BJT characteristics curve

Discussion in 'Homework Help' started by fran1942, Sep 24, 2011.

  1. fran1942

    Thread Starter Member

    Jul 26, 2010
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    Hello, on the attached curve I have drawn in a blue load line.
    Can I interpret this blue line to mean that I only need approx. 80uA to reach saturation point which will then give me my desired collector current of 60mA ?
    Therefore I do not need the full 100uA of base current to reach 60mA Ic.

    Thanks for confirmation if this correct.
     
    Last edited: Sep 24, 2011
  2. Adjuster

    Well-Known Member

    Dec 26, 2010
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    I cannot see any graphs: are you sure you have attached them or linked them correctly?
     
  3. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Ic=60mA IC and IB=80μA implies a gain of 750. Is this a Darlington device?

    The condition shown would be just approaching saturation if the characteristics were those of the individual device used, but this would allow no margin for variation between samples if the curve is a only a typical one, nor does it allow for temperature or ageing.

    Typically to guarantee saturation a lower gain is assumed, sometimes generically as low as 10 for a simple BJT. The value for a Darlington type would normally be higher, but it is best to follow the manufacturers recommendations for saturated operation if given. Using gain values specified for higher VCE is not satisfactory.
     
  4. fran1942

    Thread Starter Member

    Jul 26, 2010
    58
    0
    Hello, no this is not a real world example. It is just a concept example.
    I only drew in that arbitrary load line as an example of interpreting a characteristics graph.

    I just want to know if:
    I can interpret this blue line to mean that I only need approx. 80uA to reach saturation point which will then give me my desired collector current of 60mA ?
    Therefore I do not need the full 100uA of base current to reach 60mA Ic ?

    Is that how you interpret the curve chart ?
     
  5. Adjuster

    Well-Known Member

    Dec 26, 2010
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    You have drawn the line (67Ω fed from 4V?) rather thickly so it is a bit hard to see. The device looks to be on the verge of saturation for IB=80μA.

    For the characteristics of a real transistor, the curves in the saturation region may not come together quite as neatly as in this idealised example.
     
  6. fran1942

    Thread Starter Member

    Jul 26, 2010
    58
    0
    Yes, it is on the verge of saturation for IB=80uA. So to confirm, that would mean I would theoretically not need more than 80uA to achieve an Ic of 60mA according to my load line ?

    I understand this is idealistic and not a real-world scenario.
    it is just an understanding of the graph that I am wanting to get reinforced.
     
  7. Adjuster

    Well-Known Member

    Dec 26, 2010
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    If the transistor characteristic is exactly as shown on that graph, then it would saturate at about IB=80μA with the loading shown by your blue line. In some real transistor characteristics, the curves merge less completely than shown in your example and a lower value of saturated VCE might be obtained for a higher base current.

    This article shows a less idealised output characteristic: http://www.st-andrews.ac.uk/~jcgl/Scots_Guide/info/comp/active/BiPolar/bpcur.html

    NB. If you are answering a marked assessment, a particular formal definition of saturation may be relevant, e.g. requiring the collector potential to be below that of the base, and you must follow your instructor's requirements if you wish to get credit. In a practical engineering situation, we might consider a transistor to be saturated as long as further increase in base current would produce negligible decrease in collector voltage, even if the collector potential did not drop below that of the base.

    One reason for emphasising the need for generous base drive to guarantee saturation is that inadequate base current is a fairly common cause of failure in hobbyist projects, where transistors intended to be switched on hard fail to do so, sometimes resulting in overheating.
     
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