N Type            P Typeextra electrons   holes (missing electrons)           depletion zone
N        P        Nemitter   base   collector
N         P         Nemitter   base   collector
Electron Flow  N            P               Nemitter     base          collector
Electron Flow  N            P               Nemitter     base          collector
• Emitter is grounded (zero voltage)• Base has a small positive voltage• Collector has a large positive voltage• Electrons...
Electron Flow  N            P               Nemitter     base          collector
• Positive voltage at base and collector  attract electrons in middle P layer.• Some current flows from emitter to base.• ...
Hole Flow  N          P             Nemitter   base        collector
P         N         Pemitter   base   collector
P         N         Pemitter   base   collector
Current for Hole Flow  P                N              Pemitter         base         collector
Current for Electron Flow  P              N                Pemitter       base         collector
• Bipolar transistors act like 2 back-to-back  diodes• base-emitter diode is forward biased  – current can flow from base ...
No signal voltage on base,So transistor turned off.                 No base current                 (emitter to base)
No signal voltage on base,So transistor turned off.                                      Current can NOT flow from        ...
All supply voltage dropped on R2                                      because transistor is an openNo signal voltage on ba...
Small signal voltage applied to base,                Transistor turned on.                                        LARGE cu...
All supply voltage dropped on R2     All supply current to outputNo collector to emitter current
No voltage on base  transistor is off
Voltage on the base    turns transistor turns onsmall base current      causes LARGE                        collector curr...
Replace NPN with PNP and just      reverse the polarity
Replace NPN with PNP and just      reverse the polarity
Switches or Regulators• Transistors are manufactured to be either  switches or regulators.• Switches need to turn on and o...
Switches or Regulators• How does a transistor work as a regulator?
DCAC voltage on basecauses AC current
DCAC voltage on basecauses AC current
DC
DC     open base     allows collector     current     to flow
Small input signal current results inlarger output signal current
Output signal current is at samefrequency as input but largeramplitudeRegulator transistorscan act as amplifiers
Transistors
Transistors
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Transistors

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Transistors

  1. 1. N Type P Typeextra electrons holes (missing electrons) depletion zone
  2. 2. N P Nemitter base collector
  3. 3. N P Nemitter base collector
  4. 4. Electron Flow N P Nemitter base collector
  5. 5. Electron Flow N P Nemitter base collector
  6. 6. • Emitter is grounded (zero voltage)• Base has a small positive voltage• Collector has a large positive voltage• Electrons flow from emitter into P layer.• Base (P layer) is lightly doped, so few holes get filled by extra electrons.• Most electrons build up in base (P layer).
  7. 7. Electron Flow N P Nemitter base collector
  8. 8. • Positive voltage at base and collector attract electrons in middle P layer.• Some current flows from emitter to base.• Most current flows from emitter to collector.• Base voltage determines flow from emitter. – If base voltage is too low, no current flows from emitter to base or from emitter to collector. – The higher the base voltage, the more base current and collector current flows.• Bipolar transistors are current amplifiers.
  9. 9. Hole Flow N P Nemitter base collector
  10. 10. P N Pemitter base collector
  11. 11. P N Pemitter base collector
  12. 12. Current for Hole Flow P N Pemitter base collector
  13. 13. Current for Electron Flow P N Pemitter base collector
  14. 14. • Bipolar transistors act like 2 back-to-back diodes• base-emitter diode is forward biased – current can flow from base to emitter• base-collector diode is reverse biased – current can not flow from collector to base • some leakage current will flow – if base current flows, causes collector to emitter current to flow
  15. 15. No signal voltage on base,So transistor turned off. No base current (emitter to base)
  16. 16. No signal voltage on base,So transistor turned off. Current can NOT flow from Collector to base to input No base to emitter current.
  17. 17. All supply voltage dropped on R2 because transistor is an openNo signal voltage on base,So transistor turned off. No collector to emitter current No voltage from output to ground, so no current flows No base to emitter current.
  18. 18. Small signal voltage applied to base, Transistor turned on. LARGE current must flow to output collector current flows small base current flows
  19. 19. All supply voltage dropped on R2 All supply current to outputNo collector to emitter current
  20. 20. No voltage on base transistor is off
  21. 21. Voltage on the base turns transistor turns onsmall base current causes LARGE collector current
  22. 22. Replace NPN with PNP and just reverse the polarity
  23. 23. Replace NPN with PNP and just reverse the polarity
  24. 24. Switches or Regulators• Transistors are manufactured to be either switches or regulators.• Switches need to turn on and off FAST!• Need enough base bias voltage to overcome barrier – 0.7 V for silicon transistor – 0.3 V for germanium transistor
  25. 25. Switches or Regulators• How does a transistor work as a regulator?
  26. 26. DCAC voltage on basecauses AC current
  27. 27. DCAC voltage on basecauses AC current
  28. 28. DC
  29. 29. DC open base allows collector current to flow
  30. 30. Small input signal current results inlarger output signal current
  31. 31. Output signal current is at samefrequency as input but largeramplitudeRegulator transistorscan act as amplifiers

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