EC2251 ELECTRONIC CIRCUITS II Department of …mahalakshmiengineeringcollege.com/pdf/ece/IVsem/EC2251...Voltage shunt feedback amplifier Current series feedback amplifier Current shunt

EC2251 – ELECTRONIC CIRCUITS II Department of ECE 1

MAHALAKSHMI

ENGINEERING COLLEGE

TIRUCHIRAPALLI-621213.

UNIT I – FEEDBACK AMPLIFIERS

PART A (2 Marks)

1. Name the types of feedback amplifiers. (AUC MAY 13, DEC06)

Voltage Series feedback amplifier

Voltage shunt feedback amplifier

Current series feedback amplifier

Current shunt feedback amplifier

2. What are the advantages of negative feedback. (AUC MAY 12, MAY 10)

The advantages of negative feedback are,

Stabilization of gain

Increased bandwidth

Decreased distortion

Decreased noise

Increase in input impedance

Decrease in output impedance

3. Define CMRR and write its significance in differential amplifiers. (AUC MAY10, APR 09)

The figure of merit called Common Mode Rejection Ratio(CMRR) defines the ability of the

differential amplifier to reject common mode signal.

4. Mention the operating modes of Differential amplifier. (AUC NOV’10)

Differential amplifier with resistive loading.

Differential amplifier with active loading.

The four differential amplifier configurations are following:

Dual input, balanced output differential amplifier.

Dual input, unbalanced output differential amplifier.

Single input balanced output differential amplifier.

Single input unbalanced output differential amplifier.

5. What is positive feedback? (AUC MAY’04)

If the feedback signal φf is inphase with the input signal φs the net effect of feedback will

increase the input signal given to the amplifier φi. Hence φi= φs+ φf. Hence the input voltage

applied to the amplifier is increased thereby increasing the output φo. This type of feedback is

called positive or regenerative feedback.

6. What is negative feedback? (AUC MAY’06)

If the feedback signal φf is out of phase with the input signal φs the net effect of feedback

will decrease the input signal given to the amplifier φi. Hence φi= φs - φf. Hence the input voltage

applied to the amplifier is increased thereby decreasing the output φo. This type of feedback is

called negative or degenerative feedback.


7. Define feedback factor β. (AUC MAY’05)

Feedback factor β is defined as the ratio of φf to φo.

8. Define (i) feedback (ii) positive feedback and (iii) negative feedback.

i. Feedback: The process of combining a fraction of the output (of a Device-amplifier) back

to its input is called feedback.

ii. Positive Feedback: If the feedback is in phase to the input, it is called positive feedback. Here

iii. Negative Feedback: When the feedback is in opposition (out of phase) to the input, it is called

negative feedback.

9. What loop gain of a feedback amplifier.

In a feedback amplifier, when the signal passes through an amplifier.

10. Mention the four connections in Feedback.

1. Voltage series feedback.

2. Voltage shunt feedback 3. Current series feedback. 4. Current shunt feedback.

11. Explain the voltage series feedback.

In this case, the feedback voltage is derived from the output voltage and fed in series with input

signal. The input of the amplifier and the feedback network are in series is also known as series parallel

in parallel, hence this configuration is also known as series parallel feedback network.

12. Explain the voltage shunt feedback.

The input of amplifier and the feedback network are in parallel and known as parallel – parallel

feedback network. This type of feedback to the ideal current to voltage converter, a circulating

having very low input impedance and very low output impedance.

13. Explain the current series feedback.

When the feedback voltage derived from the load current and is fed in series with the input

signal, the feedback is said to be current series feedback, the inputs of the amplifier and the feedback

network are in series and the output are also in series. This configuration is also called as series-series

feedback configuration.

14. Explain the current shunt feedback.

When the feedback voltage is derived from the load current and a fed in parallel with the

input signal, the feedback is said to be current shunt feedback. Herein the inputs of the amplifier and

the feedback network are in parallel and the outputs are in series. This configuration is also

known as parallel series feedback.

15. Write the effects of negative feedback.

1. The gain becomes stabilized with respect to changes in the amplifier active device

parameters like hfe.

2. The non-linear distortion is reduced there by increasing the signal handling capacity or the

dynamic range of the amplifier.

16. Define feedback. (Or) What is meant by feedback?

The process of combining a fraction or part of output energy back to the input is known as

feedback.

17. What are the types of feedback? (Or) Classify the different types of feedback. (Or) What

are the 2 types of feedback amplifiers?

The different types of feedback are: 1. Positive feedback, 2. Negative feedback.


19. What is meant by positive feedback? (Or) Define direct feedback. (Or) Define

Regenerative feedback.

If feedback signal applied is in phase with the input signal and thus increases the input, it is called

as positive feedback. It is also known as regenerative feedback.

20. What is meant by negative feedback? (Or) Define inverse feedback. (Or) Define

degenerative feedback.

If the feedback signal applied to the input is out of phase with the input signal and thus

signal decrease, it is called negative feedback. It is also known as degenerative feedback.

21. How are the amplifiers classified based on the input and output.

The amplifier is basically classified into four types based on the input given and output

v obtained. They are

1).Voltage amplifier.

2). Current amplifier.

3).Transconductance amplifier.

4).Transresistance amplifier.

22. What are the effects of negative feedback? [APR-2003] (Or) What are the advantages of

negative feedback? [APR-2004]

1. It improves the stability of the circuit.

2. It improves the frequency response of the amplifier.

3. It improves the percentage of harmonic distortion.

4. It improves the signal to noise ratio (SNR).

5. It reduces the gain of the circuit.

23. What are the classifications of feedback amplifiers? (Or) List the four basic types of

negative feedback.

1. Current series feedback.

2. Current shunt feedback.

3. Voltage series feedback

4. Voltage shunt feedback.

24. Define the feedback factor β?

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

β = Vf/ Vo

Where, β is a feedback factor (or) feedback ratio, Vf is the feedback voltage, Vo is the output

voltage.

25. Define Desensitivity D? [APR – 2004]

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

voltage gain has been reduced due to feedback network.

Desensitivity factor (D) = 1+A β.

Where, A = Amplifier gain.

. β = Feedback factor.

26. Define loop gain. (Or) What is meant by return ratio?

The signal Vi is multiplied by „A‟ in passing through the amplifier is multiplier by „β‟.

In transmission through the feedback network and is multiplied by „-1‟ in the mixer. The product of

„- A β‟ is called the loop gain (or) return ratio.

27. 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 =Io / Vi

Where, Gm = Amplifier gain.

Io = Output current.

Ii = Input current.

28. What is voltage series feedback amplifier? (or) What is voltage amplifier?

In a voltage series feedback amplifier the sampled signal is a voltage and feedback signal (Which is

fed in series) is also a voltage.

A = Vo/ Vi


Vo = Output voltage.

Vi = Input voltage.

29. What is Voltage shunt feedback? (or) What is transresistance amplifier?

In voltage shunt feedback amplifier the sampled signal is a voltage and the feedback signal

(Which is fed in shunt) is a current.

Rm = Vo / Ii (or) Vo = Rm. Ii

Where, Rm = Amplifier gain.

Vo = Output voltage.

Ii = Input current.

30. What is current -shunt feedback amplifier? (or) What is current amplifier?

In a current shunt feedback amplifier, the sampled signal is a current and the feedback signal

(Which is fed in shunt) is a current.

A = Io / Ii (or) Io = A Ii.


Io = Output current.

Ii = Input current.

31. Write the expression for gain with feedback for positive and negative feedback.

For positive feedback:

Af = A / (1-A β)

For negative feedback:

Af = A / (1+A β)

Where, Af = Amplifier gain with feedback.

A = Amplifier gain without feedback.

β = Feedback factor.

32. Give an example for current-series feedback amplifier. (or) Give an example for

transconductance amplifier.

The common emitter amplifier with Re in the emitter lead and FET common source

amplifier stage with source resistor R are the best expel for current series feedback circuit.

33. Give an example for Voltage shunt feedback? (or) Give an example for transresistance

amplifier?

The collector feedback biased common emitter amplifier is an example of voltage –shunt

feedback circuit.

34. Give an example for voltage series feedback. (or) Give an example for voltage amplifier.

The common collector (or) emitter follower is an example of voltage series feedback.

35. 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.

36. What is the nature of input and output resistance in negative feedback?

(1) Voltage series feedback: Input impedance: Zif = Zi / (1+A β)

Output impedance: Zof = Zo / (1+A β)

(2) Voltage shunt feedback:

Input impedance: Rif = Ri * (1+A β)

Output impedance: Zof = Zo * (1+ A β)

(3) Current series feedback:

Input impedance: Rif = Zi / (1+A β)

Output impedance: Zof = Zo / (1+A β)

(4) Current shunt feedback:

Input impedance: Rif = Ri / (1+A β)

Output impedance: Rof = Ro / (1+A β)

37. What is meant by feedback?

A portion of the output signal is taken from the output of the amplifier and is

combined with the normal input signal. This is known as feedback.

(OR)

Feedback is a part of output is sampled and fedback to the input of the amplifier.

38. Give the different types of feedbacks used in amplifier circuits.

1. Positive feedback

2. Negative feedback.

3. Define the positive feedback.

When input signal and part of the output signal are in phase, the feedback is

calledPositive feedback.

39. Define negative feedback.

When input signal and part of the output signal are in out of phase, the feedback is

called negative feedback.

40. What type of feedback is used in oscillator?

Positive.

41. Give classification of amplifiers.

The amplifiers can be classified into four broad categories: voltage, current,

Tranconductance and Tranresistance amplifiers.

42. What is node sampling?

When the output voltage is sampled by connecting the feedback network in shunt

across the output, the connection is referred to as voltage or node sampling.

43. What is loop sampling?

When the output current is sampled by connecting the feedback network in series

with the output, the connection is referred to as current or loop sampling.

44. Define feedback factor or feedback ratio.

The ratio of the feedback voltage to output voltage is known as feedback factor or

feedback ratio.

45. What is the purpose of mixer network in feedback amplifier?

The mixer network is used to combine feedback signal and input at input of an amplifier.


46. What are the advantages of introducing negative feedback?

1. Input resistance is very high.

2. Output resistance is low.

3. The transfer gain Af of the amplifier with feedback can be stabilized against

Variations of the h-parameters or hybrid π parameters of the transistors or the Parameters of

the others active devices used in the amplifiers.

4. It improves the frequency response of the amplifiers.

5. There is a significant improvement in the linearity of operation of the feedback.

47. List the four basic feedback topologies.

1. Voltage amplifier with voltage series feedback.

2. Transconductance amplifier with current-series feedback.

3. Current amplifier with current-shunt feedback

4. Transresistance amplifier with voltage shunt feedback

48. Give the expression for gain of an amplifier with feedback.

Avf = AV/ 1+ AV β

Where, Avf – feedback voltage gain. AV – Voltage gain.

β - Feedback factor

49. What is loop gain or return ratio.

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 -A β . This gain is known as loop gain or return ratio.

50. What is sensitivity of the transfer gain?

The fractional change in amplification with feedback divided by the fractional

change without feedback is called the sensitivity of the transfer gain.

51. What is desensitivity?

The reciprocal of the sensitivity is called the desensitivity D. it is given as

D = 1+A β.

52. What is the effect of lower cut-off frequency with negative feedback?

Lower cutoff frequency with feedback is less than lower cutoff frequency without

feedback by factor (1+Amid β).

53. What is the effect of upper cut-off frequency with negative feedback?

Upper cutoff frequency with feedback is greater than upper cutoff frequency

without feedback by factor (1+Amid β).

54. What is the effect of negative feedback on bandwidth?

Bandwidth of amplifier with feedback is greater than bandwidth of amplifier

without feedback.

55. Why gain bandwidth product remains constant with the introduction of negative

feedback?

Since bandwidth with negative feedback increases by factor (1+A β) and gain

decreases by same factor, the gain-bandwidth product of an amplifier does not altered,

when negative feedback is introduced.

56. What is the effect of negative feedback on feedback distortion?

The frequency distortion is reduced with the negative feedback.


57. What is the effect of negative feedback on noise?

The noise is reduced with the negative feedback.

58. What is the effect of negative feedback on non linear distortion?

The linear distortion is reduced with the negative feedback.

59. What are the types of distortions in an amplifier?

1. Frequency

2. Noise and non linear.

60. What type of feedback is employed in emitter follower amplifier?

Voltage series feedback.

61. A feedback amplifier has an open loop gain of 600 and feedback factor β = 0.01.

Find the closed loop gain with feedback.

Avf = AV/ 1+ AV β

= 600/ (1+ 600*0.01)

= 85.714.

62. The distortion in an amplifier is found to be 3%, when the feedback ratio of

negative feedback amplifier is 0.04. When the feedback is removed, the distortion

becomes 15%. Find the open and closed loop gain.

Solution:

Given: β = 0.04

Distortion with feedback = 3%, Distortion without feedback = 15%

D = 15/3 = 5. Where D = 1+A β =5

A = 100.

63. Which is the most commonly used feedback arrangement in cascaded amplifiers

and why? Voltage series feedback is the most commonly used feedback arrangement in

cascaded amplifiers. Voltage series feedback increases input resistance and decreases

output resistance. Increase in input resistance reduces the loading effect of previous stage

and the decrease in output resistance reduces the loading effect of amplifier itself for

driving the next stage.

64. Voltage gain of an amplifier without feedback is 60dB. It decreases to 40dB with

feedback. Calculate the feedback factor.

Solution:

Given: Av = 60dB and Avf = 40 dB. We know that,

Avf = AV / 1+ AVβ

β = (AV- Avf) / ( AVAvf)

= (60-40) / (60*40)

β = 0.00833.

65. State the nyquist criterion for stability of feedback amplifiers?

1. The amplifier is unstable if the curve encloses the point -1+j0. The system is

called as unstable system.

2. The amplifier is stable if the curve encloses the point -1+jo. That system is called

as stable system.


66. What is nyquist diagram?

The plot which shows the relationship between gain and phase-shift as a function of

frequency is called as nyquist diagram.

67. Write the steps which are used to identify the method of feedback topology?

1. Identify topology (type of feedback)

a) To find the type of sampling network. b) To find the type of mixing network

2. Find the input circuit.

3. Find the output circuit.

4. Replace each active device by its h-parameter model at low frequency.

5. Find the open loop gain (gain without feedback), A of the amplifier.

6. Indicate Xf and Xo on the circuit and evaluate β = XfXO.

7. Calculate A, and β, find D, Ai, Rif, Rof, and Rof‟.

68. Write down the various characteristics of topology?

Characteristics Topology

Voltage series Current series Current shunt Voltage shunt

Sapling

signal,

XO

Voltage Voltage Current Current

Mixing

signal

Voltage Current Current Voltage

To find

input

loop, Set

V0=0 I0= 0 I0=0 V0=0

to find

output

loop, set

Ii=0 Ii = 0 Vi = 0 VI = 0

Signal

source

Thevenin Thevenin Norton Norton

β = Xf

/X0

Vf / V0 Vf/I0 If/I0 If/I0

A=X0/Xi AV=V0/

Vi

GM=I0/V

i

AI =I0/Ii RM=V0/I

i

D=1+ βA 1+ βAv 1+ βGM 1+ βAI 1+ βRM

Af AV/D GM/D AI/D RM/D

Rif RiD RiD Ri/D Ri/D

ROf R0/(1+β

AV)

R0(1+βG

M)

R0(1+βA

I)

R0/(1+βR

M)

ROf‟ R0‟/(1+β

AV)

R0(1+βA

V)

/(1+βAV)

R0(1+βA

V)/ (1+βAV)

R0‟/(1+β

RM)


PART B(16 Marks)

1. Explain the operation of voltage series feedback amplifier & current shunt feedback

amplifier. (AUC MAY 12, MAY09)

Voltage Series Feedback



Current Series Feedback:


2. Discuss the various topologies of feedback amplifier. (AUC APR’09)

Voltage Series Feedback



Current Shunt Feedback:



Voltage Series Feedback:

Voltage Shunt Feedback:

3. Explain the block diagram of feedback circuit.


• With an input signal xs, an output signal xo, a feedback signal xf, and an amplifier input signal xi, let‟s look at the basic feedback circuit illustrated above.

• The amplifier has a gain of A and the feedback network has a gain of ...

• The input to the amplifier is,

xi = xs - xf

• The output of the amplifier is,

xo = Axi

• So we can obtain an expression for the output signal in terms of the input signal and the feedback gain...

xo = A xs - xf = A xs - xo

• Rearranging,

xo = Axs - Axo xo 1 + A = Axs

• From which we obtain the negative feedback equation by solving for the overall gain x0 / xs

Afb xo

= A

1 + A

xs

• This means that the gain is almost entirely determined by the feedback circuit!!!

Afb xo

= A 1

for large A

1 + A

xs

• For positive feedback, you only need to change the “+” sign in the denominator to a “-” sign....

• It is easy to obtain the equation for the feedback signal, xf,

xf = A xs

1 + A


• If the amplifier gain and the loop gain are large (i.e. A >> 1), then the feedback signal xf becomes nearly an identical copy of the input signal xs....

This explains why the two terminals of an op-amp become nearly identical when using negative

feedback. The feedback may be classified into two types such as

1. Positive feedback

2. Negative feedback.

1. Positive feedback:

If feedback signal applied is in phase with the input signal results in increases the input and is

called positive feedback. It is also known as regenerative feedback because the voltage gain of a

feedback is greater than the open loop gain. From the general equation we get the desensitivity

factor for positive feedback is 1 – Aβ. If Aβ approaches unity then the closed loop gain becomes

infinity. It means that the circuit should give an output without any input. Thus, an amplifier

becomes an oscillator which supplies its own input. The positive feedback has the disadvantages

of increased distortion and instability and noise.

2. Negative feedback:

If the feedback signal applied to the input is out of phase with the input signal results in input

signal decreases, and is known as negative feedback. From the general equation we get the

desensitivity factor for negative feedback is (1 + Aβ). If (1 + Aβ) > 1 then the closed loop gain

reduces from the open loop gain “A”. thus it is called as degenerative feedback. Aβ >> 1 then the

closed loop gain

Avf = A/ (1 + Aβ) = A / Aβ = 1 / β.

They above equation indicates that the closed loop voltage gain entirely depends on

the feedback ratio “β”.

4. Feedback amplifier:

Signal-flow diagram of a feedback amplifier.

Open-loop gain: A

Feedback factor: β

Loop gain: Aβ

Amount of feedback: 1 + Aβ

Gain of the feedback amplifier (closed-loop gain):

Negative feedback:

The feedback signal xf is subtracted from the source signal xs

Negative feedback reduces the signal that appears at the input of the basic amplifier.

The gain of the feedback amplifier Af is smaller than open-loop gain A by a factor of (1+Aβ)


The loop gain Aβ is typically large (Aβ >>1):

The gain of the feedback amplifier (closed-loop gain) Af β 1/ β

The closed-loop gain is almost entirely determined by the feedback network β better accuracy

of Af

xf = xs(Aβ)/(1+Aβ) = xs error signal xi = xs – xf

Example

The feedback amplifier is based on an opamp with infinite input resistance and zero output

resistance

(a) Find an expression for the feedback factor.

(b) Find the condition under which the closed-loop gain Af is almost entirely determined by the

feedback network.

(c) If the open-loop gain A = 10000 V/V, find R2/R1 to obtain a closed-loop gain Af of 10 V/V.

(d) What is the amount of feedback in decibel?

(e) If Vs = 1 V, find Vo, Vf and Vi.


5. Properties of negative feedback:



6. Basic feedback topologies:



8. Nyquist stability criterion:


Points to note:

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EC2251 ELECTRONIC CIRCUITS II Department of …mahalakshmiengineeringcollege.com/pdf/ece/IVsem/EC2251...Voltage shunt feedback amplifier Current series feedback amplifier Current shunt