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Understanding BJT Saturation Mode: Operations, Applications, and Circuit Analysis

This outline explores the saturation mode of Bipolar Junction Transistors (BJTs), detailing their operational principles, especially in digital circuits where saturation and cut-off modes are crucial, and in amplifier design using the active mode. It highlights the differences between NPN and PNP transistors, explaining how Emitter-Base and Base-Collector junctions function in forward and reverse biases. The document also addresses the large-signal model of BJTs, emphasizing current assumptions, the effects of various biases, and practical design considerations. Lastly, it touches on strategies to exit a transistor from saturation.

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Understanding BJT Saturation Mode: Operations, Applications, and Circuit Analysis

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  1. BJT in Saturation Mode Section 4.5

  2. Outline • Modes of Operations • Review of BJT in the active Region • BJT in Saturation Mode

  3. Modes of Operation Applications: 1. Saturation and cut-off mode are used in digital circuits. 2. Active mode is used in the amplifier design.

  4. Extension of a PNP transistor (PNP transistor) (NPN transistor) • Emitter-base junction is forward • biased. • Holes are injected into the base. • Base-collector junction is reverse • Biased. • Injected holes in the base is swept • across the base-collector junction by • the electric field.

  5. BJT Current Assumption: BEJ: Forward Biased BCJ: Reverse Biased

  6. 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

  7. Large-Signal Model of BJT Transistors (NPN) (PNP) C C E E

  8. Experiments

  9. Saturation Mode

  10. BJT in Saturation Mode (A transistor in the active mode of operation) Key assumption so far: BE=Forward Biased BC=Reverse Biased What happens when these assumptions are not true?

  11. Review: Forward Bias Diode E Depletion region shrinks due to charges from the battery. The electric field is weaker. Majority carrier can cross the junction via diffusion; Greater diffusion current. Current flows from P side to N side

  12. Hole Current into the Collector A reverse biased BCJ keeps holes in the base. But as BCJ becomes forward biased, the strong electric field which opposes of the movement of holes into the collector is weakened. There is now a hole current into the collector. Net Result: heavy saturation leads to a sharp rise in the base current and a rapid fall in β.

  13. A Large Signal Model of the BJT The net collector current decreases as the collector enter into saturation

  14. General Rules • As a rule of thumb, we permit soft saturation with VBC <400 mV because the current in the B-C junction is negligible, provided that various tolerances in the component values do not drive the device into deep saturation. • For a device in soft saturation or active region, we approximate IC as Isexp(VBE/VT) • In the deep saturation region, the collector-emitter voltage approaches a constant value called VCE, SAT (about 200 mV).

  15. 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!

  16. Design Problem How do you get a transistor out of Saturation?

  17. Use 2n3904 npn BJT in Simulation (Error!, put 2n3904 here!)

  18. Include 2n3904 (NPN) model

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