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  1. L

    Assistance with Transistor Theory

    I thought that I had answered your question. Now I'm disappointed that the answer was not satisfactory. This is the first time I've seen the theory that collector current causes base current, although the Ebers-Moll model does include the concept of reverse current gain but that must be...
  2. L

    Assistance with Transistor Theory

    Now go back to #110 with the current driven base. If you vary the base current, then what happens to the collector current? There is no 'external circuit' there because the resistance of a current source is infinite, i.e., the BJT sees an open circuit at the base along with a forced value of...
  3. L

    Assistance with Transistor Theory

    Now how do you make the transistor work, i.e., do anything useful, with its base held at a constant voltage?
  4. L

    Assistance with Transistor Theory

    That is certainly true; however, in some circumstances the current-centric method is good enough.
  5. L

    Assistance with Transistor Theory

    Under what conditions do you believe there would be a corresponding Ic?
  6. L

    Assistance with Transistor Theory

    One way to view the base current is waste leakage. Another way is to consider it as the control current that generates Vbe across the PN junction. The BJT is current controlled because it just would not work to apply a voltage source to the base to bias the transistor.
  7. L

    Assistance with Transistor Theory

    It could also be said that the base current is a defect that demolishes the hopes and dreams of those who fervently believe in the BJT as a transconductance device, leaving them a mere ten millivolt slice of base-emitter voltage as their linear refuge.
  8. L

    Assistance with Transistor Theory

    That interpretation is likely referring to the definition of DC current gain: hFE=Ic/Ib. So Ib = Ic x (inverse BETA). I would not use the words 'waste current' to describe it.
  9. L

    Assistance with Transistor Theory

    Non-linear transconductance is not transconductance at all, it is just distortion. Gray, Hurst, Lewis and Meyer in their book Analysis and Design of Analog Integrated Circuits advise a maximum delta Vbe of 10mV to stay within an approximate linear region.
  10. L

    Assistance with Transistor Theory

    No one yet has written the magical words, 'small-signal analysis' where an operating curve is constrained to a small incremental range. If the increment is relatively small, then curves will approximate a straight line in that range. The BJT exhibits linear transconductance in that small...
  11. L

    Help with RS485

    A pure star would have 75 different network segments, but a star bus could be all one single network segment, or it could be a hybrid somewhere in between.
  12. L

    Help with RS485

    Have you thought about using star bus topology? More wire may be necessary with a star than a linear bus so the total capacitance would be greater, thus limiting network speed. But for RS485 drivers this is a rather slow network anyway. You may need several cable drivers at the hub to cover...
  13. L

    Assistance with Transistor Theory

    It should not be suprizing that a BJT must be properly biased in a circuit before it will exhibit its behavior as a current amplifier.
  14. L

    Assistance with Transistor Theory

    The FET works as a transconductance device. That is why the manufacturer's datasheet specifies the forward transconductance or transadmittance of the device. The BJT works as a current amplifier. That is why the manufacturer's datasheet specifies the current gain of the device. To the extent...
  15. L

    Assistance with Transistor Theory

    If one peers deeply into the crystal lattice around the PN junctions, it will be seen that processes like minority carrier injection are influenced by electric fields. But that is not sufficient to make the transistor work as a transconductance device. A transistor is just a three-terminal...
  16. L

    Thevenins and nortons circuit help

    Interesting, indeed. And done in 1947 before the advent of modern mathematical computational tools that can give the same results in just minutes of interaction with the PC. I wonder how many reams of paper the author used to derive those simplifications? The year 1947 seems like just a short...
  17. L

    Thevenins and nortons circuit help

    Post #26 dealt with the image file from post #20, which was a different problem. I must be just a one-trick pony. Node equations are something I can write generally without making a mistake. The algebra is not a problem when using a symbolic algebra engine. However, simplifications using...
  18. L

    Assistance with Transistor Theory

    This is what The Art of Electronics says in Section 2.10 that I find most illuminating: ...in (Section 2.01), where we said earlier that IC=hFEIB. We thought of the transistor as a current amplifier whose input circuit behaved like a diode. That's roughly correct, and for some applications it's...
  19. L

    Assistance with Transistor Theory

    Of what use is the quantity Vbe? We force a current to flow through the base-emitter junction and the semiconductor junction generates a voltage drop. It is a voltage that must be accounted for but Vbe is not otherwise used for any particular purpose. Vbe can be related to collector current...
  20. L

    Assistance with Transistor Theory

    I would just ask LvW to use the 'voltage model' to show why the collector current is proportional to the base current. Then I would use the 'current model' for doing circuit design.
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