enquire about base bias circuit stability question

Discussion in 'Homework Help' started by longcrystal, Apr 1, 2011.

  1. longcrystal

    Thread Starter New Member

    Mar 22, 2010
    20
    0
    actually i cannot understand about.

    why the base bias circuit has poor stability in relation to β variation.




    and one more question


    collector-feedback bias circuit provides bias stability with respect to temperature fluctuations.



    i already simulation two graph ICQ versus Temperature and ICQ versus β used the multisim to do it~ but in the graph in lineary function,but i still cannot understand what is this two question theory ,i dunknow how to explain about very reasonable ~someone else got any suggestion on this two question ~mind to help me ?....thank.
     
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    Simple base bias via a resistor direct from the DC supply rail gives a fairly constant (fixed) base current. This assumes no added emitter resistance. The collector current is therefore strongly dependent on β (and VBE). The unpredictable nature of the fixed bias circuit therefore makes it relatively useless if the designer needs a consistent outcome with respect to a predictable collector current.

    Collector feedback bias offers some improvement over simple fixed bias with respect to both temperature variations in VBE and β variation effects. In this case, the tendency for the collector current to change in response to either VBE or β changes is partially compensated by the tendency of collector voltage to decrease as Ic tries to increase - or Vc tends to increase if Ic tends to decrease. This is due to the presence of the collector resistor Rc. In either case the base current is partially adjusted by the negative feedback obtained by connecting the base bias resistor from collector to base. This gives somewhat better control of Ic compared with directly biasing the base through a resistor from the supply rail.
     
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  3. longcrystal

    Thread Starter New Member

    Mar 22, 2010
    20
    0
    okay thank for"TNK" brief explain ~ ~~.
     
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