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Circuit Element Verification: BJT

  1. Jan 6, 2012 #1
    Hello all,
    Just recently my one Professor has been depicting the BJT transistor with a capacitor joining the base and emitter portion of the transistor. The transistor is of npn configuration, however, he has yet to say anything regarding the capacitor. Is this "capacitor" a characteristic of the physical transistor, or..?
  2. jcsd
  3. Jan 6, 2012 #2


    Staff: Mentor

  4. Jan 6, 2012 #3


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    There is a small capacitance in that position, but its effect is usually swamped by the forward biassed diode of the base emitter junction.

    More likely, he is preparing to tell you about Miller Effect which is an amplified capacitance effect which severely limits the frequency response of audio amplifiers.

    The base collector junction has capacitance and this is multiplied by the gain of the amplifier and it appears as a shunt capacitor across the input of the amplifier stage. Very large capacitances can be produced in this way and it is a serious effect.
  5. Jan 6, 2012 #4
    No, it was not a voltage source. There were voltage sources, but he drew the capacitor similar to how these are drawn for the MOSFET, except it was a BJT and there was one capacitor. Unless he was meaning to draw a MOSFET and accidentally drew a BJT.
  6. Jan 6, 2012 #5
    Interesting. Thanks! I couldn't seem to find anything regarding it.
  7. Jan 10, 2012 #6
    Every pn junction has depletion capacitance and diffusion capacitance (for ac). No one attaches extra cap across base emitter. It shorts the ac path to ground.
  8. Jan 10, 2012 #7
    If this is a normal electronics class, I don't understand why he worry about the Cbe!!! The on resistance is so low that usually nobody ever worry about it. It is totally different from the MOSFET you show. For MOSFET, the Cds is everything, it is the limiting factor for speed and the drive requirement. But BJT is totally different animal. I have studied enough books to confirm this. Yes, there is always a capacitor there, but so what? If your transistor is running 1mA, the on resistance is only 26Ω, takes quite a capacitor to make any different on this baby.

    The only thing that this might get important is only when you approach the [itex]f_T\;[/itex] of the transistor that the gain start to drop off because at that frequency the reactance of the Cbe get so low that Z=j26Ω and the gain roll off. But most transistor class don't talk about this. Confirm with the professor is this what he use it for, if not, there is no reason to consider this capacitor.
    Last edited: Jan 10, 2012
  9. Jan 10, 2012 #8
    My notes have confirmed 4 instances where he has has drawn the capacitor as described. This class is also a digital design class
  10. Jan 10, 2012 #9


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    Have you tried asking your prof?
  11. Jan 10, 2012 #10
    There is nothing wrong to draw the capacitor as it is really there!!! The question is did the professor tell you to worry about it or use it to do calculation? That's where both me and Vk6kro said the cap is not important even though it's there.

    If it is a digital class, that already answer my question. He is not talking about [itex]f_T\;[/itex]. So just let it be there in the model. They still use BJT for digital IC? I thought that is so yesterdecade!!!
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