifier input (i.e. it is 1800 out of phase), than it is called as
negative feedback.
Other names for it are degenerative or inverse feedback. RC Coupled
Amplifier
A two-stage AC coupled amplifier circuit using identical transistors is as
shown In fig.
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A coupling capacitor C is used to convert the output of first stage to
the base (i.e. input) of second stage and so as the coupling from one
stage to next is achieved by a coupling capacitor followed by a
connection to a shunt resistor, therefore, such amplifiers are called
resistance capacitance coupled amplifiers.
Operation V is the input signal applied to the base of first transistor
Q1 its amplified brm appears across the collector resistor R.
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This amplified signal developed across Rc is ed to the base of the next
transistor Q2 through coupling capacitor C, The second stage further
amplifies the signal.
This is how, the cascaded stages amplify the signal and the overall
gain (G = G1 x G2 x G) is considerably increased.
It may be noted that the overall ga is always less than the product of
gains of individual stages It is because of effect,ve load resistance of
each stage decreases due to shunting effect of the input resistance of
the next stage.
Voltage V frequency response is as shown in fig
1. At low frequencies (< 50 Hz) the gain is small.
2. At mid frequencies (50 Hz to 20 kHz) also called as audio frequency range
the gain is almost constant.
3. At high frequencies (> 20 kHz) the gain drops off as shown in fig clearly.
Advantages:
1. It is the most convenient and least expensive multistage amplifier.
2. It has a wide frequency response i e its gain versus frequency curve
remain constant for a wide frequency range.
3. It gives less frequency distortion.
Disadvantages:
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1. Th& overall gain of the amplifier is comparatively small because of the
loading effect of number of stages.
2. it bcomes noisy wth the passage of long time
3. It provides poor impedance matching between the stages
Applications
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R-C coupled amplifiers provide excellent audio-fidelity over a wide
range of frequencies.
Therefore they are widely used as voltage amplifiers e g in the initial
stages of Public Address System.
Q 26. Explain types of negative feedback in transistor circuits?
Ans.
Transistor circuit with negative voltage freedback
The
components
and
necessary feedback voltage
is
.
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act
as
feedback
circuit
and
provide
the
to the input. Let the voltage feedback
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This circuit reduces the output impedance and also decreases the gain of the
amplifier.
all the modern high pass. transistor ampllfier negative feedback is employed.
Transistor circuit with negative current feedback
1.Common-emitter circuit with by-pass capacitor
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When an ac signal v is applied at the input and voftage increases
during positive halt the emitter-base junction becomes more forward
biased.
This develops an ac voltage across the emitter resistor. Thus, a
negative voltage is feedback to the input.
Since, the ye voltage feedback to the input is proportional to the
output current
hence the irrent feedback circuit.
Emitter follower :
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It has high input impedance and low output impedance.
In theon-emitter amplifier circuit, if the collector resistance R is
reduced to zero an output is from the emitter terminal, instead of
collector terminal.
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When ac signal Vs is applied at the input, resulting current
flows
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through
and produces an output voltage across it
This voltage opposes the signal voltage thus provides negative
feedback.
Q 27. Find the loop gain and overall gain for negative feedback if A =
200 and B = 0 02
Ans.
Loop gain of feedback amplifier is A x M
200 x 0.02=4
and gain with feedback
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Q 28. The gain of an amplifier without feedback is 100 and with
negative feedback is 50. If due to ageing the amplifier gain falls to
60, find the %age reduction in stage
(i) without feedback and
(ii) with negative feedback.
Ans.
(i) %age reduction in stage gain without feedback
(ii) %age reduction in stage gain with feedback
As, negative feedback
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Q 29. The input impedance of an amplifier is 4 Ks2 and voltage gain is
2000. If negative feedback of m 0.006 is employed than what will be
the input impedance of a circuit 25%.
Q 30. The voltage gain of an amplifier is 100 and with negative
feedback circuit m= 006. Calculate the %age change in the gain with
feedback if A falls as 10.
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Ans.
Gain of the amplifier with negative feedback,
And now absoftite gain of the amplifier without negative feedback after
ageing is say A1.
A1 = A—A’
= 100—20
=80
And gain with negative feedback after ageing
Now, %age change in system gain
Q 31 The gain of amplifier is 200 with its bandwidth as 20 KHz If
20% of negative feedback is applied in amplifier then determine the
feedback gain and new bandwidth of amplifier
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Ans.
Feedback fraction m=20%=02
gain with negative feedback
Q 32. An
frequency
output is
frequency
amplifier has gain = 200 without feedback the cut off
= 20 KHz and input resistance is 20 K If one percentage of
applied as negative feedback, what is the new cut off
and input resistance
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Q 33. For emther fouower circuit shown n figure.
Measure the input resistance.
Ans.
The voftage at base terminal of transistor is
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Q 34. Write the amplifier classification, companson signal and output
signai for cases of positive (P) and negative (N) voltage series
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current series, voltage shunt and current shunt feedback amplifiers.
Ans.
1. Voltage series (Series shunt):
Negative and positive feedback
(i) Amplifier classification — Voltage
(ii) Comparison signal — Voltage
(iii) Output signal —* Voltage
2. Current series (Series series):
Negative and positive feedback
(i) Amplifier classification -+ Voltage to current converter
(ii) Comparison signal —* Voltage
(iii) Output signal -4 Current
3. Voltage shunt (Shunt shunt)
Negative and positive feedback
(i) Amplifier classification —+ Current to voltage converter
(ii) Comparision signal —+ Current
(iii) Output signal —* Voltage
4. Current shunt (shunt series):
Negative and positive feedback:
(i) Amplifier classification -4 Current
(ii) Comparison signal -+ Current
(iii) Output signal —* Current –
Q 35. Explain the general feedback system arrangement.
Ans.
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A feedback system consists of the following blocks
1. Basic amplifier to amplify the signal.
2. Sampling network to obtain current or voltage proportional to the output.
3. Feedback network (usually two part network) comosed of passive
components such as resistance, óapacitor and inductor.
4. Mixing network to get the mixed signal of input and feedback signal for the
error output.
Q 36. Explain how the negative feedback amplifier improves stability,
reduces noise and increases the input impedance.
Ans.
(i) Negative feedback improves stability by making gain of the amplifier
independent of change in power supply or change in the parameters of
transistor.
The gain of an amplifier with negative feedback is
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By taking Am>>1
This gain of feedback is independent of the internal gain A.
Hence the negative feedback give, more stability.
(ii) Negative feedback reduces noise (or distortion) by employing reverse
direction distortion whch is added to the original distortion as
Thus, distortion or noise is reduced by factor (1 + Am).
(iii) Following fig. shows the typical feedback (negative) connection
Effect of negative feedback on input impedance
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Thus, series voltage negative feedback increases the input impedance of an
amplifier
by a factor
Q 37. An amplifier with 60 dB gain without feedback has output
impedance equal to 12 k2. It is required to modify its output
impedance to 600 2 by the feedback. Calculate feedback factor and
percentage change in overall gain for 10% change in the gain of
basic amplifier.
Ans. Open-loop voltage gain, A = 60 dB or antilog
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Output impedance with feedback,
Now, feedback factor is given by
For 10% change in openoop gain, we have new open-loop gain as
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