1. Chapter 8: BJT Small-Signal Analysis
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

2. BJT Small Signal Analysis
Slide 1
The re and hybrid models will be used to analyze AC small-signal transistor circuits.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

3. Common-Emitter (CE) Fixed-Bias Configuration
Slide 2
The input (Vi) is applied to the base and the output (Vo) is from the collector.
The Common-Emitter is characterized as having high input impedance and low output impedance with a high voltage and current gain.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

4. Removing DC effects of VCC and Capacitors
Slide 3
Removing DC effects of VCC and Capacitors
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

5. re Model Slide 4 Determine , re, and ro:
 and ro: look in the specification sheet for the transistor or test the transistor using
a curve tracer.
re: calculate re using dc analysis:
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

6. Impedance Calculations
Slide 5
Input Impedance:
[Formula 8.1]
[Formula 8.2]
Output Impedance:
[Formula 8.3]
[Formula 8.4]
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

7. Gain Calculations Slide 6 Voltage Gain (Av): [Formula 8.5]
Current Gain (Ai):
[Formula 8.7]
[Formula 8.8]
Current Gain from Voltage Gain:
[Formula 8.9]
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

8. Phase Relationship Slide 7
The phase relationship between input and output is 180 degrees. The negative sign used in the voltage gain formulas indicates the inversion.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

9. CE – Voltage-Divider Bias Configuration
Slide 8
CE – Voltage-Divider Bias Configuration
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

10. re Model Slide 9 You still need to determine , re, and ro.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

11. Impedance Calculations
Slide 10
Input Impedance:
[Formula 8.10]
[Formula 8.11]
Output Impedance:
[Formula 8.12]
[Formula 8.13]
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

12. Gain Calculations Slide 11 Voltage Gain (Av): [Formula 8.14]
Current Gain (Ai):
[Formula 8.16]
[Formula 8.17]
[Formula 8.18]
Current Gain from Voltage Gain:
[Formula 8.19]
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

13. Phase Relationship Slide 12
A CE amplifier configuration will always have a phase relationship between input and output is 180 degrees. This is independent of the DC bias.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

14. CE Emitter-Bias Configuration
Slide 13
Unbypassed RE
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

15. re Model Slide 14 Again you need to determine , re.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

16. Impedance Calculations
Slide 15
Input Impedance:
[Formula 8.20]
[Formula 8.21]
[Formula 8.22]
[Formula 8.23]
Output Impedance:
[Formula 8.33]
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

17. Gain Calculations Slide 16 Voltage Gain (Av): [Formula 8.25]
or [Formula 8.27]
Current Gain (Ai):
[Formula 8.28]
Current Gain from Voltage Gain:
[Formula 8.29]
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

18. Phase Relationship Slide 17
A CE amplifier configuration will always have a phase relationship between input and output is 180 degrees. This is independent of the DC bias.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

19. CE Emitter-Bias Configuration
Slide 18
Bypassed RE
This is the same circuit as the CE fixed-bias configuration and therefore can be solved using the same re model.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

20. Emitter-Follower Configuration
Slide 19
You may recognize this as the Common-Collector configuration. Indeed they are the same circuit.
Note the input is on the base and the output is from the emitter.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

21. re Model Slide 20 You still need to determine , re, and ro.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

22. Impedance Calculations
Slide 21
Input Impedance:
[Formula 8.37]
[Formula 8.38]
[Formula 8.39]
[Formula 8.40]
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

23. Impedance Calculations (cont’d)
Slide 22
Output Impedance:
[Formula 8.42]
[Formula 8.43]
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

24. Gain Calculations Slide 23 Voltage Gain (Av): [Formula 8.44]
Current Gain (Ai):
[Formula 8.46]
Current Gain from Voltage Gain:
[Formula 8.47]
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

25. Phase Relationship Slide 24
A CC amplifier or Emitter Follower configuration has no phase shift between input and output.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

26. Common-Base (CB) Configuration
Slide 25
The input (Vi) is applied to the emitter and the output (Vo) is from the collector.
The Common-Base is characterized as having low input impedance and high output impedance with a current gain less than 1 and a very high voltage gain.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

27. re Model Slide 26 You will need to determine  and re.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

28. Impedance Calculations
Slide 27
Input Impedance:
[Formula 8.54]
Output Impedance:
[Formula 8.55]
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

29. Gain Calculations Slide 28 Voltage Gain (Av): [Formula 8.56]
Current Gain (Ai):
[Formula 8.57]
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

30. Phase Relationship Slide 29
A CB amplifier configuration has no phase shift between input and output.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

31. CE Collector Feedback Configuration
Slide 30
This is a variation of the CE Fixed-Bias configuration.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

32. re Model Slide 31 You will need to determine  and re.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

33. Impedance Calculations
Slide 32
Input Impedance:
[Formula 8.58]
Output Impedance:
[Formula 8.59]
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

34. Gain Calculations Slide 33 Voltage Gain (Av): [Formula 8.60]
Current Gain (Ai):
[Formula 8.61]
[Formula 8.62]
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

35. Phase Relationship Slide 34
This is a CE amplifier configuration; therefore there is a 180-degree phase shift between input and output.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

36. Approximate Hybrid Equivalent Circuit
Slide 35
The h-parameters can be derived from the re model:
hie = re hib = re
hfe =  hfb = -
hoe = 1/ro
The h-parameters are also found in the specification sheet for the transistor.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

37. CE Fixed-Bias Configuration
Slide 36
CE Fixed-Bias Configuration
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

38. Hybrid Equivalent Circuit
Slide 37
Impedances: [Formula 8.83]
[Formula 8.84]
Gain: [Formula 8.85]
[Formula 8.86]
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

39. CE Voltage-Divider Configuration
Slide 38
CE Voltage-Divider Configuration
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

40. Hybrid Equivalent Circuit
Slide 39
Impedances: [Formula 8.87]
[Formula 8.88]
Gain: [Formula 8.89]
[Formula 8.90]
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

41. CE Unbypassed Emitter-Bias Configuration
Slide 40
CE Unbypassed Emitter-Bias Configuration
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

42. Hybrid Equivalent Circuit
Slide 41
Impedances: [Formula 8.91]
[Formula 8.92]
[Formula 8.93]
Gain: [Formula 8.94]
[Formula 8.95]
[Formula 8.96]
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

43. CC or Emitter-Follower Configuration
Slide 42
CC or Emitter-Follower Configuration
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

44. Hybrid Equivalent Circuit
Slide 43
Impedances: [Formula 8.97]
[Formula 8.98]
[Formula 8.99]
Gain: [Formula 8.100]
[Formula 8.101]
[Formula 8.102]
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

45. CB Configuration Slide 44 Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

46. Hybrid Equivalent Circuit
Slide 45
Impedances: [Formula 8.103]
[Formula 8.104]
Gain: [Formula 8.105]
[Formula 8.106]
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

47. Summary Table Slide 46a Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

48. Summary Table Slide 46b Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

49. Troubleshooting Slide 47
1. Check the DC bias voltages – if not correct check power supply, resistors, transistor. Also check to ensure that the coupling capacitor between amplifier stages is OK.
2. Check the AC voltages – if not correct check transistor, capacitors and the loading effect of the next stage.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

50. Practical Applications
Slide 48
• Audio Mixer
• Preamplifier
• Random-Noise Generator
• Sound Modulated Light Source
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.