Bjt Baising???

Discussion in 'General Electronics Chat' started by acelectr, Mar 17, 2011.

  1. acelectr

    Thread Starter Member

    Aug 28, 2010
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    Hi I've just started on working about bjt amplifications etc... I did a bunch of readings from different kinds of sources. But the thing, the term that I still did not understand is the term bias. What is bias or biasing? What is the meaning of a biased bjt circuit, for instance I've got this problem saying blablabla the bjt is biased with a 1mA collector current blabla etc...

    If I'll get a clear explanation for the term I'll be really glad.
    thnx for any help:rolleyes:
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    Bias is turning a component on in a very controlled manner. It applies to transistors and diodes. Many cases bias is to put the component in the middle of it's linear region. BJTs operate both as digital switches and linear amplifiers, with BJT linear amps the bias is critical.

    If you have a specific question don't forget to include a schematic. I have several predrawn to help with discussions.

    [​IMG]

    The main bias components with this design is R1 and R2. R3 provides negitive feedback. The total effect is to create a circuit whose values are very stable in the DC region. This is more important than it sounds, as when BJTs get warm or hot they tend to conduct more, which can create more heat, which creates more conduction. When this happens it is called thermal run away. This configuration prevents that.
     
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  3. acelectr

    Thread Starter Member

    Aug 28, 2010
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    Hi, by the middle of the linear region did you mean the quiescent point? When saying "properly biased" does it mean that the quiescent point is in a proper position so for applying proper amplification? and secondly what do you mean by "stable in the DC region"??

    Thnx for your concern
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    As to the second question, look up thermal runaway. That is when it is not stable.

    I'm not sure what you mean by quiescent point, think in terms where the output has a symmetrical sine wave output, or if there is clipping, it is equally on the top and bottom. This is a correctly biased amplifier, it is harder to achieve than it may sound.
     
  5. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    No.
    R4 (the emitter resistor) provides negative feedback.
     
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