Unit - V feedback amplifiers and oscillator

Electronics devices and circuits

1. FEEDBACK AMPLIFIERS 1 Electronic devices & circuits UNIT-V Dr. mohammed mahaboob basha Associate Professor Department of ECE

2. Contents of Feed back Amplifier Introduction to Feedback Feedback Amplifier – Positive & Negative Advantages/Disadvantages of Negative Feedback Basic Feedback Concept Classification of Amplifiers Series – Shunt Configuration Shunt – Series Configuration Series - Series Configuration Shunt – Shunt Configuration

3. Classification of Basic Amplifiers Amplifiers can be classified broadly as, Voltage amplifiers. Current amplifiers. Transconductance amplifiers. Transresistance amplifiers.

4. Voltage Amplifier

5. Current Amplifier

6. Transconductance Amplifier

7. Transresistance Amplifier

8. Parameter Comparisons

9. Feedback Amplifier Feedback is a technique where a portion of the output of a system (amplifier) is fed back and recombined with input The output quantity (either voltage or current) is sampled by suitable sampler. The output of feed back network which has a fraction of the output signal combined with input signal through mixer There are 2 types of feedback amplifier: Positive feedback Negative feedback

10. Block diagram of a basic Feedback Amplifier

11. Positive Feedback Positive feedback is the process in which the portion of output is added to the input, amplified again, and this process continues. Positive feedback is used in the design of oscillators and other applications.

12. Feedback V o is the input voltage for the feedback network • V f is output voltage of the feedback network Feedback ratio Basic Amplifier gain

13. Gain in Positive Feedback Amplifier Feedback Amplifier gain • From the circuit,  

14. Gain in Negative Feedback Amplifier Feedback Amplifier gain • From the circuit,

15. Effects of Negative Feedback • Stabilization of Gain Increased Bandwidth Decreased Distortion Decreased Noise Increase in Input Impedance Decrease in output Impedance

16. Advantages of Negative Feedback Gain Sensitivity – variations in gain is reduced. Bandwidth Extension – larger than that of basic amplifier. Noise Sensitivity – may increase S-N ratio. Reduction of Nonlinear Distortion Control of Impedance Levels – input and output impedances can be increased or decreased.

17. Disadvantages of Negative Feedback Circuit Gain – overall amplifier gain is reduced compared to that of basic amplifier. 2. Stability – possibility that feedback circuit will become unstable and oscillate at high frequencies.

18. Classification of Amplifiers Classify amplifiers into 4 basic categories based on their input (parameter to be amplified; voltage or current) & output signal relationships: Voltage-Series Feed back amplifier Voltage- Shunt Feed back amplifier Current – Series Feed back amplifier Current – Shunt Feed back amplifier

19. Sampling Network

20. Feedback Network It may consists of resistors, capacitors and inductors. Most often it is simply a resistive configuration. It provides reduced portion of the output as feedback signal to the input mixer network. It is given as Where is a feedback factor or feedback ratio.

21. Mixer Network

22. Feedback Configuration

23. Voltage- Series Feedback Amplifier Input to the feedback network is in parallel with the output of amplifier. A fraction of the output voltage is applied in series with the input voltage through a feedback network. The shunt connection at the output reduces the output resistance. The series connection at the input increases the input resistance. The voltage feedback factor is given by It is a true voltage amplifier

24. Voltage - Series Feedback amplifier Voltage gain:

25. Voltage - Series Feedback amplifier Input Impedance:

26. Voltage - Series Feedback amplifier Output Impedance:

27. Voltage- Shunt Feedback Amplifier Input to the feedback network is in parallel with the output of amplifier. A fraction of the output voltage is applied in parallel with the input voltage through a feedback network. The shunt connection at the output reduces the output resistance. The Shunt connection at the input reduces the input resistance. The voltage feedback factor is given by It is also called a Trans-Resistance amplifier

28. Voltage- Shunt Feedback Amplifier Trans-Resistance gain:

29. Voltage- Shunt Feedback Amplifier Input Impedance:

30. Voltage- Shunt Feedback Amplifier Output Impedance:

31. Current- Series Feedback Amplifier Input to the feedback network is in series with the output of amplifier. A fraction of the output current is applied in series with the input voltage through a feedback network. The series connection at the output increases the output resistance. The series connection at the input increases the input resistance. The feedback factor is given by It is also called a Trans-conductance amplifier

32. Current- Series Feedback Amplifier Trans-Conductance:

33. Current- Series Feedback Amplifier Input impedance:

34. Current- Series Feedback Amplifier Output impedance:

35. Current- Shunt Feedback Amplifier Input to the feedback network is in series with the output of amplifier. A fraction of the output current is applied in parallel with the input voltage through a feedback network. The series connection at the output increases the output resistance. The Shunt connection at the input reduces the input resistance. The voltage feedback factor is given by It is also called a Current amplifier

36. Current- Shunt Feedback Amplifier Current gain:

37. Current- Shunt Feedback Amplifier Input Impedance:

38. Current- Shunt Feedback Amplifier Output Impedance:

39. Feedback Amplifier Input and output Impedances Summary For a series connection at input or output, the resistance is increased by (1+  A). For a shunt connection at input or output, the resistance is lowered by (1+  A).

40. Feedback Amplifier

41. Comparison Between Positive and Negative Feed Back

42. Q1 .The voltage gain of an amplifier without feedback is 3000. Calculate the voltage gain of the amplifier if negative voltage feedback is introduced in the circuit. Given that feedback fraction β  = 0.01.

43. Q2. The overall gain of a multistage amplifier is 140. When negative voltage feedback is applied, the gain is reduced to 17.5. Find the fraction of the output that is feedback to the input.

44. Q3 . When negative voltage feedback is applied to an amplifier of gain 100, the overall gain falls to 50. (i) Calculate the fraction of the output voltage feedback. (ii) If this fraction is maintained, calculate the value of the amplifier gain required if the overall stage gain is to be 75.

45. Q4. With a negative voltage feedback, an amplifier gives an output of 10 V with an input of 0.5 V. When feedback is removed, it requires 0.25 V input for the same output. Calculate (i) Gain without feedback (ii) feedback fraction β .

46. Q5. 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. It may be seen that by the application of negative voltage feedback, the amplifier distortion is reduced from 5% to 0.313%.

47. Q6. An amplifier has a gain of 1000 without feedback and cut-off frequencies are f1 = 1.5 kHz and f2 = 501.5 kHz. If 1% of output voltage of the amplifier is applied as negative feedback, what are the new cut-off frequencies ? Note the effect of negative voltage feedback on the bandwidth of the amplifier. The lower cut-off frequency is decreased by a factor (1 + Aν β ) while upper cut-off frequency is increased by a factor (1 + Aν β ). In other words, the bandwidth of the amplifier is increased approximately by a factor (1 + Aν β ). Solution : Given, , f1 = 1.5 kHz and f2 = 501.5 kHz

48. Problem 7

49. OSCILLATORS

50. RC Phase Shift Oscillator

51. RC Phase Shift Oscillator

52. Wien-Bridge Oscillator

53. Wien-Bridge Oscillator

54. LC Oscillators

55. Hartley Oscillator colpitts oscillator

56. For Hartley Oscillator

57. We know that

58. Colpitts

59. Crystal oscillator

60. Crystal Oscillator Used when accuracy and stability of f o is utmost important. Where do you need such high stability of frequency of oscillations ? Instead of an inductor, it uses a crystal of quartz, tourmaline , or Rochelle salt . Piezoelectric effect. The crystal is suitably cut and then mounted between two metallic plates.

61. Crystal oscillator is most commonly used oscillator with high-frequency stability. They are usually, fixed frequency oscillators where stability and accuracy are the primary considerations. In order to design a stable and accurate LC oscillator for the upper HF and higher frequencies it is absolutely necessary to have a crystal control; hence, the reason for crystal oscillators. In crystal the primary frequency determining element is a quartz crystal . Because of the inherent characteristics of the quartz crystal the crystal oscillator may be held to extreme accuracy of frequency stability. Crystal Oscillator

62. Crystal Oscillator The crystal size and cut determine the values of L, C s , R and C m . The resistance R is the friction of the vibrating crystal capacitance C s is the compliance, and inductance L is the equivalent mass. The capacitance C m is the electrostatic capacitance between the mounted pair of electrodes with the crystal as the dielectric.

63. Crystal Oscillator Piezoelectric Effect The quartz crystal is made of silicon oxide (SiO 2 ) and exhibits a property called the piezoelectric When a changing an alternating voltage is applied across the crystal, it vibrates at the frequency of the applied voltage. In the other word, the frequency of the applied ac voltage is equal to the natural resonant frequency of the crystal. The thinner the crystal, higher its frequency of vibration. This phenomenon is called piezoelectric effect.

64. Oscillators 97 गुरुवार, 10 फरवरी 2022 C m (mounting capacitance) = 3.5 pF; C s = 0.0235 pF; L = 137 H; R = 15 k Ω

65. Crystal Oscillator Characteristic of Quartz Crystal The crystal can have two resonant frequencies; One is the series resonance frequency f 1 which occurs when X L = X C . At this frequency, crystal offers a very low impedance to the external circuit where Z = R. The other is the parallel resonance (or antiresonance) frequency f 2 which occurs when reactance of the series leg equals the reactance of C M . At this frequency, crystal offers a very high impedance to the external circuit. R L C C M

66. Crystals have incredibly high Q. For the given values, Q = 5500. Q as high as 100000 can be possible. An LC circuit has Q not greater than 100. The extremely high value of Q makes f o highly stable. Crystal Oscillator

67. Series and Parallel Resonance First, resonance occurs at f s for the series combination of L and C s . Above f s the series branch L,C s , and R has inductive reactance. It then resonates at f p , with C m . For this parallel resonance, equivalent series capacitance is C p . Crystal Oscillator

68. Series Resonance Parallel Resonance Crystal Oscillator

69. Normally, C s is much smaller than C m . Therefore, C p is slightly less than C s . Hence, the frequency f p is slightly greater than f s . The crystal is inductive only between the frequencies f s and f p . The frequency of oscillation must lie between these frequencies. Hence the stability. Crystal Oscillator

70. Oscillators 103 गुरुवार, 10 फरवरी 2022 The f o is between 411 kHz and 412 kHz.

71. Thank You