Explain the result for BJT characteristic

Discussion in 'Homework Help' started by simpsonss, Oct 6, 2010.

  1. simpsonss

    Thread Starter Active Member

    Jul 8, 2008
    173
    0
    hi,

    I'm doing an experiment for BJT 2n3904 regarding the characteristic of my BJT.

    The circuit is as below.
    untitled1.JPG

    The result i get is as below.
    When Vout =12V Ic =3.10mA Ib=0.014mA beta value = 221
    when Vout = 2v Ic =13mA Ib=0.077mA beta value =169

    So is it mean that when more voltage apply on V1 then Ib will be higher and hence more current is allow to pass through Vce and this makes Ic also higher. Am i correct?

    And when Ic higher beta value increase but will start decreasing when Ic=7mA. i get this by referring to the datasheet as shown below.
    untitled2.JPG

    My conclusion is when Ib higher, the BJT is more biased and thus Ic higher.
    Am i correct till this state? So is it the voltage at Vout is not a point to mention when we talk about characteristic of a BJT?

    thank you.
     
  2. Georacer

    Moderator

    Nov 25, 2009
    5,142
    1,266
    You are correct. Just one note on the β parameter and its behaviour. As you know \beta=\frac{I_C}{I_B}. When the transistor works in the active region, it can amplify you base current by the nominal value of β and allow it to pass from the collector to the emitter.

    When you enter saturation region however, the transistor cannot produce the current you are asking. The high base current cannot produce a high enough collector current to confirm the above relation. The ratio of collector to base current is now much smaller than the nominal β, and is called forced β. The more you increase base current in saturation region, collector current stays the same at its maximum I_{sat}, and the more the forced β diminishes. This is what the constructor's diagram shows.
     
  3. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    You are correct up to this point.

    This statement is correct, but it might be better to realise that the action of the transistor is to adjust its collector voltage (=Vout) to draw a current of beta times the base current, in conjunction with the other resistors in the collector circuit.
    So the collector voltage is not determined by the transistor alone, but also by the circuit.
     
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