1. BJT amplifier & small-signal concept
Lecture 8

2. Preview Basic characteristics of BJT amplifiers
DC and ac analysis & equivalent circuits
Small-signal hybrid-π equivalent circuits
Small-signal voltage gain, input resistance & output resistance
Common-emitter amplifiers
dc & ac load lines

3. Linear analog amplifier

4. Notation

5. Basic characteristics of an amplifier

6. Basic characteristics of an amplifier (cont’d)

7. Basic BJT amplifier

8. Graphical analysis of BJT amplifiers

9. Graphical analysis of BJT amplifiers (Cont’d)

10. Analysis of BJT amplifiers

11. dc analysis and equivalent circuit

12. ac analysis and equivalent circuit

13. Small-signal hybrid-π equivalent circuit

14. Small-signal hybrid-π equivalent circuit (Cont’d)

15. Small-signal voltage gain

16. Input and output resistances

17. Example: dc circuit analysis

18. Example: dc circuit analysis (Cont’d)

19. Example: dc circuit analysis (Cont’d)-

20. Small-signal hybrid-π equivalent circuit with Early effect

21. Example: determine BJT amplifier parameters
Determine the small-signal voltage gain, input resistance, and output resistance of the BJT amplifier circuit in previous example with the early effect. Assume that the early voltage is 50V.

22. Common-emitter amplifiers (with voltage-divider biasing & coupling capacitor)

23. Common-emitter amplifiers (with voltage-divider biasing & coupling capacitor)- Cont’d

24. Common-emitter amplifiers (with voltage-divider biasing & coupling capacitor & emitter resistor)

25. dc & ac load lines Dc load line is used to find Q-point
Ac load line is used to determine graphically the operation of a BJT amplifier
Dc and ac load lines are essentially different since capacitors appear as an open circuit for a de operation but a short circuit for an ac operation

26. dc load line

27. ac load line

28. Maximum output symmetrical swing

29. Saturation

30. Cutoff

31. Example: Calculate BJT circuit parameters
Determine the small-signal voltage gain, input resistance, and output resistance of the circuit shown in attached Figure. Assume the transistor parameters are: β=100, VBE(on) =0.7V, and VA=100V

34. end

35. (a) Determine the small-signal hybrid- parameters rπ, gm, and r0
TQ8.1
In Figure 8.1, let β=150,VA=200V, VCC=7.5V, VBE(on)=0.7V, RC=15KΩ, RB=100kΩ, and VBB=0.92V.
(a) Determine the small-signal hybrid- parameters rπ, gm, and r0
(b) Find the small-signal voltage gain Av
Fig. 8.1