EMT 112 / 4 ANALOGUE ELECTRONICS

1. EMT 112 / 4ANALOGUE ELECTRONICS Lecture I Multistage Amplifiers [ Intro & Cascade Connections] March 10, 2010 1600 – 1700DKP 2

2. CASCADED BJT AMPLIFIER • Many applications cannot be handled with single-transistor amplifiers in order to meet the specification of a given amplification factor, input resistance and output resistance. • For example, the required voltage gain may exceed that which can be obtained in a single-transistor amplifier circuit. Some particular design required low output resistance. • As a solution – transistor amplifier circuits can be connected in series or cascaded amplifiers. • This can be done either to increase the overall small-signal voltage gain or provide an overall voltage gain greater than 1 with a very low output resistance

3. CASCADED BJT AMPLIFIER • This can be done either to increase the overall small-signal voltage gain or provide an overall voltage gain greater than 1 with a very low output resistance. • The overall voltage or current gain is not simply the product of the individual amplification factors. • Each stage in the cascaded amplifier contribute to have a better gain however the loading effect must be taken into account.

4. CASCADED BJT AMPLIFIER (i) Cascade /RC coupling

5. CASCADED BJT AMPLIFIER (ii) Cascode (series)

6. CASCADED BJT AMPLIFIER (iii) Darlington/Direct coupling

7. CASCADED BJT AMPLIFIER (iv) Transformer coupling

8. CASCADED BJT AMPLIFIER i) Cascade connection • RC coupling • Coupling capacitor couples the signal from one stage to the another stage and block dc voltage from one stage to the another stage. • The signal developed across the collector resistor of each stage is coupled into the base of the next stage • The overall gain = product (multiplication)of the individual gain

9. CASCADED BJT AMPLIFIER i) Cascade connection Example 1 Draw the AC equivalent circuit and calculate Av, Ri and Ro.

10. CASCADED BJT AMPLIFIER Example 1 – Solution DC analysis The circuit under DC condition (stage 1 and stage 2 are identical) RB = R1 || R2 = 15 k || 4.7 k = 3.58 kΩ VBB = (R2 / R1 + R2) x VCC = (4.7k/15k + 4.7k) x 20 = 4.77 V

11. CASCADED BJT AMPLIFIER Example 1 – Solution (cont’d) Applying Thevenin’s theorem, the circuit becomes; IBQ1 =VBB – VBE / RBB + (1 + β)RE =4.77 – 0.7 / 3.58 k (201)1k = 19.89 uA

12. CASCADED BJT AMPLIFIER Example 1 – Solution (cont’d) AC analysis The small-signal equivalent circuit (mid-band);

13. CASCADED BJT AMPLIFIER Example 1 – Solution (cont’d) Hint: Find the output for both stage!

14. CASCADED BJT AMPLIFIER Example 1 – Solution (cont’d) The small-signal voltage gain; Substituting values;

15. CASCADED BJT AMPLIFIER Example 1 – Solution (cont’d) The input resistance; The output resistance;

16. CASCADED BJT AMPLIFIER Direct couple (DC) The first stage is directly coupled to the next stage without going through a coupling capacitor A common-emitter stage driving another common-emitter stage

17. CASCADED BJT AMPLIFIER Direct couple (DC) The biasing network must be be suitably designed otherwise the dc bias of one stage will upset the bias voltage of the other

18. CASCADED BJT AMPLIFIER Direct couple (DC) A common-emitter driving an emitter-follower (common-collector)

19. CASCADED BJT AMPLIFIER Example 2 Perform a dc analysis and hence calculate the voltage gain Av where; Assume 1 = 170, 2 = 150 and VBE(ON) = 0.7 V.

20. CASCADED BJT AMPLIFIER Example 2 - Solution DC analysis

21. CASCADED BJT AMPLIFIER Example 2 – Solution (cont’d) The base-emitter loop of Q1 Rearranging;

22. CASCADED BJT AMPLIFIER Example 2 – Solution (cont’d) Substituting values;

23. CASCADED BJT AMPLIFIER Example 2 – Solution (cont’d)

24. CASCADED BJT AMPLIFIER Example 2 – Solution (cont’d) For the RC1 – collector of Q1 – base of Q2 – RE2 loop; and

25. CASCADED BJT AMPLIFIER Example 2 – Solution (cont’d) Substituting values;

26. CASCADED BJT AMPLIFIER Example 2 – Solution (cont’d)

27. CASCADED BJT AMPLIFIER Example 2 – Solution (cont’d)

28. CASCADED BJT AMPLIFIER Example 2 – Solution (cont’d) AC analysis

29. CASCADED BJT AMPLIFIER Example 2 – Solution (cont’d)

30. CASCADED BJT AMPLIFIER Example 2 – Solution (cont’d)

31. CASCADED BJT AMPLIFIER Example 2 – Solution (cont’d)

32. CASCADED BJT AMPLIFIER Example 2 – Solution (cont’d) Substituting values;

33. CASCADED BJT AMPLIFIER Example 2 – Solution (cont’d)

34. CASCADED BJT AMPLIFIER Example 2 – Solution (cont’d)

35. CASCADED BJT AMPLIFIER Example 3 Determine; (a) Q-points (b) Av; (c) Ri; (d) Ro. The parameters for Q1 and Q2 are;  = 125; VBE(on) = 0.7 V; ro = .

36. CASCADED BJT AMPLIFIER Example 3 – Solution DC analysis The circuit under DC condition: Applying Thevenin’s theorem at the input;

37. CASCADED BJT AMPLIFIER Example 3 – Solution (cont’d) The circuit becomes as shown; Taking the loop VBB – B1 – E1 – V–; Substituting values;

38. CASCADED BJT AMPLIFIER Example 3 – Solution (cont’d)

39. CASCADED BJT AMPLIFIER Example 3 – Solution (cont’d) Taking the loop V+ – RC1 – B2 – E2 – RE2 – V–; But; and;

40. CASCADED BJT AMPLIFIER Example 3 – Solution (cont’d) Hence; Substituting values; Or;

41. CASCADED BJT AMPLIFIER Example 3 – Solution (cont’d) Hence; and;

42. CASCADED BJT AMPLIFIER Example 3 – Solution (cont’d)

43. CASCADED BJT AMPLIFIER Example 3 – Solution (cont’d)

44. CASCADED BJT AMPLIFIER Example 3 – Solution (cont’d)

45. CASCADED BJT AMPLIFIER Example 3 – Solution (cont’d) (a) The Q-points are; and;

46. CASCADED BJT AMPLIFIER Example 3 – Solution (cont’d) The circuit under ac condition

47. CASCADED BJT AMPLIFIER Example 3 – Solution (cont’d) Small–signal equivalent circuit

48. CASCADED BJT AMPLIFIER Example 3 – Solution (cont’d)

49. CASCADED BJT AMPLIFIER Example 3 – Solution (cont’d)

50. CASCADED BJT AMPLIFIER Example 3 – Solution (cont’d) The voltage gain of the second stage is;