Beta variation of Transistor

Discussion in 'General Electronics Chat' started by Jebasingh, Mar 23, 2011.

  1. Jebasingh

    Thread Starter New Member

    Mar 1, 2011
    16
    0
    Hi all,

    The variation of the beta of my transitor is from 100 to 150 depending on the temperature and my collector current should be regulated 8mA
    i am controlling the collector current by adusting the base current ( typically i am controlling the base voltage )
    How i can control the supply voltage tolerances ( base & bias ) to maintain a constant current 8mA irrespective of the changes in temperature & supply voltage tolerances

    Regards
    Jebasingh
     
  2. awright

    Well-Known Member

    Jul 5, 2006
    84
    7
    Attempting to control the collector current of a bipolar transistor by controlling the base current and relying on the beta of the transistor is a thankless task because beta is not a stable factor. It varies with current and temperature and with the specific transistor. There is no feedback to control the current. I do not understand what you mean by your statement, "( typically i am controlling the base voltage )."

    Why are you trying to regulate collector current by controlling base current, anyway? What are trying to accomplish?

    Look up the many good examples of constant current circuits.

    awright
     
  3. dha

    New Member

    Dec 25, 2008
    6
    1
    Here's how you do it. Fix the base voltage by placing a zener from base to ground. Let's say that is a 5 V zener. Place a resistor from supply voltage to that zener with a value calculated to give you about 10 ma of current through the zener and don't worry about what goes into the base of the transistor. With this set-up, about 4.3 volts will appear at junction of the emitter and emitter resistor. To get a fixed current of 8 ma through the transistor the emitter resistor must be 4.3.V divided by .008 amps or 537 ohms. It won't matter what load you hang on the collector. The current will be 8 ma even if you short the collector to the supply. You will get less than 8 ma if the collector load resistance is so high it, in itself, limits the current.
     
  4. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    One of the most common configuration for a simple BJT with negitive feedback to make the BJT beta independent looks like this...

    [​IMG]
     
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