FEEDBACK AMPLIFIERS UNIT-I

UNIT – I

FEEDBACK AMPLIFIERS

PART - A

1. Classify negative feedback amplifiers?        (MAY 2010 & NOV 2010)

            (i) Voltage series feedback (ii) Voltage shunt feedback (iii) Current series feedback (iv) Current shunt  feedback.

2. What are merits of negative feedback amplifiers?(MAY2010&JUN 2009)

            The performance measures of negative feedback amplifiers are as follows(i) Improved stability   ii) Reduction in gain    iii) Reduction in distortion and noise iv) Increase the input Impedance and bandwidth  v) Decrease the output impedance

3. Define Feedback factor?                 (MAY 2010, MAY 2012)

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

4. Discuss the effect of feedback on noise.            (MAY 2011)

            Effect of feedback on noise can be explained with the following consideration:Let A₁ and A₂ be two amplifier feedback circuit .N be the noise introduced after stage1.The output voltage can be expressed as,

 V₀  =                                           A₁A₂V_s          =          NA₂

                                             1 + A₁A₂2                 1 + A₁ A₂2

Thus the overall noise of two stage amplifier is reduced by the factor 1 + A₁A₂b.

5. “The gain bandwidth product of an amplifier is not altered, when negative feedback is introduced”-Justify the statement.    (NOV 2004)

            When negative feedback is employed in an amplifier, its bandwidth increases by the same factor (1 + Ab) by which its gain reduces .Hence the product of gain and bandwidth called gain bandwidth product  remains the same.

6. Draw the block diagram of voltage feedback amplifier and give its input and output resistance.                         (MAY 2005)

 

7. Define loop gain. (NOV 2007)

            The product of the feedback factor b and the gain of the amplifier multiplied by –1 (for negative feedback) is called loop gain or return ratio or the loop transmission feedback factor.

8.  What is loop gain? (May 11)

            A path of a signal from input terminals through basic amplifiers, through the feedback network and back to the input terminals from a loop. The gain of this loop is the product –Aβ. This gain is known as loop gain.

9.  What is return ratio? (May 11)

            A path of a signal from input terminals through basic amplifiers, through the feedback network and back to the input terminals from a loop. The gain of this loop is the product –Aβ. This gain is known as loop gain or return ratio.

10. Compare the input and output resistance of the voltage and current shunt feedback amplifiers.                       (NOV 2008, NOV 2011)

	Input Resistance	Output Resistance
Voltage Shunt	Rif = Ri / (1+Aβ)	R0f = R0 / (1+Aβ)
Current Shunt	Rif = Ri / (1+Aβ)	R0f = R0 .(1+Aβ)

11. If an amplifier has the gain of 400 and feedback ratio of 0.1, find the gain with negative feedback.                (NOV 2008 , 2010, 2011)

            Negative feedback gain Af = A / (1+Aβ), Given  A=400, β=0.      

Therefore, gain = 400 / (1+(400 X 0.1)) = 9.7

12. State Nyquist stability criterion.                                   (MAY 2010)

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

13. Define sensitivity in feedback amplifiers .                    (MAY 2011)

            The fractional change in the amplification with feedback divided by the fractional change without feedback is called the sensitivity.

 

15. Define desensitivity.   (MAY 2004 & NOV 2010, NOV 2011, MAY 2012)

            The fractional change in the amplification with feedback divided by the fractional change without feedback is called the sensitivity. The reciprocal of the sensitivity is called the desensitivity.  D= 1+Aβ.

16.Define positive feedback.

If the feedback signal is in phase with input signal, then the net effect of the feedback will increase the input signal given to the amplifier. This type of feedback is said to be positive or regenerative feedback.

17. Give the effect of negative feedback on amplifier characteristics.

Characteristics	Type of feedback
	Current-series	Voltage-series	Voltage-shunt	Current-shunt
Voltage gain	Decreases	Decreases	Decreases	Decreases
Bandwidth	Increases	Increases	Increases	Increases
Input resistance	Increases	Increases	Decreases	Decreases
Output resistance	Increases	Decreases	Decreases	Increases

18. Give an example for voltage-series feedback.

        The Common collector or Emitter follower amplifier is an example for voltage series feedback.

 

19. Define negative feedback?

          If the feedback signal is out of phase with the input signal then the input voltage applied to the basic amplifier is decreased and correspondingly the output is decreased. This type of feedback is known as negative or degenerative feedback.

20. Define 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.

21.Write the expression for input  resistance of voltage series feedback amplifier.

                               Input resistance with feedback, Rif  = Ri 

 22. Write the expression for output  resistance of voltage series feedback amplifier.

               Output resistance with feedback, Rof = Ro

 23. What type of feedback used oscillators?

                                    Positive feedback.

24. What is the purpose of mixer network in feedback amplifiers?

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

25. Define feedback factor or feedback ratio.

            The ratio of feedback voltage to output voltage is known as feedback factor.

26. What is nyquist diagram?

            The plot which shows the relationship between gain and phase shift as a function of frequency is called nyquist diagram.

27. State the Nyquist criterion for stability of feedback amplifiers.

            * The amplifiers is unstable if the curve enclose the point -1+j0. The system is called unstable system.

            * The amplifiers is stable if the curve enclose the point -1+j0. The system is called stable system.

28. What is the effect of voltage series feedback on circuit impedance?

            The effect of voltage series feedback on circuit impedance increases the input impedance but decreases the output impedance.

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

            The noise is reduced with the negative feedback.

30. What is the effect of  negative feedback on frequency distortion?

            The frequency distortion is reduced with  the negative feedback.

31. Which type of negative feedback always produces an increase in input resistance and decrease in output resistance?

            Negative voltage series feedback always produces an increase in input resistance but decrease in output resistance.

32. What are the types of distortion in an amplifiers?

* Frequency

* Non linear

33. Why is positive feedback rarely used in practice?

            Positive feedback is rarely used in practice because it has the disadvantage of increased distortion and instability.

34. Give two examples of Voltage series feedback amplifier.

            The examples of voltage series feedback amplifier are (i) emitter follower   (ii) source follower.

35. Distinguish between series and shunt feedback.

SERIES FEEDBACK	SHUNT FEEDBACK
i)The feedback signal is connected  in series with the input signal.	i) The feedback signal is connected in parallel with the input signal.
ii) It increases the input resistance.	ii) It decreases the input resistance.

36. Discuss the effect of feedback on output impedance.

            Any amplifier with low output impedance is capable of delivering power to the load without much loss and it can be easily achieved with the help of of feedback techniques. In this case, the output impedance of the feedback amplifier is given as

       Z_of  =                                                   Zo

                                                              1 + Ab

37. Explain how reduction in distortion is carried out in case of feedback amplifier.

            Let D^| be the distortion in amplifier without feedback and D be the distortion in amplifier with       feedback. After amplification, the distortion is in anti phase with the original distortion voltage D and      expressed as                

                                       D^| =        D

                                                 1 + Ab

38. Discuss the disadvantage of negative feedback.

            The main disadvantage of negative feedback is that it reduces the input to the amplifier which in turns reduces the output of the amplifier.

39. What are the two types of feedback?

             If the sampled output adds to the input of the amplifier, it is called positive or direct or regenerative feedback. A feedback is said to be negative or inverse or degenerative if the sampled output reduces the input to the amplifier.

40. Compare the amplifier gain with and without feedback.

            Gain of the amplifier without feedback is,A= X₀ / X_i        Where X₀ – Output voltage, X_i – Input Voltage Gain of the with feedback (negative) is given as,A_f  =  A /(1 + Ab) Where  A = amplifier gain without feedback, b= feedback gain, A_f  = gain with feedback.

41. Define sampling and mixing.

            Sampling is the process of taking a part of output voltage or current. The process of adding or subtracting

this sampled value to the input of the amplifier is called mixing.

42. What are the types of distortion that occur in amplifiers?

            Non-linear distortion, phase distortion, frequency distortion and noise distortion are the types of distortion had occur in amplifiers.

43. Give some applications of voltage feedback.

            (i) Public address system, (ii) transistor radio receivers etc..,

44. What type of feedback is employed in a common collector amplifier?

            Negative voltage series feedback is employed in a common collector amplifier.

45. What is the basic difference between feedback in biasing circuits and amplifier circuits?

            In biasing circuits, we employ dc negative feedback while in amplifiers, it is ac negative feedback.

46. Why negative feedback is employed in high gain amplifiers?

            The negative feedback reduces the gain of the amplifiers but improves the circuit performance in terms of stability distortion, etc. Hence it is employed in high gain amplifier.

47. What is phase margin?

            The phase margin evaluated at the frequency to where the magnitude of A(jw) b reaches unity. The phase margin is defined as the difference between the angle of A(jw) b at w and the angle-180°.

48. What is gain margin?

            The stability of a feedback loop can also be assessed by examining its bode plot and evaluating one of two related parameters. One parameter called the gain margin is defined as the difference between unity and magnitude of A(jw) b at w 180°.

49. What is feedback?

            When we sample a part of the output voltage or current of an amplifier by a suitable circuit and apply this to the input so as to modify the performance and operation of the circuit, this is called feedback.

50. What is a feedback amplifier?

            If an amplifier output voltage or current is sampled and this sampled output is applied to the input through a two port network, the network becomes a feedback  amplifier.

51. List the types of feedback.

            Broadly classifying, the feedback can be divided as positive or regenerative or direct and negative or  degenerative or inverse. Based on the type of sampling and mixing, it can be further classified into voltage  series or shunt and current series or shunt.

52. What is negative feedback?

            When the effect of the feedback signal on the action of the circuit is opposite to that produced by the input   signal, then the feedback is called negative, degenerative or inverse.

PART - B

1. (a) Draw the block diagram of current sampling - series mixing feedback configuration and derive an expression for transfer gain, input resistance and output resistance.            (MAY 2010)

    (b) How the negative feedback amplifier improves stability, reduces noise and increases the input impedance?

2. (a) Identify four possible topologies of a feedback network.

    (b) Identify the o/p signal & f/b signal for each topology.

    (c) Identify the transfer gain & feedback factor for each topology.             

                                                                                    (MAY 2010)

3. (a) Write notes on nyquist criterion.

    (b) Writs notes on a feedback amplifier, using a block diagram.                                                                                                  (MAY 2010)

4. With block diagram of current series feedback and derive the expression for R_if and R_of..                                                                                                           (DEC 2009)

5. Design a two stage voltage series feedback amplifier so as to reduce the gain to 75. Assume β=0.01, Vcc=20,h_fe1=h_fe2=100. Draw the designed circuit.                                                                                  (DEC 2009)

6. (a) Using block diagram, derive the closed loop forward transfer ratio of feedback system in terms of the open gain.                   (NOV 2011)

    (b) Discuss the effect of negative feedback on the frequency response of an amplifier.                                                  (JUN 2009,  MAY 2012)

 7. Draw the circuit of emitter follower. Identify the type of negative feedback. Calculate the gain, input and output resistance with and without feedback.                                                     (DEC 2008 & NOV 2010)

8. (a) Discuss the classification of feedback amplifiers with schematic(topology). How is impedance level modified in each type?                                                                                  (MAY 2008, MAY 2012)

    (b) Derive expression for Avf with positive feedback and state condition for stability in negative feedback  amplifiers.

9. Draw the circuit of a current series feedback amplifier and explain. Derive expressions for input and output  impedance. How does it improve the stability of the amplifier.                                                                        (DEC 2007)

10. Give the block diagram of feedback amplifier and discuss the effect of negative feedback with respect to closed loop gain, bandwidth and distortion.                                                                                        (DEC 2006)

11. (a) Describe briefly the general characteristics of negative feedback amplifier.

      (b) Draw the basic circuit of the voltage shunt feedback amplifier and describe the concepts involved in such an amplifier.                   (DEC 2005)

12. (a) Using a block diagram, derive the expression of closed loop forward transfer ratio with positive and negative feedbacks introduced in an amplifier.         (MAY 2005, NOV 2011, MAY 2012)

(b) List out the steps that are carried out in obtaining the complete analysis of a feedback amplifier.

13. Define and explain the following types of amplifier (i) voltage amplifier (ii) Current amplifier (iii) trans-conductance amplifier (iv) trans-resistance amplifier. Derive a general expression for gain with feedback. What is an ideal feedback amplifier?                                               (APR 1995)

14. Derive the expressions for A_vf, R _if,  R_of, R_o for voltage series feedback transistor amplifier?

15. Perform the general analysis of current shunt feedback amplifier.

16. Explain the general analysis of Voltage shunt feedback.

17. (a) Using an example, explain voltage series feedback and analyze the circuit to determine the gain,

input and output resistance.

      (b) Explain Nyquist criterion to analyze the stability of feedback amplifiers.     (MAY 2011)

18. The gain bandwidth product of an amplifier is not altered, when negative feedback is introduced. Justify the statement.                             (NOV 2011)

19. For, a feedback amplifier, derive the expressions for (1) the gain with feedback (2) Lower cut off  frequency (3) Upper cut off frequency. (MAY 2012)

20. If an amplifier has a bandwidth of 300 kHz and a voltage gain of 100, what will be the new bandwidth and gain if 10% negative feedback is introduced? What will be the gain – bandwidth product before and after feedback? What should be the amount of feedback if the bandwidth is to be limited to 800 kHz?

21. A current series feedback amplifier is shown in the figure below:

It has the following parameters: R1 =20K Ω, R2=20K Ω, h_ie = 2 K Ω, R_L=1K Ω, R_e=100 Ω, h_fe=80, h_re=h_oe=0 Calculate A_v, β, R_if  and A_{vf .}      (MAY 2012)