base-emitter voltage

Discussion in 'General Electronics Chat' started by i_nour, Sep 29, 2016.

  1. i_nour

    Thread Starter New Member

    Nov 9, 2015
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    is the base-emitter voltage constant? if yes,then what is the mean of controlling the collector current by this constant voltage?
     
  2. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    It is not a constant. It varies significantly with base current and moderately with temperature. The collector current is not controlled by the base voltage, it is controlled by the base current. The ratio of the collector current to the base current is the gain of the transistor, h(fe).

    ak
     
  3. #12

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    Nov 30, 2010
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  4. AlbertHall

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  5. i_nour

    Thread Starter New Member

    Nov 9, 2015
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    isn't the B-E junction look like a forward-biased diode?
     
  6. #12

    Expert

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    Yes, but that doesn't mean forward biased diodes have the same voltage for all currents.
    Top right graph on page 2
     
  7. joeyd999

    AAC Fanatic!

    Jun 6, 2011
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    Ebers-moll disagrees with you. And current mirrors would not work, otherwise.
     
  8. crutschow

    Expert

    Mar 14, 2008
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    This has been an on-going discussion whenever the operation of a BJT is discussed.
    The purists insist that it is a voltage controlled source, and the grunts claim it's current controlled.
    I resolve that dichotomy by using both views, depending upon the application.

    Ignoring the solid-state physics theory and looking at the BJT as a black box the following is observed:

    The BJT collector current is related to the base current by the current gain (hfe or β).
    The input of a BJT base-emitter junction looks like a forward biased diode so its voltage does vary with current with the same exponential relation between voltage and current as a diode.
    This gives a small change in Vbe over the normal range of AC base currents (a few tens of mV riding on the dc bias diode forward drop voltage).
    So you can argue that the collector current is related to either the base current or base voltage, depending upon your view.

    In practice, the current gain point-of-view is easier to use for large-signal design, such as determining DC bias points and when using the transistor as a switch.
    For that, the transistor basically looks like a current-controlled current-source and the current gain is used for the calculations.
    In that case the active base-emitter voltage is normally assumed to be around 0.6-0.7V.

    For AC small-signal calculations the voltage-controlled current-source (transconductance) view is often used.
    In that case it's only the small change in AC base voltage (riding on the DC bias voltage of about 0.65V) that's of interest when calculating circuit operation.
    For that the transconductance gain of the BJT is used, not the current gain.
     
    Last edited: Sep 29, 2016
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  9. DickCappels

    Moderator

    Aug 21, 2008
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    To expand a little on the voltage controlled current source model, the change in collector current as a function of change in forward base-emitter voltage can be compared to the Gm (amps/volt) of a vacuum tube.
     
  10. #12

    Expert

    Nov 30, 2010
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    Yay! Great description, crutschow.:)
    A person who can't (or won't) work with more than one model is limiting himself. It's like throwing one of your tools away to insist that one model fits all applications. You might make any valid model fit if you work hard enough, but learning to think in different ways can make the job so much easier!
    Is a transistor controlled by base current? Yes. (It's called hfe.) Is that current described by an equation for Vbe/Ib? Yes. (It's called Ebers-Moll.) Both of them work, but each of them is easier for what they are best at. Put 'em in your tool box. Both of them work.;)
     
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  11. k7elp60

    Senior Member

    Nov 4, 2008
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    Another interesting thing about the base emitter junction is the fact that the voltage across the junction will vary with temperature, as I recall it is something like 26mv/degree. So the junction can be used as a way to observe temperature.
     
  12. MrAl

    Well-Known Member

    Jun 17, 2014
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    Hi,

    Actually the change in base emitter voltage with temperature is more like -2.2mv per degree C. But i digress...


    The way i like to explain the 'control' argument is that the transistor can not be controlled by either current (ib) or voltage (Vbe) alone because current cant do anything on it's own and neither can voltage, it takes both. Therefore to resolve the argument i like to say that the transistor is controlled by ENERGY.

    For example, to get the current to change in the base emitter the voltage has to be changed and/or the temperature has to be changed, either of which requires a change in energy. And, to get the voltage to change across the base emitter the current has to change in order to change the charge. That's what leads to a third argument called "Charge controlled". However, that can be resolved on the basis of energy too for the same reasons. If we want to change the voltage we have to change the current (or temperature) and that means a change in energy.

    So the ENERGY control idea covers all three sub cases and two more:
    1 Current controlled
    2. Voltage contolled
    3. Charge controlled.
    4. Temperature controlled.
    5. Photon controlled.

    However, each of these five views has application where it is convenient to the calculations.
    For example, current control is good when we want to understand how the input current affects the output current, such as in an amplifier or constant current circuit.
    Voltage control is good when we need to understand how the input voltage is going to affect the operation such as in a voltage reference design.
    Charge control is good when we want to understand how the transistor switches on and off.
    Temperature control is good when we need to measure temperature.
    Photon control is good when we need to measure light intensity.

    All of these require some change in energy, so the energy idea covers all bases yet we often need to break that down into the five sub cases because they help understand certain things in a more distinct way.

    I think the main question here though was about how we can get a constant current out with a constant voltage in. The answer lies within the voltage controlled view. However, the input voltage is then held constant by the external circuit not by the transistor itself.
     
    Last edited: Sep 30, 2016
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