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Small Signal Model PNP Transistor

Small Signal Model PNP Transistor. Section 4.1-4.4,4.6. Schedule. Overview. Review. Small Signal Model. Section 4.4. Schematic of an Audio Amplifier. Microphone produces a small signal. How does the amplifier circuit respond to a small change in the input signal?

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Small Signal Model PNP Transistor

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  1. Small Signal ModelPNP Transistor Section 4.1-4.4,4.6

  2. Schedule

  3. Overview

  4. Review

  5. Small Signal Model Section 4.4

  6. Schematic of an Audio Amplifier Microphone produces a small signal. How does the amplifier circuit respond to a small change in the input signal? How is the analysis performed? Small signal model

  7. Small Signal Analysis (For a Circuit You have not Seen Before) • Replace each ideal DC voltage source with a small signal ground. • Replace each ideal DC current source with an open circuit. • Replace each transistor by its small signal model • Analyze the small signal equivalent circuit.

  8. Small Signal Analysis (For a Circuit You have not Seen Before) • Analyze the Circuit by Inspection

  9. Voltage Source RS should be 0 for a good battery! DC Voltage Source in Small Signal Analysis

  10. Current Source RS should be infinity for a good battery!

  11. Small Signal Model (NPN) (PNP) • Statements that are always true for both NPN and PNP. • rπ is between B and E. • the direction of the dependent current source always points • from the collector to emitter. • 3. ro is always between B and C.

  12. Question Replace Q1 and Q2 by their small equivalent circuit.

  13. Answer

  14. Question

  15. Answer

  16. Derivation of the Small Signal Model

  17. Change in the Collector Current Due to a Small Change in Base-Emitter Voltage If a signal changes the base-emitter voltage by a small amount, how much change is produced in the collector current?

  18. Derivation of Transconductance Small signal model of Q1 If a signal changes the base-emitter voltage by a small amount, how much change is produced in the collector current?

  19. But there is something else…. Small signal model A change in VBE creates a change in base current!

  20. 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?

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

  22. 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.

  23. Example Small Signal Model

  24. Summary

  25. Output Resistance Due to 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.

  26. Early Effect

  27. James M. Early

  28. Modeling of Early Effect

  29. What Doesn’t Change with Early Effect?

  30. Modification of the Small Signal Model

  31. Slides to Cover During the Lab

  32. A Simple Cadence Example

  33. 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.

  34. DC Bias of the Amplifier

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

  36. Display the Transconductance

  37. Display Transconductance

  38. Verify Transconductance (1)

  39. Verify Transconductance (2)

  40. Transconductance

  41. Introduce a Small Signal

  42. Calculate Peak to Peak Voltage

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

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