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General questions regarding transistors in different modes.

Discussion in 'Electronics Homework Help' started by dlevsApiJ, May 20, 2016.

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  1. dlevsApiJ


    May 19, 2016

    I am currently doing a course in electronics and came to the chapters about transistors. I am currently in the last chapter and still have some remaining questions. I searched on the internet but what is still not fully clear to me:

    1) A transistor is in saturation mode when Vce = 0.1V. What is a typical value for a transistor that is in active / amplification mode? Is this difference in Vce the distinction between placing a transistor in any of these two modes?

    2) When a transistor is in saturation mode, its hFE is less high than it would normally be in active mode, than the minimal value in the datasheet. Right? How can you determine the exact value?

    I hope the questions are clear. For some reason I have a hard time with fully understanding transistors; especially the difference between the two modes.

    Thanks in advance.
    Last edited: May 20, 2016
  2. dorke


    Jun 20, 2015
    One of the "definitions" of transistor saturation is :
    The linear ratio relation that holds in the active region, IC=beta*IB no longer holds .

    We have infect IC<beta*IB ,thus in saturation an increase in IB no longer increases IC.
    In this respect we can say that the max IC possible for the specific circuit is reached.
    The VCE voltage is called VCEsat and is low.

    To answer your questions:

    1.In the active region we have VCEsat<VCE<VCC .
    There is no such thing as a " typical value" it depends on what you want to achieve from
    the specific circuit.

    2. As explained above ,in saturation there is no more "meaning " for HFE.
    Last edited: May 21, 2016
  3. dlevsApiJ


    May 19, 2016
    Thanks, that actually makes a lot clear to me! (the course I am doing has some simple information missing on this topic in my opinion..).

    Two final questions to get it absolutely clear:

    1) When the transistor is in saturation mode, the collector current is determined by the Rc and the Vcc and has no specific relation to Ib anymore?

    2) When I am looking in the datasheet of a BC337 transistor it states Ic = 800 mA as absolute maximum rating.
    So, when I am using a BC337 transistor in saturation mode, I guess 800mA is the maximum Collecter Current I can use before destroying the transistor?
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    in answer to the second question, 800mA may lead to the transistor overheating and failing. However the current won't cause failure of the transistor by itself.
  5. dorke


    Jun 20, 2015
    1. Yes ,to be more precise ,it also depends on VCEsat which is transistor related.
    2.Yes,but to be more precise that absolute max IC is constant(DC) current.
    dlevsApiJ likes this.
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