1. Elektronica II
Ch.5 BJT AC Analysis

2. 5.1 Introduction
Ch.3 Transistor: basic construction, appearance, characteristics
Ch.4 Transistor: biasing
Ch.5 AC response BJT amplifier by reviewing the (small-signal) models re, hybrid π and hybrid equivalent

3. 5.2 Amplification in the AC domain

4. 5.3 BJT transistor modeling
hybrid equivalent model: specification sheets include parameters, defined for a specific set of operating conditions (Ic, Vce, f)
re model: important parameter determined by actual operating conditions
hybrid π model: high-frequency analysis

5. 5.3 BJT transistor modeling (cont’d)
AC equivalent model is obtained by:
Setting all dc sources to zero and replacing them by a short-circuit equivalent
Replacing all capacitors by short-circuit equivalent
Removing all elements bypassed by the short-circuit equivalents introduced by steps 1 and 2
Redrawing the network in a more convenient and logical form
Parameters: Vi, Ii, Zi, Vo, Io, Zo, Av, Ai

6. 5.3 BJT transistor modeling (cont’d)

7. 5.4 The re transistor model
Common-Base Configuration
Common-Emitter Configuration
Common-Collector Configuration (use model defined by CE configuration)

8. 5.4 The re transistor model (cont’d) Common-Base Configuration

9. 5.4 The re transistor model (cont’d) Common-Base Configuration
Typical values of Zi range from a few ohms to a maximum of about 50 Ω
Typical values of Zo are in the mega-ohm range
In general, the input impedance is relatively small and the output impedance quite high

10. 5.4 The re transistor model (cont’d) CB-Amplifier

11. 5.4 The re transistor model (cont’d) CB-Amplifier (example 5.11)

12. 5.4 The re transistor model (cont’d) Common-Emitter Configuration

13. 5.4 The re transistor model (cont’d) Common-Emitter Configuration
Typical values of Zi defined by βre range from a few hundred ohms to the kilo-ohm range, with a maximum of about 6 kΩ to 7 kΩ
Typical values of Zo are in the range of 40 kΩ to 50 kΩ

14. 5.4 The re transistor model (cont’d) CE-Amplifier: Fixed-bias

15. 5.4 The re transistor model (cont’d) CE-Amplifier: Fixed-bias

16. 5.4 The re transistor model (cont’d) CE-Amplifier: Fixed-bias

17. 5.4 The re transistor model (cont’d) CE-Amplifier: Fixed-bias (example 5.4)

18. 5.4 The re transistor model (cont’d) CE-Amplifier: Voltage-divider

19. 5.4 The re transistor model (cont’d) CE-Amplifier: Voltage-divider

20. 5.4 The re transistor model (cont’d) CE-Amplifier: Voltage-divider (Example 5.5)

21. 5.4 The re transistor model (cont’d) CE-Amplifier: Voltage-divider (Example 5.8)

22. 5.4 The re transistor model (cont’d) CE-Amplifier: Voltage-divider (Example 5.9)

23. 5.4 The re transistor model (cont’d) Other configurations
=> 5.8 common-emitter fixed-bias: Zi, Zo, Av, phase relationship, example 5.4
=> 5.9 voltage-divider bias:
=> Re bypassed, with ro : Zi, Zo, Av, phase relationship, example 5.5 en 5.9
=> Re unbypassed, with ro : example 5.8
=> 5.10 ce emmitter-bias:
=> Re unbypassed, without ro : Zi, Zo, Av, phase relationship
=> Re unbypassed, with ro: Zi, Zo, Av
=> bypassed => zie ce fixed-bias
-> example 5.6 (unbypassed, with ro) en 5.7 (bypassed, with ro)
=> 5.11 emitter-follower:
=> without ro : Zi, Zo, Av, phase relationship
=> with ro : Zi, Zo, Av
-> example 5.10: without en with ro
=> variaties: with voltage-divider biasing en with collector resistor Rc
=> 5.12 common-base configuration: Zi, Zo, Av, Ai: example 5.11
=> 5.13 collector feedback:
=> without Re:
=> without ro: Zi, Zo, Av, phase relationship
=> with ro: Zi, Zo, Av
-> example 5.12: without en with ro
=> with Re: exercise
=> 5.14 collector dc feedback: with ro: Zi, Zo, Av, phase relationship, example 5.13

24. 5.15 Determining the current gain
For each transistor configuration, the current gain can be determined directly from the voltage gain, the defined load and the input impedance

25. 5.15 Determining the current gain (cont’d)

26. 5.16 Effect of RL and RS Two approaches can be used:
By inserting the (re model) equivalent circuit and use methods of analysis to determine the quantities of interest
By defining a two-port equivalent model and use the parameters determined for the no-load situation

27. 5.16 Effect of RL and RS (cont’d) 1st approach

28. 5.16 Effect of RL and RS (cont’d)

29. 5.16 Effect of RL and RS (cont’d)

30. 5.16 Effect of RL and RS (cont’d) Fixed-bias

31. 5.16 Effect of RL and RS (cont’d) Fixed-bias

32. 5.16 Effect of RL and RS (cont’d) Fixed-bias: example 5.14
Voltage divider
Emitter follower

33. 5.17 2nd (Two-port systems) approach (effect of RL and RS)

34. 5.17 Two-port systems approach (effect of RL and RS) (cont’d)

35. 5.17 Two-port systems approach (effect of RL and RS) (cont’d)

36. 5.17 Two-port systems approach (effect of RL and RS) (cont’d)

37. 5.17 Two-port systems approach (effect of RL and RS) (cont’d) Example 5.15

38. 5.18 Summary tables

39. 5.18 Summary tables (cont’d)

40. 5.19 Cascaded systems