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Understanding Bipolar Junction Transistors

Learn about the principles and operation of bipolar junction transistors, including their construction, operating regions, currents, voltage ratings, and testing. Explore the differences between PNP and NPN transistors, and discover integrated and discrete transistor options.

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Understanding Bipolar Junction Transistors

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  1. Chapter 6 Bipolar Junction Transistors

  2. Transistors • Transistor – A three-terminal device whose output current, voltage, and/or power are controlled by its input. • Often used in amplifiers (circuits designed to effectively increase the strength of an input signal). Also used as high-speed electronically controlled switches, and in many power supply regulator circuits. • The two basic types of transistor are the bipolar junction transistor (BJT) and the field-effect transistor (FET).

  3. Bipolar Junction Transistors • The npnand pnptransistors are shown here. • The collector and emitter are made of the same material, and the base is made of the other. • The arrow on the schematic symbol identifies the emitter terminal and the type of component.

  4. BJT Terminal Currents • IE is normally has the greatest value, followed closely by IC. • The BJT is a current-controlled device. The value of IC is normally some multiple of the value of IB.

  5. Transistor Current Gain (b) • Transistor current gain (b) – The factor by which current increases from the base of a transistor to its collector. • Represented using the Greek letter Beta (b).

  6. Transistor Terminal Voltages VCC, VBB, and VEE are power supply voltages, each connected (directly or indirectly) to the identified terminal. VC, VB, and VE are terminal voltages, each measured from the identified terminals to ground. VCE, VBE, and VCB are inter-terminal voltages, each measured from the first identified terminal to the second. (For example, VCE is the voltage measured from the collector terminal to the emitter terminal.)

  7. BJT Construction • The transistor contains two pn junctions: The base-emitter junction and the collector-base junction. • The junctions can be represented as diodes.

  8. Transistor Operating Regions • Cutoff – Both junctions are reverse biased • Active – The base-emitter junction is forward biased and the collector-base junction is reverse biased. • Saturation – Both junctions are forward biased.

  9. Cutoff • Both transistor junctions are reverse biased. • All terminal currents are approximately equal to 0 A.

  10. Saturation • Both transistor junctions are forward biased. • IC reaches its maximum value (determined by the components in the collector and emitter circuits) and is independent of the values of IB and b. • VCE < VBE

  11. Active Operation • The base-emitter junction is forward biased and the collector-base junction is reverse biased. • All terminal currents have some measurable value. • VBE is approximately 0.7 V. • VCE is in the range of VBE < VCE< VCC.

  12. Transistor Currents

  13. DC Beta (b) • Beta (b) – The ratio of collector current ( IC ) to base current ( IB ).

  14. Current Relationships Beta can be used to calculate any of the transistor terminal currents.

  15. DC Alpha (a) • Alpha (a) – The ratio of collector current ( IC ) to emitter current ( IE ). • Also referred to as collector current efficiency. • IE is greater than IC, so the value of a is always less than 1.

  16. The Relationship Between Beta (b) and Alpha (a)

  17. Transistor Voltage Ratings • VCBO – The maximum allowable reverse voltage across the collector and base terminals. • VCEO – The maximum allowable reverse voltage across the collector and emitter terminals. • VEBO – The maximum allowable reverse voltage across the base and emitter terminals.

  18. Transistor Collector Curves • The portion to the left of VK represents saturation. • The IB = 0 mA line and the space below it represents cutoff. • The center portion of the curve represents the active region of operation. • The right end of the curve represents the device characteristics when it is driven into breakdown.

  19. Composite Collector Curves

  20. Beta Curves • Beta curves show how the value of beta varies with both temperature and dc collector current.

  21. Transistor Off Characteristics • Collector cutoff current (ICEX) – The maximum value of IC when the device is in cutoff. • Base cutoff current (IBL) – The maximum value of IB when the device is in cutoff.

  22. Transistor On Characteristics • DC current gain (hFE ) – The ratio of dc collector current to dc base current. The dc beta rating of the component. • Collector-emitter saturation voltage, VCE(sat) – The rated value of VCE when the component is saturated. • Base-emitter saturation voltage, VBE(sat) – The rated value of VBE when the component is saturated.

  23. Transistor Testing I

  24. Transistor Testing II

  25. PNP vs NPN Transistors The primary differences between PNP and NPN circuits are found in the current directions and voltage polarities.

  26. Integrated Transistors • Integrated Transistor – An IC that contains two or more transistors with identical characteristics. • Discrete Transistor – A term used to describe a transistor that is housed in an individual casing.

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