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Understanding Current-Voltage Characteristics of Transistors

Explore circuit symbols, biasing, and I-V characteristics of NPN and PNP transistors in active modes. Learn about equivalent circuit models, Early Effect, and more.

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Understanding Current-Voltage Characteristics of Transistors

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  1. ECE 255 Jan 30, 2018 Lecture Instructor: Weng Cho Chew

  2. Current-Voltage CharacteristicsI-V Characteristics • Circuit symbols for BJT’s:

  3. Different Transistor Biasing • NPN and PNP transistors in active modes:

  4. Biasing Needs of NPN and PNP Transistors in Active Modes Parallel case for PNP.

  5. Equivalent circuit models. Digression: Collector-Base Reverse Current: ICBO The above models assumes that this current is zero. This current is small and has a large leakage component. Doubles for every 10oC rise in temperature.

  6. Graph of I-V Characteristics of a Transistor

  7. The I-V Characteristic is Temperature Dependent

  8. The Early Effect • First, iC vs vCBcurve for the ideal case. • Also, reflected in the equivalent circuit model.

  9. The Early Effect, Contd. • First, iC vs vCEcurve for the actual case. Common-emitter configuration and common-emitter characteristics. First studied by J.M. Early. Base-width modulation effect.

  10. Reminder Slide for Carrier Transport vCB depends on vCE Depletion region width depends on vCE Early effect as a base-width modulation effect.

  11. Some Math

  12. The Modified Equivalent Circuit The modification is illustrated to the PI (P) model. Corresponding modification is done to the T model.

  13. Another Way to Characterize a Transistor Use a constant current source. Beta can be calculated at the operating point Q as In the saturation region, iC increases rapidly with vCE. One can define an incremental resistance for this saturation region called the collector-to-emitter resistance:

  14. An Equivalent Circuit Model of a Saturated Transistor Both junctions are forward biased.

  15. Clicker Question 1 (1 Point) Which is not the major reason for the Early effect? Base width modulation Increase of the depletion region in the CBJ Leakage of the current across the CBJ

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