1. Chapter3:Bipolar Junction Transistors (BJTs)1

2. Figure 3.1 A simplified structure of the npn transistor.
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Figure 3.2 A simplified structure of the pnp transistor.

3. Figure 3.4 Cross-section of an npn BJT.
Figure 3.3 Current flow in an npn transistor biased to operate in the active mode. (Reverse current components due to drift of thermally generated minority carriers are not shown.)
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Figure 3.4 Cross-section of an npn BJT.

4. sedr42021_0504.jpg
Figure 3.5 Profiles of minority-carrier concentrations in the base and in the emitter of an npn transistor operating in the active mode: vBE > 0 and vCB ³ 0.

5. Figure 3.5 Large-signal equivalent-circuit models of the npn BJT operating in the forward active mode.
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6. sedr42021_0509.jpg
Figure The iC –vCB characteristic of an npn transistor fed with a constant emitter current IE. The transistor enters the saturation mode of operation for vCB < –0.4 V, and the collector current diminishes.

7. Figure 3.8 Circuit symbols for BJTs.
Figure 3.7 Large-signal model for the pnp transistor operating in the active mode.
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Figure 3.8 Circuit symbols for BJTs.

8. Figure 3.10 Circuit for Example 5.1.
Figure 3.9 Voltage polarities and current flow in transistors biased in the active mode.
Example: β=100, VBE=0.7v at Ic=1mA
For Ic=2mA and Vc= 5v, what are the different values of Rc=?, VBE=? IE=?, RE=?
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Figure Circuit for Example 5.1.

9. Application 1:
In the circuit shown below (Fig E3.15), the voltage at the emitter was measured and found to be -0.7v. If β=50, find IE?, IC?, IB?, VC?
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Figure 3.11

10. Application 1: P 423
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Figure 12 Analysis of the circuit for Example : (a) circuit; (b) circuit redrawn to remind the reader of the convention used in this book to show connections to the power supply

11. Application 2: P 422
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Figure 12 Analysis of the circuit for Example 12. Note that the circled numbers indicate the order of the analysis steps.

12. Figure 3.13 Example 5.7: (a) circuit
Application 3: Determine the voltage at all nodes and the current through all branches
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Figure 3.13 Example 5.7: (a) circuit

13. Application 4:
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Figure 14 Example : (a) circuit; (b) analysis with the steps indicated by the circled numbers.

14. Figure 5.15 Example: (a) circuit; (b) analysis with steps numbered.
Application 5: For the emitter bias network determine
a. lB
b. IC
c. VCE
d. VC
e. VE
f. VB
g. VBC
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Figure Example: (a) circuit; (b) analysis with steps numbered.

15. Figure 5.16 Circuits for Example
Application 5:
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Figure Circuits for Example

16. Transistor Switch
A transistor when used as a switch is simply being biased so that it is in cutoff (switched off) or saturation (switched on). Remember that the VCE in cutoff is VCC and 0 V in saturation.
Fig 4-22
VCE(cutoff) = VCC
IC(sat) = (VCC – VCE(sat))/βDC
IB(min) = Ic (sat)/βDC
Figure 3.17

17. Biasing in BJT Amplifier circuits: Transistor circuit under examination in this introductory discussion.
sedr42021_0543a.jpg
Figure 3.18

18. The Classical biasing for BJTs using a single power supply: (a) circuit; (b) circuit with the voltage divider supplying the base replaced with its Thévenin equivalent.
Figure 3.19: (a) circuit; (b) circuit with the voltage divider supplying the base replaced with its Thévenin equivalent

19. Common-base re equivalent circuit.
Figure 3. 20

20. Defining Zo.
Figure 3. 21

21. Defining Av = Vo/Vi for the common-emitter configuration.
Figure 3. 22

22. re model for the common-emitter transistor configuration.
Figure 3. 23

23. Common Emitter Configurations
Common-emitter fixed-bias configuration.
Figure 3. 24

24. Network of the previous figure following the removal of the effects of VCC, C1 and C2.

25. Substituting the re model into the network
Figure 3. 26

26. Determining Zo for the network
Figure 3. 27

27. Demonstrating the 180° phase shift between input and output waveforms.
Figure 3. 28

28. Example
Figure 3. 29

29. Voltage-divider bias configuration.
Figure 3. 29

30. Substituting the re equivalent circuit into the ac equivalent network
Figure 3. 30

31. Example
Figure 3. 31

32. CE emitter-bias configuration.
Figure 3. 32

33. Substituting the re equivalent circuit into the ac equivalent network
Figure 3. 33

34. Defining the input impedance of a transistor with an un-bypassed emitter resistor.
Figure 3. 34

35. Example
Figure 3. 35

36. Common Collector Configurations
Emitter-follower configuration.
Figure 3. 36

37. Substituting the re equivalent circuit into the ac equivalent network
Figure 3. 37

38. Defining the output impedance for the emitter-follower configuration.
Figure 3. 38

39. Example
Figure 3. 39

40. Emitter-follower configuration with a voltage-divider biasing arrangement.
Figure 3. 40

41. Emitter-follower configuration with a collector resistor RC
Figure 3. 41

42. Collector feedback configuration.
Figure 3. 42

43. Substituting the re equivalent circuit into the ac equivalent network
Figure 3. 42

44. Defining Zo for the collector feedback configuration.
Figure 3. 43

45. Example
Figure 3. 44

46. Collector feedback configuration with an emitter resistor RE.
Figure 3. 45

47. Homework: Present the AC equivalent circuit
Figure 3. 46

48. Resolve the all the numerical parameters for the circuit shown below
Figure 3. 47

49. Common-base configuration.
Figure 3. 48

50. Substituting the re equivalent circuit into the ac equivalent network
Figure 3. 49

51. Example
Figure 3. 50

52. General overview of the small signal Analysis (CE, CC, CB
Common Emitter
Input resistance
moderate/small
Output resistance
large
Open Circuit Voltage gain
large
Short Circuit Current gain
Figure 3.51: A common-emitter amplifier using the structure (b) Equivalent circuit obtained by replacing the transistor with its hybrid-p model.
large
Large voltage and current gain but Rin and Ro not good for voltage amplifier.

53. Common Emitter with RE
Re increases Rin but reduces open circuit voltage gain. Current gain and output resistance are unchanged.
Input resistance
Open Circuit Voltage gain
reduced
increased
Rib greatly increased by resistance reflection rule (Miller)
Voltage gain reduced by ~ (1+gmRe); Rib increased by this factor.
Figure (a) A common-emitter amplifier with an emitter resistance Re. (b) Equivalent circuit obtained by replacing the transistor with its T model.

54. Common Base Input resistance Open Circuit Voltage gain
Short Circuit Current gain
small
large
unity
Output resistance
Non-inverting version of common emitter.
large
Good for unity gain current buffer.
Figure (a) A common-base amplifier using the structure (b) Equivalent circuit obtained by replacing the transistor with its T model.

55. Common Collector
Figure (a) An emitter-follower circuit based on the structure (b) Small-signal equivalent circuit of the emitter follower with the transistor replaced by its T model augmented with ro. (c) The circuit in (b) redrawn to emphasize that ro is in parallel with RL. This simplifies the analysis considerably.

56. Open Circuit Voltage gain Current gain
Input resistance
large
Output resistance
small
Open Circuit Voltage gain
Current gain
~unity
large
Voltage gain ~1 so emitter follows base input voltage (emitter follower)
Good for amplifier output stage: large Rin, small Rout.