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Transistor working

  1. Apr 26, 2012 #1
    I'm trying to understand working of transistor( common emmiter ) and there is one line that i dont understand and i ahve found that line in many books :- " A small change in base current controls the larger collector current" . I dont understand how the base current controls the collector current.
    Can someone explain. Also if someone has a link explaining transistors it would be helpful
     
  2. jcsd
  3. Apr 26, 2012 #2
    We typically control the collector current by controlling the emitter current value. Then Ic = alpha*Ie. If we control base current Ib, we get a collector current given by Ic = beta*Ib. This type of method is called "beta-dependent", and is usually avoided. Alpha is very consistent and predictable over speciman, current value, and temperature, having values from 0.98 to 0.998. Hence if we take the value of alpha at 0.99, we are at most off by 1%.'

    Beta, OTOH, can be as low as 50 & as high as 500. It varies with temperature, current value, and speciman. The most common application where base current is the contrrolled input is saturated switch. By overdriving the base, we force the collector to saturate. If minimum beta is 50 worst case, but we force base current to be 1/10th of the collector current, the device saturates reliably.

    Devices with beta of 50, 100, 200, & 400, all saturate with near identical performance. For most applications, we design a network so that Ie is a specific value, then we know that Ic = alpha*Ie. That is how we control the bjt.

    I would recommend university textbooks on electronics and peer-reviewed publications. Under no circumstances do I advise "web surfing". The web is filled with amateur electronics hobbyists who seem to have an insatiable urge to lecture on electronics as if they were a professor. OEM web sites like On Semi, Fairchild, etc. are also a good source of info. Avoid self-proclaimed "experts". Anybody who claims that the uiversities and OEMs got it wrong should be ignored.

    I hope I've helped.

    Claude
     
  4. Apr 26, 2012 #3
    ok thanx
     
  5. Apr 26, 2012 #4

    dlgoff

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    Science Advisor
    Gold Member

    I concur about the web having lots of trash, but this site is good.

    Transistors
    Transistor Operation

    With interactive images such as this:

    tran10.gif
     
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