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Learning Outcome

ECA1212 Introduction to Electrical & Electronics Engineering Chapter 5: Bipolar Junction Transistor by Muhazam Mustapha, October 2011. Learning Outcome. Be able to explain some basic physical theory and operation of BJT Be able to do calculation on DC and AC analysis on BJT circuit.

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Learning Outcome

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  1. ECA1212Introduction to Electrical &Electronics EngineeringChapter 5: Bipolar Junction Transistorby Muhazam Mustapha, October 2011

  2. Learning Outcome • Be able to explain some basic physical theory and operation of BJT • Be able to do calculation on DC and AC analysis on BJT circuit By the end of this chapter students are expected to:

  3. Chapter Content • Theory of BJT • BJT Operation • DC Analysis • AC Analysis

  4. Bipolar Junction Transistor CO1

  5. Bipolar Junction Transistor • If diodes are made by fabricating one PN junction, BJT are made by fabricating two PN junctions. • It involves fabrication of 3 layers of P-N-P (pnp) or N-P-N (npn) types: • The middle layer has to be very thin • 3 terminals are attached to the 3 layers p n n p p n CO1

  6. Terminals • The middle layer is called BASE (B) • The top and bottom layers are not symmetrical • Top layer is called COLLECTOR (C) – doped more lightly than the bottom layer (emitter) • Bottom layer is called EMITTER (E) – doped more heavily than the top layer (collector) Collector Collector p n Base Base n p + + p n Emitter Emitter CO1

  7. Circuit Symbol and Notations • pnp BJT symbol: • npn BJT symbol: C C C C B B B B E E E E • The direction of the arrow is the direction of the current when the BE junction is put on forward bias CO1

  8. Circuit Symbol and Notations • In normal operation, BE junction is put to forward bias • For that reason, npn is more popular since E is normally put to the lowest voltage on the BJT • Hence, B has to be at higher voltage in order to put BE junction in forward bias CO1

  9. Circuit Symbol and Notations • Notation for currents and voltage for npn: C + vCB iC − iB + vCE B + − iE vBE − E KCL: iE = iB + iC KVL: vCE = vCB + vBE • For pnp, the polarities are reversed CO1

  10. Transport Phenomena • Transistor can be considered as two diode joined back to back with the joint at base • Diode of BE junction is forward biased, hence there will be current flowing • Diode of CB junction is reverse biased, hence there is no current C B E CO1

  11. Transport Phenomena • So how do we get current flowing through C? • Current manages to get through C due to the fact that B layer is very thin • Since B layer is very thin, the reversely flowing transport (electron or hole) at BE junction will overshoot into the depletion region on the reverse biased CB junction CO1

  12. Transport Phenomena overshooting electrons across reverse biased junction causing large avalanche current N P hole movement forward biased electron movement N CO1

  13. Transport Phenomena • These overshot transport will further collide with the covalence bond in depletion region and produce more holes and electrons • The newly produced electrons and holes will further collide with other bonds and produce more and more new free electrons and holes • The whole process explained above is called avalanche • These avalanche produced electrons and holes will too move under the influence of the external field (voltage of vCE) CO1

  14. Transport Phenomena • Hence the current through C (iC) is contributed by the overshooting and avalanched transport • Since the overshooting current is due to iB, and since the amount of the avalanche current is due to the overshooting current, then the amount of avalanche current would be proportional to iB • So as to say, iB actually controls iC by some multiplication factor • This factor is called the CURRENT GAIN, β CO1

  15. I-V Characteristic • Since iC can be controlled by iB, we can consider BJT like an input-output transfer box • The current and voltage input parameter of BJT are iB and vBE respectively • While the current and voltage output parameter of BJT are iC and vCE respectively C • The I-V characteristic of BJT is featured by the I-V characteristics of these input and output iC iB + vCE B OUTPUT INPUT + − vBE E − CO1

  16. BE (Input) Characteristic • Since BE junction is just like a forward biased diode, the I-V characteristic is so like that too 500 400 300 IB (μA) 200 100 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 VBE (V) CO1

  17. CE (Output) Characteristic • Since CB junction is reversed biased, the I-V characteristic of CE is flat (zero) unless IB > 0 • With some values of IB, we get a family of I-V flattening curves for CE 50 IB = 300μA 40 IB = 250μA IB = 200μA 30 IC (mA) IB = 150μA 20 IB = 100μA 10 IB = 50μA IB = 0 1 2 3 4 5 6 7 8 9 10 CO1 VCE (V)

  18. Operation Region • BJT may be put to operate at 4 different operation mode – for our class we will be covering only 3 modes • The 3 modes are called operation region • The region is defined by the areas in CE (output) I-V characteristic graph: • Active • Saturation • Cutoff CO1

  19. Operation Region SATURATION REGION 50 IB = 300μA 40 IB = 250μA ACTIVE REGION IB = 200μA 30 IC (mA) IB = 150μA 20 IB = 100μA 10 IB = 50μA IB = 0 1 2 3 4 5 6 7 8 9 10 VCE (V) CUTOFF REGION CO1

  20. Cutoff State / Region • The BJT is basically in OFF condition with no current flowing because IB is zero • Uses: • OFF state in digital circuit • OFF state for analog switch • Detailed features: • IB = 0 • IC = ICEO ≈ 0 • VCE ≥ 0 • VBE < VD CO1

  21. Saturation State / Region • The BJT is basically in full ON condition with very low VCE whereby the BJT may be considered to have a very low output resistance • Uses: • ON state in digital circuit • ON state for analog switch • Detailed features: • IB > 0 • IC < βIB • VCE = Vsat ≈ 0.2V • VBE = VD CO1

  22. Active State / Region • The BJT is in linear analog amplification mode whereby IC is almost proportional to IB • Uses: • Analog signal amplication • Detailed features: • IB > 0 • IC = βIB • VCE > VD • VBE = VD CO1

  23. Detecting Operation Region • It’s easy: • If VBE < VD (means IB is 0), then the BJT is in cutoff regardless of VCE • If VBE = VD and VCE = Vsat, then the BJT is in saturation • Otherwise it is in active region – BE junction forward biased and BC junction reverse biased CO1

  24. Biasing (DC Analysis) CO1

  25. DC Analysis (Biasing) Refer to Giorgio Rizzoni’s Principles and Applications of Electrical Engineering: Chapter 10.4, Example 10.7, Example 10.9 • Biasing of a transistor means putting the transistor’s VCE and IC into a desired position in the IC-VCE graph • This is done normally if we want the transistor to operate in active region • Cutoff and saturation region normally don’t require much biasing since the area is limited • The biasing process is a little tricky since IC is controlled by IB – not directly by VCE CO1

  26. DC Analysis (Biasing) Refer to Giorgio Rizzoni’s Principles and Applications of Electrical Engineering: Chapter 10.4, Example 10.7, Example 10.9 • The position of the biased BJT’s VCE and IC is called Q point • The value of IB is also required for the biasing • There are a few biasing configuration exist, but for the purpose of non-EE class, we will only study the most popular configuration called self-bias common emitter configuration • Refer to Giorgio Rizzoni’s Fundamentals of Electrical Engineering Figure 10.28 CO1

  27. DC Analysis (Biasing) Refer to Giorgio Rizzoni’s Principles and Applications of Electrical Engineering: Figure 10.29 RB = R1 || R2 IC IC R1 RC RC RB + + IB IB VCE VCE VCC VCC + + − − VBE VBE VBB − − IE IE R2 RE RE Thevenin’s Equivalent VBB = (VCC)(R2)/(R1+R2) CO1

  28. DC Analysis (Biasing) Refer to Giorgio Rizzoni’s Principles and Applications of Electrical Engineering: pages 573 – 575 • The target of biasing process is to find the value of the resistors so that Q point is position at around VCC/2 in the IC-VCE characteristic graph • R1, R2 and RE will determine IB • IB will determine IC – either by IC = βIB, or by an IC-VCE graph • Then from KVL, VCE = VCC−ICRC−IERE • This equation is what called load-line equation CO1

  29. DC Analysis (Biasing) Refer to Giorgio Rizzoni’s Principles and Applications of Electrical Engineering: pages 573 – 575 • Steps: • R1 and R2 will be combined using Thevenin’s theorem to form RB • Use KVL on BE loop to get IB from RB and IE • Use β or IC-VCE graph to get IC • Use KVL on CE loop (load-line equation) with the required VCE for the Q point to get RC CO1

  30. DC Analysis (Biasing) • Class discussion: Giorgio Rizzoni’s Principles and Applications of Electrical Engineering: pages 573 – 575, Example 10.9 CO1

  31. AC Analysis CO1

  32. AC Analysis Refer to Giorgio Rizzoni’s Principles and Applications of Electrical Engineering: Example 10.8, page 570-574 • AC analysis is done to determine the performance of transistor amplifier circuit • There are a few parameters of interest, like input and output resistance, but for the purpose of non-EE class, we will do only AC gain (current and voltage) • AC analysis is done after biasing is completed and assuming there is some AC signal being introduced into the circuit as superimpose on top of the DC values (biasing) CO1

  33. AC Analysis Refer to Giorgio Rizzoni’s Principles and Applications of Electrical Engineering: Example 10.8, page 570-574 • The oscillation of the input and output signals will be denoted by Δ (delta) • For this class we will consider the I-V characteristic of the sinusoidal input and output signals will be the same as the DC relationship • next slide CO1

  34. AC Analysis Refer to Giorgio Rizzoni’s Principles and Applications of Electrical Engineering: Example 10.8, page 570-574 R1 RC ΔVO ΔVB VCC R2 RE CO1

  35. AC Analysis Refer to Giorgio Rizzoni’s Principles and Applications of Electrical Engineering: Example 10.8, page 570-574 Output Input Voltage Gain CO1

  36. ΔVO Formula Refer to Giorgio Rizzoni’s Principles and Applications of Electrical Engineering: Example 10.8, page 570-574 In the formula for ΔVO, it only depends on ΔIC even though from the KVL at the output it should also depends on ΔIE. The reason for this is in real circuit we put a capacitor across RE which effectively SHORTS circuit RE when AC current flows – means we can disregard RE in AC analysis formula. R1 RC ΔVO ΔVB VCC R2 RE CO1

  37. AC Analysis • Class discussion: Giorgio Rizzoni’s Principles and Applications of Electrical Engineering: pages 570 – 574, Example 10.8 CO1

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