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Transistor beta(dc current gain)

  1. Jul 5, 2010 #1
    i am tring to use transistor tip 41c , and i want to calculate Ic ,,, knowing that ( Ic=B*Ib)
    but when i try to know BETA (B) from the data sheet of the transistor i found that it vary with the collector current (Ic) and the graph show the relation between (Ic) and beta in case Vce=4 volt only,what if i ve to set Vce at different value ?????????.. plz if any one has experince in that field help,my circuit and the datasheet are in attach.

    Attached Files:

  2. jcsd
  3. Jul 5, 2010 #2
    When employing the bjt, we generally set the emitter current, Ie, to a desired value, & then collector current Ic, is computed as Ic = alpha*Ie. For a small signal bjt, alpha can range from 0.98 to 0.998, very narrow range of values, less than 2% total variation over temperature, specimen, & current level.

    The relationship between alpha & beta is alpha = beta/(beta+1), or beta = alpha/(1-alpha).

    The problem with setting Ib to a specific value is known as "beta dependency". The beta value can go as low as 50, up to as much as 500. If we wish to set Ic to a known stable, consistent value, we should rely on alpha, not beta.

    A good text on basic electronics will show you how to do this. Also, there are many web sites dying to teach people about electronics, but if you are a beginner in the learning stage, you need to differentiate between sound advice, & heresy propagated by 2nd rate hackers, crackpots & wanna-be's. I hope this helps.

  4. Jul 5, 2010 #3
    It is impossible to set Vce to anything with the circuit shown in your picture.

    Or if you like Vce is permanently set to the supply voltage.

    Redraw your circuit, including the load (you have not shown any load) and then we can discuss what you want to achieve.
  5. Jul 5, 2010 #4
    ys Vce is set to the supply voltage which is Vcc = 9volt.
  6. Jul 5, 2010 #5
    Which means that all you will achieve unless you are very very careful with the base resistor is a blown transistor.
  7. Jul 5, 2010 #6
    The circuit in your diagram is unstable because beta varies substantially not only among transistors of the same type but also with temperature. As your transistor begins conducting it will get warmer increasing the beta which increases the current it conducts. If the current is high enough it will go into thermal runaway and destroy itself.

    You need some sort of negative feedback to prevent that. The easiest is to put a resistor between the emitter and ground. The current is then controlled by the voltage at the base which is equal to Ie*R + Vbe.
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