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Saturation region of a BJT

  1. Sep 21, 2014 #1
    From the output characteristics of a BJT, it is observable that in the active region, the collector current becomes constant or independent of collector emitter voltage. So, in true terms, it is saturation, then why do we call it the active region?
    Also, we have the equation:-
    Ic = (Vcc – Vce)/Rc
    Then, accordingly, with varying Vce, Ic should vary. Please clarify.
     
  2. jcsd
  3. Sep 22, 2014 #2

    analogdesign

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    You have it backwards. In a BJT in saturation the Vce of the BJT is roughly constant (and about 0.1 or 0.2 V). You are confusing saturation in a MOSFET with saturation in a BJT.
     
  4. Sep 23, 2014 #3
    One of the things that get people super confused (including myself at one point) is the presumption that the base current is being magnified to form the collector current. This notion is perpetrated so much in books that saturation of the transistor almost seems impossible. What is happening is that the base current is controlling the amount of current that Vcc supplies. If you understand this very simple fact, you've unlocked the mystery of the transistor. Here's further explanation :

    Assume you have Vcc connected to Rc in series without the transistor, directly to ground. Whats the current that Vcc can supply Rc ? Its Vcc/Rc. This is the maximum current that Vcc can supply.
    Now put the transistor in place. The base current now dictates the collector current. So even though the source would like to supply Vcc/Rc, the transistor prevents that from happening by dropping some of Vcc across its terminals, Vce. The larger is Vce, the smaller is the current that Vcc can supply. The larger is the base current, the smaller is Vce. This means that Vcc can now supply more current than before. So lets say that you have a base current that causes the transistor to supply Vcc/Rc. What happens if you increase the base current ? Nothing ! The transistor cannot supply Vcc cannot supply more than Vcc/Rc.

    In short:
    Keeping Vcc and Rc constant, a small base current will cause Vcc to supply a small collector current. This also means that Vcc drops most of its voltage across its terminals as Vce.
    Increasing the base the current causes Vcc to supply more collector current. This also means that Vce is getting smaller. As long as the base current can control what Vcc can supply, the transistor is said to be operating in the linear in the linear region.
     
  5. Feb 10, 2016 #4
    amenhotep, thank you so much for that explanation! I stumbled around on the web and books for an hour before coming to your post which just clarified my understanding so well :)
     
  6. Feb 11, 2016 #5

    LvW

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    Just because you have mentioned the "unlocked mystery" of the transistor :
    I am aware that many people believe (believe, without any proof) that the base current would "control" the collector current.
    And for some applications this assumption does work. But not for all!
    Therefore, I think it is helpful to mention again that the collector current of a bipolar transistor is controlled/determined by the base-emitter voltage Vbe only. The base current Ib is an (unwanted, but unavoidable) by-product.
    This is not only an assertion - it is a fact which can be verified.
    I do not want to start this discussion today again - because this subject was discussed already intensively also in this forum.
    See here, for example:

    https://www.physicsforums.com/threads/amplifiers-transistors-and-opamps.828847/

    https://www.physicsforums.com/threa...istor-saturation-current.824127/#post-5175698
     
    Last edited: Feb 11, 2016
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