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COMSATS Institute of Information Technology Virtual campus Islamabad

COMSATS Institute of Information Technology Virtual campus Islamabad. Dr. Nasim Zafar Electronics 1 EEE 231 – BS Electrical Engineering Fall Semester – 2012. Bipolar Junction Transistors - BJTs. Lecture No: 14 Contents: Introduction Bipolar Transistor Currents

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COMSATS Institute of Information Technology Virtual campus Islamabad

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  1. COMSATS Institute of Information TechnologyVirtual campusIslamabad Dr. Nasim Zafar Electronics 1 EEE 231 – BS Electrical Engineering Fall Semester – 2012

  2. Bipolar Junction Transistors-BJTs Lecture No: 14 Contents: • Introduction • Bipolar Transistor Currents • Bipolar Transistor Characteristics and Parameter • Early Effect Nasim Zafar.

  3. References: • Microelectronic Circuits: Adel S. Sedra and Kenneth C. Smith. • Electronic Devices : Thomas L. Floyd ( Prentice Hall ). • Integrated Electronics Jacob Millman and Christos Halkias (McGraw-Hill). • Electronic Devices and Circuit Theory: Robert Boylestad & Louis Nashelsky ( Prentice Hall ). • Introductory Electronic Devices and Circuits: Robert T. Paynter. Nasim Zafar.

  4. Reference: Chapter 4 – Bipolar Junction Transistors: Figures are redrawn (with some modifications) from Electronic Devices By Thomas L. Floyd Nasim Zafar.

  5. C B E Bipolar Junction Transistors BJTs-Circuits Nasim Zafar.

  6. Transistor Types • MOS - Metal Oxide Semiconductor • FET - FieldEffectTransistor • BJT - Bipolar Junction Transistor ◄ Nasim Zafar.

  7. Transistor Current Characteristics Nasim Zafar.

  8. An Overview of Bipolar Transistors: • While control in a FET is due to an electric field. • Control in a bipolar transistor is generally considered to be due to an electric current. • current into one terminaldetermines the currentbetween two others • as with an FET, abipolar transistorcan be used as a‘control device’ Nasim Zafar.

  9. Transistor Biasing Configurations: • Common-Base Configuration (CB) : input = VEB & IE ; output = VCB & IC 2. Common-Emitter Configuration (CE): input = VBE & IB ; output = VCE & IC • Common-Collector Configuration (CC): input = VBC & IB ; output = VEC& IE Nasim Zafar.

  10. Operation Modes: • Active: • Most importance mode, e.g. for amplifier operation. • The region where current curves are practically flat. • Saturation: • Barrier potential of the junctions cancel each other out causing a virtual short. • Ideal transistor behaves like a closed switch. • Cutoff: • Current reduced to zero • Ideal transistor behaves like an open switch. Nasim Zafar.

  11. Operation Modes: • Active: BJT acts like an amplifier (most common use). • Saturation: BJT acts like a short circuit. • Cutoff: BJT acts like an open circuit. Nasim Zafar.

  12. Common Emitter Characteristics: • We consider DC behaviour and assume that we are working in the normal linear amplifier regime with the BE junction forward biased and the CB junction reverse biased. Nasim Zafar.

  13. Common-Emitter Output Characteristics IC Output Characteristic Curves - (Vc- Ic Active Region IB VCE Saturation Region Cutoff Region IB = 0 Nasim Zafar.

  14. Common-Base-Configuration (CBC) NPN Transistor Circuit Diagram: NPN Transistor Nasim Zafar.

  15. Common-Base Output Characteristics: Although the Common-Base configuration is not the most common configuration, it is often helpful in understanding the operation of BJT Output Characteristic Curves - (Vc- Ic ) IC mA Breakdown Region 6 Active Region IE 4 IE=2mA Saturation Region 2 IE=1mA Cutoff IE = 0 VCB 0.8V 2V 4V 6V 8V Nasim Zafar.

  16. Transistor Currents - Output characteristics: Nasim Zafar.

  17. Common-Collector Output Characteristics: Emitter-Current Curves IE Active Region IB VCE Saturation Region Cutoff Region IB= 0 Nasim Zafar.

  18. 21.4 Bipolar Transistor Characteristics • Behaviour can be described by the current gain, hfe or by the transconductance, gm of the device Nasim Zafar.

  19. Conventional View & Current Components:NPN Transistor-CEC Nasim Zafar.

  20. Current Components: NPN Transistor-CEC Nasim Zafar.

  21. BJT Characteristics and Parameters Nasim Zafar.

  22. BJT-Current Gain Parameters: • Two quantities of great importance in the characterization of transistors are the so-called common-base current gain .. • and the so-called common-emitter gain. • DC  and DC   = Common-emitter current gain  = Common-base current gain Note:  and  are sometimes referred to as dc and dc because the relationships being dealt within the BJT are DC. Nasim Zafar.

  23. BJT-Current Gain Parameters: • Common-base current gain  , is also referred to as hFB and is defined by:  = hFB = IC / IE • Common-emitter current gainβ, is also referred ashFE and is defined by:  = IC/IB Thus: Nasim Zafar.

  24. Beta () or amplification factor: • The ratio of dc collector current (IC) to the dc base current (IB) is dc beta (dc ) which is dc current gain where IC and IB are determined at a particular operating point, Q-point (quiescent point). • It’s define by the following equation: 30 < dc < 300  2N3904 • On data sheet, dc=hFE with h is derived from ac hybrid equivalent circuit. FE are derived from forward-current amplification and common-emitter configuration respectively. Nasim Zafar.

  25. In the dc mode the level of IC and IE due to the majority carriers are related by a quantity called alpha: = IC = IE + ICBO • It can then be summarize to IC = IE(ignore ICBO due to small value) • For a.c situations where the point of operation moves on the characteristics curve, an a.c alpha defined by • Alpha a common base current gain factor that shows the efficiency by calculating the current percent from current flow from emitter to collector. The value of  is typical from 0.9 ~ 0.998. Nasim Zafar.

  26. BJT-Current Gain Parameters:  = Common-Base Current Gain (typical 0.99) Nasim Zafar.

  27. BJT-Current Gain Parameters:  = Common-emitter current gain (10-1000; typical 50-200) Nasim Zafar.

  28. DC  and DC   = Common-base current gain (0.9-0.999; typical 0.99)  = Common-emitter current gain (10-1000; typical 50-200) • The relationship between the two parameters are: Nasim Zafar.

  29. Performance Parameters for PNP: Common emitter dc current gain, dc: But, Note that  is large (e.g.  = 100) For NPN transistor, similar analysis can be carried out. However, the emitter current is mainly carried by electrons. Example: Nasim Zafar.

  30. Performance Parameters for PNP: Emitter efficiency: Fraction of emitter current carried by holes. We want  close to 1. Base transport factor: Fraction of holes collected by the collector. We want T close to 1. Common base dc current gain: Nasim Zafar.

  31. Example: NPN Common-Base Configuration: C • Given: IB= 50  A , IC= 1 mA • Find: IE,  , and  • Solution: • IE = IB + IC = 0.05 mA + 1 mA = 1.05 mA • = IC / IB = 1 mA / 0.05 mA = 20  = IC / IE = 1 mA / 1.05 mA = 0.95238 VCB + _ IC B IB IE VBE + _ E Nasim Zafar.

  32. Nasim Zafar.

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