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Small Signal Model PNP Transistor

Small Signal Model PNP Transistor. Section 4.4. BJT in the active region. Electrons cross the forward biased BE junction and are swept reverse biased BC junction. Large Signal Model of a BJT. Called “large” signal model because this model is applicable even if V BE

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Small Signal Model PNP Transistor

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  1. Small Signal ModelPNP 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 VBE 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 VBE creates a change in base current!

  8. Example 4.10 Signal Generated By a microphone Small Signal Equivalent Circuit VBE=800 mV β=100 IS,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. IC 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 • The DC at Vout is 0.9 V • gm=1 mS Gain is approximately equal to –gmRC. Bias current is IC=gmVt 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

  28. Introduce a Small Signal

  29. Calculate Peak to Peak Voltage

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

  31. PNP Transistors

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

  33. 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!

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

  35. Equations

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

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