1. Small Signal Model PNP Transistor Section 4.4

2. BJT in the active region Electrons cross the forward biased BE junction and are swept reverse biased BC junction.

3. Large Signal Model of a BJT Called “large” signal model because this model is applicable even if V BE changes from 300 mV to 800 mV

4. Experiments

5. Transconductance If a signal changes the base-emitter voltage by a small amount, how much change is produced in the collector current ?

6. Illustration of Transconductance

7. But there is something else…. A change in V BE creates a change in base current!

8. Example 4.10 Signal Generated By a microphone Small Signal Equivalent Circuit V BE =800 mV β=100 I S,Q1 =3 x 10 -16 A Question: If a microphone generates a 1 mV signal, how much change is observed in the collector and base current ?

9. A Simple Amplifier Determine the output signal level if the microphone produces a 1 mV signal.

10. AC Ground The voltage produced by a voltage source is constant. The small signal model is concerned only with changes in quantities. Therefore, a DC voltage source must be replaced with a ground in small signal analysis.

11. Example

12. Early Effect A larger reverse bias voltage leads to a larger BC depletion region. The effective base width (WB) is reduced. The slope of the electron profile increases. I C increases as VCE is increased.

13. Early Effect

14. James M. Early

15. Modeling of Early Effect

16. What Doesn’t Change with Early Effect ?

17. Modification of the Small Signal Model

18. Summary

19. A Simple Cadence Example

20. Assumption Assume that 1.The DC at Vout is 0.9 V 2.g m =1 mS Gain is approximately equal to –g m R C. Bias current is I C =g m V t R=(1.8V-0.9V)/26uA=34.6 Kohms Gain is -34.6.

21. DC Bias of the Amplifier

22. Sweep the Base Voltage to Get the IC=26 uA

23. Display the Transconductance

24. Display Transconductance

25. Verify Transconductance (1)

26. Verify Transconductance (2)

27. Transconductance VBIC 562.5 mV25.64 uA 563.5 mV26.64 uA ∆VBE1 mV ∆IC1 uA gm=∆VBE/∆IC1 mS

28. Introduce a Small Signal

29. Calculate Peak to Peak Voltage

31. Peak to Peak Voltage=67.78 mV 67.78 mV/2=33.9

33. PNP Transistors

34. Structures of BJT Transistors (NPN transistor)(PNP transistor)

35. Voltage and Current Polarities of NPN and PNP transistors A “fat” voltage between collector and emitter voltage places a transistor in the active region! A “skinny” voltage between collector and emitter voltage places a transistor in the active region!

36. Large-Signal Model of BJT Transistors (NPN)(PNP)

37. Equations

38. Small Signal Model of BJT Transistors (NPN)(PNP)