Where does the name Diode-Connected BJT come from?

Discussion in 'Homework Help' started by Finaleblue, Sep 21, 2015.

  1. Finaleblue

    Thread Starter New Member

    Sep 22, 2014
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    In npn BJT, when the collector is shorted with the base we call it a diode-connected bjt.
    How I understand it is, when we put a voltage source in shorted collector-emitter node and ground the emitter, the bjt acts like a small resistor.
    What happens when we connect the voltage source on the emitter end and ground the collector in diode-connected bjt?
    Assuming from the name "diode" I assume there should be no current until the voltage reaches some breakdown point.

    So, long story short, can someone explain me why we call such bjt diode-connected?
    Thanks
     
  2. WBahn

    Moderator

    Mar 31, 2012
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    What does the I-V characteristic of a diode-connected BJT look like?
     
  3. Finaleblue

    Thread Starter New Member

    Sep 22, 2014
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    Shouldn't it be just linear since it acts like a resistor?
     
  4. WBahn

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    Mar 31, 2012
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    On what basis to you claim that it acts like a resistor?

    If it acts like a resistor, then if you double the current through it, you double the voltage across it. How does this reconcile with your understanding of the voltage across the base-emitter junction?
     
  5. crutschow

    Expert

    Mar 14, 2008
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    Why would you think something called a diode-connected BJT act like a resistor? :confused:

    It actually acts like a diode which conducts current in the forward direction (the direction of the base-emitter arrow) with the logarithmic current versus voltage relationship of a diode, and blocks in the reverse direction until the reverse breakdown voltage of the base-emitter junction is exceeded.
    The advantage of this connection is that it acts very close to an ideal diode at low currents, more so than a typical standard diode.
     
  6. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    This configuration actual provided a constant current source; it's prevalent inside most op-amps. It's called diode connected because there are only two terminals from the "outside" universe.

    Eric
     
  7. Finaleblue

    Thread Starter New Member

    Sep 22, 2014
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  8. WBahn

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    Mar 31, 2012
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    ????

    As I read it, the configuration he is talking about is a voltage source connected directly across a diode-connected BJT. How does that provide a constant current source?
     
  9. WBahn

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    Mar 31, 2012
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    Your confusion comes from not distinguishing between large-signal and small-signal models.

    Hint: What is the small-signal model of a regular old diode?
     
  10. Finaleblue

    Thread Starter New Member

    Sep 22, 2014
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    Thanks WBahn!
     
  11. crutschow

    Expert

    Mar 14, 2008
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    You are confusing that with a current mirror, half of which consists of a diode-connected BJT.
     
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