1. EMT 112/4 ANALOGUE ELECTRONICS 1 Power Amplifiers Syllabus Power amplifier classification, class A, class B, class AB, amplifier distortion, class C and D, transistor power dissipation, thermal management.

2. Part II Classification of Power Amplifiers POWER AMPLIFIERS

3. POWER AMPLIFIER – Classification They are grouped together based on their Q-points on the DC load line.

4. POWER AMPLIFIER – Classification In class-A; the transistor conducts during the whole cycle of sinusoidal input signal

5. POWER AMPLIFIER – Classification In class-B; the transistor conducts during one-half cycle of input signal

6. POWER AMPLIFIER – Classification In class-AB; the transistor conducts for slightly more than half a cycle of input signal

7. In class-C; the transistor conducts for less than half a cycle of input signal POWER AMPLIFIER – Classification

8. POWER AMPLIFIER – Class-A Operation For maximum swing ( +ve and –ve), transistor is biased such that the Q point is at centre of the load line. The transistor conducts for a full cycle of the input signal

9. POWER AMPLIFIER – Class-A Operation Instantaneous power dissipation in transistor is; For sinusoidal input signal; And;

10. POWER AMPLIFIER – Class-A Operation For maximum possible swing; And; Therefore;

11. POWER AMPLIFIER – Class-A Operation When the input signal = 0, the transistor must be capable of handling a continuous power of; Efficiency; P L = average ac power to the load P S = average power supplied by the source ( V CC )

12. POWER AMPLIFIER – Class-A Operation For maximum possible swing; Power supplied by the source; The efficiency; Maximum theoretical efficiency of class A amplifier is therefore 25%

13. POWER AMPLIFIER – Class-B Operation Consists of complementary pair electronic devices One conducts for one half cycle of the input signal and the other conducts for another half of the input signal Both devices are off when the input is zero (See Figure)

14. POWER AMPLIFIER – Class-B Operation The complementary pair When v I = 0, both A and B are OFF and therefore v O = 0.

15. POWER AMPLIFIER – Class-B Operation When v I > 0, A is ON and B is OFF and v O > 0.

16. POWER AMPLIFIER – Class-B Operation When v I < 0, A is OFF and B is ON and v O < 0.

17. POWER AMPLIFIER – Class-B Operation The transfer characteristic of the complementary pair

18. POWER AMPLIFIER – Class-B Operation Approximate Class-B : Complementary push-pull circuit

19. POWER AMPLIFIER – Class-B Operation Approximate Class-B : Complementary push-pull circuit Assuming ideal transistor; when v I = 0; both Q n & Q p are off and v O = 0

20. POWER AMPLIFIER – Class-B Operation Approximate Class-B : Complementary push-pull circuit v O = v I  v BEn When v I > 0; Q p is off & Q n conducts and becomes an emitter follower, sourcing the current I L to R L.

21. POWER AMPLIFIER – Class-B Operation Approximate Class-B : Complementary push-pull circuit When v I < 0; Q n is off & Q p conducts and becomes an emitter follower, sinking the current I L from R L.

22. POWER AMPLIFIER – Class-B Operation Approximate Class-B : Complementary push-pull circuit The problem with Class- B: transistors are not ideal.

23. POWER AMPLIFIER – Class-B Operation Crossover Distortion Assuming cut-in voltage of transistor is 0.6 V, v O = 0 for a range 0.6 V < v I < 0.6 V. The transfer characteristic becomes non-linear (See Figure). The range where both transistors are simultaneously off known as the dead band.

24. POWER AMPLIFIER – Class-B Operation The output will be distorted – crossover distortion (Figure) Crossover Distortion

25. POWER AMPLIFIER – Class-B Operation Crossover Distortion Crossover distortion can be eliminated by biasing the transistor with small quiescent current – class-AB

26. POWER AMPLIFIER – Class-B Operation Idealized Power Efficiency

27. POWER AMPLIFIER – Class-B Operation Idealized Power Efficiency

28. POWER AMPLIFIER – Class-B Operation Idealized Power Efficiency The Q-point is at the cutoff point of both transistors (zero collector current)

29. POWER AMPLIFIER – Class-B Operation Idealized Power Efficiency The output voltage of the idealized class-B is; The maximum possible value of V p is V CC The instantaneous power dissipation in Q n is;

30. POWER AMPLIFIER – Class-B Operation Idealized Power Efficiency For, the collector current is; For the collector current is;

31. POWER AMPLIFIER – Class-B Operation Idealized Power Efficiency The collector-emitter voltage is; Therefore the instantaneous power dissipation in Q n is; for

32. POWER AMPLIFIER – Class-B Operation Idealized Power Efficiency And; for Therefore, the average power dissipation in Q n is;

33. POWER AMPLIFIER – Class-B Operation Idealized Power Efficiency Plotting P Qn versus V p as given by the equation; gives us the following curve:

34. POWER AMPLIFIER – Class-B Operation Idealized Power Efficiency Because of symmetry; Differentiating which occurs when with respect to V p, for maximum P Qn gives us;

35. POWER AMPLIFIER – Class-B Operation Idealized Power Efficiency Since each power source supplies half sinewave of current, the average value is; The total power supplied by the two sources is;

36. POWER AMPLIFIER – Class-B Operation Idealized Power Efficiency The power delivered to the load is; The efficiency is; Maximum efficiency occurs when

37. POWER AMPLIFIER – Class-B Operation Idealized Power Efficiency Under this condition; Maximum theoretical efficiency of class B amplifier is therefore 78.5% The efficiency obtained in practice is less than the maximum value because of other circuit losses and because the peak output voltage must remain less than V CC to avoid transistor saturation which can cause distortion in the output signal.

38. POWER AMPLIFIER – Class-B Operation Idealized Power Efficiency The maximum transistor power dissipation occurs at; Substituting in the expression for efficiency;

39. POWER AMPLIFIER – Class-AB Operation Crossover distortion can be virtually eliminated by applying small quiescent bias on each transistor (See Figure) If Q n and Q p are matched, each emitter-base junction is biased with V BB /2 when v I is zero. Hence v O is also zero. The quiescent collector currents are;

40. POWER AMPLIFIER – Class-AB Operation As v I increases, the voltage at the base of Q n increases and v O increases. Q n operates as an emitter follower supplying current to R L. The output voltage is given by; The collector current of Q n is; (Neglecting the base currents)

41. POWER AMPLIFIER – Class-AB Operation Since i Cn must to supply the load current, v BEn increases which causes v BEp to decrease because V BB is constant. The decrease in v BEp results in a decrease in i Cp.

42. POWER AMPLIFIER – Class-AB Operation When v I goes negative, the base voltage of Q p decreases followed by a decrease in v O. Q p operates as emitter follower, sinking the load current. As i Cp increases v EBp increases causing a decrease in v BEn and i Cn.

43. POWER AMPLIFIER – Class-AB Operation Transfer characteristics ( v O versus v I )

44. POWER AMPLIFIER – Class-AB Operation

45. POWER AMPLIFIER – Class-AB Operation

46. POWER AMPLIFIER – Class-AB Operation relationship Using the relationship, the above expression can be written as; Hence; The produc of i Cn and i Cp is constant, therefore if i Cn increases i Cp decreases but does not to zero

47. POWER AMPLIFIER – Class-AB Operation M n and M p are matched transistors with the following parameters; Example 8.8 If V DD = 10 V and R L = 8 , find the bias voltage V BB /2 for I DQ = 0.05 A. Find also V GSn, V SGp and v I if v O = 5 V.

48. POWER AMPLIFIER – Class-AB Operation Example 8.8 – Solution Since the MOSFETs are matched, at quiescent point; and Hence;

49. POWER AMPLIFIER – Class-AB Operation Example 8.8 – Solution (cont’d) Substituting values; which yields; From the expression we have;

50. POWER AMPLIFIER – Class-AB Operation Example 8.8 – Solution (cont’d) When and

51. POWER AMPLIFIER – Class-AB Operation Example 8.8 – Solution (cont’d) Since; then; And;

52. POWER AMPLIFIER – Class-C Operation B – E junction is reverse-biased to obtain Q-point beyond cut-off. Transistor conducts for less than half a cycle of input signal Tuned circuit is required. Used for RF amplifier. Efficiency > 78.5%