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2. AMPLIFIERS AND OSCILLATORS 6/9/2015www.noteshit.com2

3. Differential amplifier An amplifier which amplifies the difference between the two input signals is called differential amplifier Fig 9.1. 6/9/2015www.noteshit.com3

4. CLASSIFICATION Differential amplifier is classified as 1 Voltage amplifier 2. Current amplifier 3. Tran conductance amplifier 4. Tran resistance amplifier 6/9/2015www.noteshit.com4

5. CHARACTERISTICS The characteristics of differential amplifier 1.Infinite input resistance 2.Zero output resistance 3.Infinite gain 4.Infinite Bandwidth 6/9/2015www.noteshit.com5

6. BASIC DIFFERENTIAL AMPLIFIER Basic differential amplifier has two input voltage signals. 6/9/2015www.noteshit.com6

7. Cont V O =A d (V 1 -V 2 ) Where A d is called differential gain, (V 1 -V 2 ) Is called differential voltage V d = (V 1 -V 2 ). A d =V O / V d if V 1 =V 2 then V d, and V O would become zero. In practical differential Amplifier depends on both difference and average common level of two inputs. 6/9/2015www.noteshit.com7

8. Cont…. Common mode signal is V C = (V 1 +V 2 ) /2 V O =A C (V 1 +V 2 ) /2 or V O =A C V C. Where V C is called as common mode gain. The Total output voltage is V O = A d V d +A C V C. 6/9/2015www.noteshit.com8

9. COMMON MODE REJECTION RATIO The ability of a differential amplifier to reject a common mode signal is expressed by CMMR =ρ=20 log A d/ A C Differential amplifier has two modes Common mode and differential mode. 6/9/2015www.noteshit.com9

10. DIFFERENTIAL MODE 6/9/2015www.noteshit.com10

11. Cont…  The input signals are same in magnitude but 180 degree phase shift  Q 1 is positive going and Q 2 is negative going.  R E has same current but 180 degree phase shift,so AC current in R E is zero.  The output of collector one and two are equal but opposite in phase. 6/9/2015www.noteshit.com11

12. Cont…..  Therefore the total output voltage of differential mode is V d = (V 1 -V 2 ) V 1 = Voltage across collector one V 2 = Voltage across collector two  Therefore total output voltage is twice as large as signal voltage from either collector to ground. 6/9/2015www.noteshit.com12

13. COMMON MODE 6/9/2015www.noteshit.com13

14. Cont..  The input signals of Q 1 and Q 2 are equal in magnitude and phase.  RE carries current (2I E ) and provides negative feedback.  The output voltage at collector one and two are same in phase and magnitude  Therefore Vo= 0 in common mode. 6/9/2015www.noteshit.com14

15. Feedback amplifiers and oscillators Feedback is a process of injecting some energy from the output and then return it to the input.The amplifier which uses this is called feedback amplifier. Classification of feedback amplifiers 1. Positive Feedback amplifiers 2. Negative Feedback amplifiers 6/9/2015www.noteshit.com15

16. Feedback amplifiers and oscillators 6/9/2015www.noteshit.com16

17. Feedback amplifiers and oscillators 6/9/2015www.noteshit.com17

18. Feedback amplifiers and oscillators Types of feedback connections 6/9/2015www.noteshit.com18

19. Feedback amplifiers and oscillators 6/9/2015www.noteshit.com19

20. Feedback amplifiers and oscillators 6/9/2015www.noteshit.com20

21. Feedback amplifiers and oscillators 6/9/2015www.noteshit.com21

22. Feedback amplifiers and oscillators 6/9/2015www.noteshit.com22

23. Feedback amplifiers and oscillators 6/9/2015www.noteshit.com23

24. Feedback amplifiers and oscillators 6/9/2015www.noteshit.com24

25. Feedback amplifiers and oscillators 6/9/2015www.noteshit.com25

26. Feedback amplifiers and oscillators 6/9/2015www.noteshit.com26

27. Feedback amplifiers and oscillators 6/9/2015www.noteshit.com27

28. Feedback amplifiers and oscillators 6/9/2015www.noteshit.com28

29. Feedback amplifiers and oscillators 6/9/2015www.noteshit.com29

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33. Oscillators 6/9/2015www.noteshit.com33

34. Oscillator Introduction of Oscillator Linear Oscillator –Wien Bridge Oscillator –RC Phase-Shift Oscillator –LC Oscillator Stability 6/9/2015www.noteshit.com34

35. Oscillators Oscillation: an effect that repeatedly and regularly fluctuates about the mean value Oscillator: circuit that produces oscillation Characteristics: wave-shape, frequency, amplitude, distortion, stability 6/9/2015www.noteshit.com35

36. Application of Oscillators Oscillators are used to generate signals, e.g. –Used as a local oscillator to transform the RF signals to IF signals in a receiver; –Used to generate RF carrier in a transmitter –Used to generate clocks in digital systems; –Used as sweep circuits in TV sets and CRO. 6/9/2015www.noteshit.com36

37. Linear Oscillators 1.Wien Bridge Oscillators 2.RC Phase-Shift Oscillators 3.LC Oscillators 4.Stability 6/9/2015www.noteshit.com37

38. Integrant of Linear Oscillators For sinusoidal input is connected “Linear” because the output is approximately sinusoidal A linear oscillator contains: - a frequency selection feedback network - an amplifier to maintain the loop gain at unity 6/9/2015www.noteshit.com38

39. Basic Linear Oscillator and If V s = 0, the only way that V o can be nonzero is that loop gain A  =1 which implies that (Barkhausen Criterion) 6/9/2015www.noteshit.com39

40. Wien Bridge Oscillator Frequency Selection Network Let and Therefore, the feedback factor, 6/9/2015www.noteshit.com40

41.  can be rewritten as: For Barkhausen Criterion, imaginary part = 0, i.e., Supposing, R 1 =R 2 =R and X C1 = X C2 =X C, 6/9/2015www.noteshit.com41

42. Example By setting, we get Imaginary part = 0 and Due to Barkhausen Criterion, Loop gain A v  =1 where A v : Gain of the amplifier Therefore, Wien Bridge Oscillator 6/9/2015www.noteshit.com42

43. RC Phase-Shift Oscillator  Using an inverting amplifier  The additional 180 o phase shift is provided by an RC phase-shift network 6/9/2015www.noteshit.com43

44. Applying KVL to the phase-shift network, we have Solve for I 3, we get Or 6/9/2015www.noteshit.com44

45. The output voltage, Hence the transfer function of the phase-shift network is given by, For 180 o phase shift, the imaginary part = 0, i.e., and, Note: The –ve sign mean the phase inversion from the voltage 6/9/2015www.noteshit.com45

46. LC Oscillators  The frequency selection network (Z 1, Z 2 and Z 3 ) provides a phase shift of 180 o  The amplifier provides an addition shift of 180 o Two well-known Oscillators: Colpitts Oscillator Harley Oscillator 6/9/2015www.noteshit.com46

47. For the equivalent circuit from the output Therefore, the amplifier gain is obtained, 6/9/2015www.noteshit.com47

48. The loop gain, If the impedance are all pure reactances, i.e., The loop gain becomes, The imaginary part = 0 only when X 1 + X 2 + X 3 =0  It indicates that at least one reactance must be –ve (capacitor)  X 1 and X 2 must be of same type and X 3 must be of opposite type With imaginary part = 0, For Unit Gain & 180 o Phase-shift, 6/9/2015www.noteshit.com48

49. Hartley OscillatorColpitts Oscillator 6/9/2015www.noteshit.com49

50. Colpitts Oscillator Equivalent circuit In the equivalent circuit, it is assumed that:  Linear small signal model of transistor is used  The transistor capacitances are neglected  Input resistance of the transistor is large enough 6/9/2015www.noteshit.com50

51. At node 1, where, Apply KCL at node 1, we have For Oscillator V  must not be zero, therefore it enforces, 6/9/2015www.noteshit.com51

52. Imaginary part = 0, we have Real part = 0, yields 6/9/2015www.noteshit.com52

53. THANK U 6/9/2015www.noteshit.com53