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Cause and effect confusion, BJT

  1. Jul 17, 2012 #1
    Lets take a simple BJR transistor having an Rc.
    The Ic = beta*Ib, of course. But Ic=(Vcc-Vce)/Rc as well right?
    Now the confusion, does Ib cause Ic or is Ib possible without Ic (and thus no Vcc)? If we have Vcc and thus Vce than we shall have Ic thus we can have Ib=Ic/beta? What causes Vce, is it Ib or Ic. The books don't go into these other perspectives. I can see the equations and that is what is making me confused.

    I am confused, where does the process start from...

    Why does Vce vary when we put in a signal?
     
  2. jcsd
  3. Jul 17, 2012 #2
    You need to read about what region the BJT is working in. In linear circuit, the BJT work in forward reverse mode where the base collector diode is reverse biased and the base emitter diode is forward biased.

    You should not look at the collector voltage alone, more importantly, it's the emitter current that set up collector current. When you forward bias the base emitter diode and reverse bias the base collector diode, and if you assume beta is very high, the collector current is very close to the emitter current. Usually people put a resistor Re between the emitter to ground. For NPN, the emitter is about 0.7V below the base voltage you put on it when it is forward biased. you setup the emitter current by biasing the base and the Ie= (Vb-0.7)/Re. If the base collector diode is reverse biased, the Ic≈Ie.

    You work on Ie, not Ic.
     
  4. Jul 17, 2012 #3

    vk6kro

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    Yes, you can have a base current without a collector current.

    You could leave the collector wire disconnected and pass a current through the base emitter diode.

    (If fact, if you reverse the voltage, the base emitter diode of some transistors behaves like a Zener diode with a breakdown of about 8 volts.)

    Now if you apply a collector voltage of more than a volt or so, the collector current will depend on the base current. Doing this has almost no effect on the base current.

    If you vary the base current, the collector current, (which is much bigger than the base current) varies in proportion to the size of the base current.

    If there is a collector resistor, this varying collector current causes a varying voltage across the collector resistor. This voltage is the output voltage of the amplifier.
     
  5. Jul 17, 2012 #4
    Ic = β x Ib is the basic relationship between base and collector current. Rc will impose boundary condition on the maximum collector current. Ic = β x Ib will be true up to the point where Ic is less or equal Vcc/Rc. If you increase base current further collector current will not change. So, Ic = β x Ib ≤ Vcc / Rc.
     
  6. Jul 18, 2012 #5

    jim hardy

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    BJR transistor? maybe BJT(adjacent key) ?
    Rc being external to transistor i guess you meant, between collector and supply?

    Yes of course.

    I would say Ib ALLOWS Ic so long as there's a Vcc to pull the current out of the collector(or push it in whichever you prefer)
    Vce is caused by supply, Vcc. Vce is how much supply is left after the Rc takes away its share.

    Sorry i sometimes get literal. Used to hang out with English teachers.
    But i remember struggling with these concepts myself - i hope this helps.
    Not to suggest other answers were not complete or good - just i find if several approaches are offered usually one of them will help make another one click .
    That's Synergy.

    old jim
     
  7. Jul 18, 2012 #6
    I really think looking only at the few equation is too simplistic. I think it is more important to first identify the mode where the BJT is operating in first and foremost. As everyone know, if the Rc is too big, the collector can go into forward bias and all bets are off. OP has to really learn about the forward reverse bias thingy before even talk about the equations.

    Cases where collector is open, forward biased ( saturated) is totally different and non of the equation really matter.

    For basic understanding of forward reverse biaing, it is the Ie that matter, everything start with the Ie. In this rough approximation, just assume β is very high and Ic≈Ie. All the design is to control Ie, and Ic just follow.

    In real life design, we avoid controlling Ic by Ib all together. Instead we use Re ( emitter resistor) to degenerate and stabilize the drift of the transistor. This is too long to explain and I think everyone is trying to answer a specific point here. Get a good basic book like

    https://www.amazon.com/Electronic-Principles-Albert-Malvino/dp/0028028333/ref=sr_1_1?ie=UTF8&qid=1342652479&sr=8-1&keywords=malvino+electronic+principles

    This book really explain how to design BJT circuits in a very simplistic way and it work. It is easy to understand, I read it like story books and I use the concept to even design IC in my job before.
     
    Last edited: Jul 18, 2012
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