Beta for Voltage Divider Bias Circuit

Discussion in 'The Projects Forum' started by chaosweapon, Nov 14, 2008.

  1. chaosweapon

    Thread Starter Member

    Nov 14, 2008
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    I'm designing a voltage divider bias circuit for a BC548 transistor. The HFE varies between 110 and 800. The problem is I don't know which value of HFE to take in order to help me design the circuit. As an approximation for R2 I'm using:

    R2 = (beta x RE) / 10
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Manufacturers only guarantee a minimum hFE for a specified Vce and collector current; and what the maximum may go up to.

    Download a datasheet from a manufacturer and look at the typical hFE curves. Fairchild Semiconductor's datasheet looks pretty decent; go to their website and download it. Note that the values plotted are typical, and not what you're guaranteed to get out of each and every transistor.

    hFE varies quite a bit between individual transistors, even in the same lot. Variation over temperature is another can o' worms.
     
  3. chaosweapon

    Thread Starter Member

    Nov 14, 2008
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    0
    So my best bet is to use the minimum hFE ie 110?
     
  4. steveb

    Senior Member

    Jul 3, 2008
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    This is the common problem in transistor circuit design. You must design your circuit so that the important performance attributes are not significantly depended on beta. All electronics courses and books dealing with transistor design talk about this issue and the design approaches to accomplish this.

    I'm not exactly sure of your circuit, but it looks like you are using one of the design formulas which is a rule of thumb that helps you make the circuit relatively independent of beta. Hence, you want to choose the beta value that makes the rule work best, which I believe is the minimum value of beta in your case.

    Of course, I'm just guessing at what you are doing, but you should make sure you understand the reason for any design formulas, or rules of thumb that you use.
     
  5. chaosweapon

    Thread Starter Member

    Nov 14, 2008
    15
    0
    Yes, I'm using a rule of thumb. Isn't this the purpose for it:
    Code ( (Unknown Language)):
    1. (beta + 1)R[SIZE=1]E[/SIZE] >> R[SIZE=1]1[/SIZE] || R[SIZE=1]2[/SIZE]
     
  6. steveb

    Senior Member

    Jul 3, 2008
    2,433
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    Ok, I thought you were using that one. Yes, so it's clear that you want the minimum beta for your calculations. If your rule is obeyed for the minimum beta, then it is certain to be obeyed for any larger beta.
     
  7. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The range of beta for a BC548 is huge. Buy a BC548A, BC548B or BC548C for a narrower range of about 2:1. A 2N3904 transistor has almost the same performance (but its pins are the reverse) and has a 3:1 range of beta.
     
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