1. BJT, AC behavior
Bollen

2. AGENDA BJT AC behaviour DC & AC signals Characteristics
DC input characteristics
AC input characteristics
How to draw and ac circuit
Configurations
Common Emitter
Examples
Calculating
BJT
Impedance reflection p 171
Input/output resistance
Source resistiance
CE example
Common Collector
Example
Calculations
Common Base
Bollen

3. BJT, AC behavior
Signal operation can be seen as a small variation on the DC bias of a circuit. If the signals are very small to the DC bias, transistor parameters can be considered as constant. While the BJT is a non-lineair behavior component this appoximation is only valid for very small variations on the DC-bias point. The first thing to do is to explain the AC model of a BJT, afterwards we make applications an calculations on a complete AC circuit.
P.J.F. Paap
P.J.F. Paap
Bollen

4. BJT, DC & AC signals
Bollen

5. BJT, DC & AC signals
Bollen

6. BJT, characteristics DC model ac model
DC model; Vbe = 0V Ube, Uce, Ic, Ib, Ie Capitals
ac model; re = 26mV/Ie ube, uce, ic, ib, ie Low cases
Bollen

7. BJT, DC input characteristics
Vbe = 0V7
Bollen

8. BJT, AC input characteristics
re = 26mV/Ic
The dynamic resistor can be
calculated by the DC current Ic
Bollen

9. BJT, characteristics
Bollen

10. BJT, how to draw an ac circuit 1/4
1 Kill all DC sources (ΔU = 0 V)
replacing each dc voltage source with a short circuit
and
each dc current source with an open circuit
2 Replace all Capacitors ( xc = 1/2.pi.f )
3 Use the ac model of the active devices
T model for BJT
Bollen

11. BJT, how to draw an ac circuit 2/4
1 Kill all DC sources (ΔU = 0 V)
replacing each dc voltage source with a short circuit
and
each dc current source with an open circuit
Bollen

12. BJT, how to draw an ac circuit 3/4
2 Replace all Capacitors
( xc = 1/2.pi.f )
Bollen

13. BJT, how to draw an ac circuit 4/4
3 Use the ac model of the active devices,
T model for BJT
Bollen

14. BJT, configurations 1 Common Emitter Input = base Output = collector
2 Common Collector Input = base Output = emitter
or Emitter follower
3 Common Base Input = emitter Output = collector
Bollen

15. BJT, CE examples
Bollen

16. BJT, CE calculating
Law of Bollen
here ix = ib
Bollen

17. BJT, CE calculating Ib does NOT flow through Rb
When Rb changes in value,
base current will NOT change
Through re,
base current & collector current
If input voltage increases,
output voltage decreases
Bollen

18. BJT, CE calculating
There is NO Rb !!
Bollen

19. BJT, CE example
Bollen

20. BJT, CE example
NO Rb1 en Rb2
in the formula !!
Bollen

21. BJT, CE example
Bollen

22. BJT, impedence reflection page 171
If you stand in base you see all resistors in the emitter (ß+1) magnified
If you stand in emitter you see all resistors in the base 1/(ß+1) magnified
Bollen

23. BJT, input / output resistance
Bollen

24. BJT, input / output resistance
Bollen

25. BJT, source resistance
Rs causes an attenuation in combination with the input resistance of the circuit. Then the signal is amplificated !
Bollen

26. BJT, CE example
Bollen

27. BJT, CC example
Bollen

28. BJT, CC
Bollen

29. BJT, CC
Bollen

30. BJT
Bollen

31. BJT, CB
Bollen

32. BJT, CB
Bollen