Question regarding the saturation region of a bjt?

Discussion in 'General Electronics Chat' started by nPab, Sep 16, 2014.

  1. nPab

    Thread Starter New Member

    Sep 16, 2014
    5
    0
    I always come across the statement that the collector current of a bjt is maximum when it is in saturation region and also people say that it is because the bjt is fully on when it is in the saturation region..

    But when you observe the Ic vs Vce graph of a bjt for a particular Ib, we see that the linear region has a higher Ic compared to the saturation region (Obvious from the graph!). So how is the above statement correct?
    Also why is it said that the bjt is fully on in the saturation region?
     
  2. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    Care to post the particular datasheet, so that we can talk about the same exact figure?
     
  3. nPab

    Thread Starter New Member

    Sep 16, 2014
    5
    0
    Isn't that general for all bjts...?
    I needed a grasp on the concept...I don't understand why it is said that saturation region carries the highest collector current when the linear region is further up in the graph..
    I'm talking about Ic vs Vce characteristics of a bjt...
     
  4. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    This statement is true only when we have a circuit with resistors and BJT.
    http://forum.allaboutcircuits.com/threads/which-method-npn.97963/#post-731172
     
  5. nPab

    Thread Starter New Member

    Sep 16, 2014
    5
    0
    Sorry jony130...I don't understand....how is the Ic vs Vce graph affected in presence of Rc?
    Also shouldn't the linear region infact be called saturation region (I know I sound crazy..but bear with me) because isn't in the linear region where current changes negligibly with Vce?Also current in the linear region is higher than the saturation region right?(from the graph)...
    Please post a reply....
     
  6. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    Add a Rc resistor change a lot.
    The load line is the answer. For example for Rc = 4.6K

    [​IMG]
    As you can see now the maximum collector current is equal to
    Ic_max = Vcc/Rc = 10V/4.6k = 2.18mA and nothing you can do about this.
    And for example for Ib = 400uA the collector current cannot be greater than 1.75mA

    http://www.electronics-tutorials.ws/transistor/tran_2.html
    http://pcbheaven.com/wikipages/Transistor_theory/?p=5

    No, but don't worry, your logic is used in MOS. In MOS transistor linear/active region is cold a saturation region.
     
  7. crutschow

    Expert

    Mar 14, 2008
    13,014
    3,234
    As noted, you seem to be confusing the saturation region of a MOSFET with the saturation region of a BJT. Unfortunately, just to confuse us all, they are defined differently (and somewhat oppositely).

    The linear region of a MOSFET is similar to the saturation region of a BJT. (load current changes proportional to the voltage).

    The saturation region of a MOSFET is similar to the active region of a BJT (load current doesn't change much with a change in voltage).

    The definition of BJT saturation is that the magnitude of the collector-emitter voltage (Vce) is equal to or less than the magnitude of the base-emitter voltage (Vbe). That's the region used when the transistor is performing as a switch.
     
    Last edited: Sep 16, 2014
  8. Cal Rico

    New Member

    May 1, 2012
    15
    1
    How do you guys keep this stuff in your head??
    I have to go to Malvino or Electronics for Inventors and keep the book at my lab desk as I work through this question.
     
  9. crutschow

    Expert

    Mar 14, 2008
    13,014
    3,234
    When you've been in the electronics design business for 40 odd years you eventually retain some of the basics (and become an expert at finding what you don't remember on Google). ;)
     
    nPab, kubeek and tom_s like this.
Loading...