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Bipolar Junction Transistors

Bipolar Junction Transistors. EE314. History of BJT First BJT Basic symbols and features A little bit of physics… Currents in BJT’ Basic configurations Characteristics. Chapter 13: Bipolar Junction Transistors. Current Flow in BJT. pnp BJT. -i C. i E. -V CE. -i B.

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Bipolar Junction Transistors

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  1. Bipolar Junction Transistors EE314

  2. History of BJT • First BJT • Basic symbols and features • A little bit of physics… • Currents in BJT’ • Basic configurations • Characteristics Chapter 13: Bipolar Junction Transistors

  3. Current Flow in BJT pnp BJT -iC iE -VCE -iB • Injected h+ current from E to B • 2. e- injected across the forward-biased EB junction (current from B to E) • 3. e- supplied by the B contact for recombination with h+ (recombination current) • 4. h+ reaching the reverse-biased C junction • 5,6.Thermally generated e- & h+ making up the reverse saturation current of the C junction

  4. Now, you can try… npn BJT

  5. BJTs – Basic configurations

  6. npn BJTs – Operation Modes Forward & reverse polarized pn junctions Different operation modes:

  7. npn BJTs – Operation Modes • When there is no IB current almost no IC flows • When IB current flows, IC can flow • The device is then a current controlled current device Operational modes can be defined based on VBE and VBC

  8. BJT-Basic operation pnp BJT npn BJT (n+), (p+) – heavy doped regions; Doping in E>B>C

  9. BJTs – Current & Voltage Relationships Operation mode: vBE is forward & vBC is reverse Einstein relation The Shockley equation IES–saturation I (10-12-10-16A); VT=kT/q -thermal V (26meV) D – diffusion coefficient [cm2/s] m – carrier mobility [cm2/Vs] The Kirchhoff’s laws It is true regardless of the bias conditions of the junction Useful parameter the common-emitter current gain for ideal BJT b is infinite

  10. BJTs – Current & Voltage Relationships Useful parameter the common-base current gain for typical BJT a is ~0.99 The Shockley equation once more If we define the scale current A little bit of math… search for iB Finally…

  11. BJTs – Characteristics Schematic Common-Emitter Output Input VBC<0 or equivalently VCE>VBE If VCE<VBE the B-C junction is forward bias and IC decreases Remember VBE has to be greater than 0.6-07 V Example 13.1

  12. BJTs – Load line analysis Common-Emitter Amplifier smaller vin(t) Input loop if iB=0 if vBE=0

  13. BJTs – Load line analysis Common-Emitter Amplifier Output loop Example 13.2

  14. Circuit with BJTs Our approach: Operating point - dc operating point Analysis of the signals - the signals to be amplified Circuit is divided into: model for large-signal dc analysis of BJT circuit bias circuits for BJT amplifier small-signal models used to analyze circuits for signals being amplified Remember !

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