BJT - Ideal maximum reverse beta

Discussion in 'General Electronics Chat' started by xxxyyyba, Feb 21, 2015.

  1. xxxyyyba

    Thread Starter Member

    Aug 7, 2012
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    Hi!

    Ideal maximum reverse beta for BJT NPN 2N2222 is 0.1, according to Multisim.
    Here is my circuit:
    ttl.jpg

    Q1 and Q2 are in reverse active mode. It should be Ic=Beta_reverse*Ib, Ie=(Beta_reverse+1)*Ib.
    We can calculate Beta_reverse as Ic/Ib. We get Beta_reverse=0.2504. However, this is higher than Ideal maximum reverse beta=0.1. How to explain it? What would happen in real life if I build this circuit and exceed Ideal maximum reverse beta like this?
     
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Likely, not much. Possibly a little smoke. If you're interested, it is worth trying.
    Do you have some bigger/better idea that you are trying to test/model with this circuit?
     
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  3. WBahn

    Moderator

    Mar 31, 2012
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    Where are you getting the max reverse beta value of 0.1 from?

    Keep in mind that beta is a moving target and depends on a number of things. Also, the collector current for a given base current is dependent on not just beta, but also other things such as the Early voltage.

    What is the point of this effort? Just grins and giggles? If so, that's fine. You can learn lots of things by exploring such thoughts. Built it and see what happens. My guess is that it will be somewhat close to the simulation results, though another thing to keep in mind is that the simulation model is primarily intended to be accurate in the normal mode of operation and they may not have given a lot of effort into getting it very accurate in the reverse mode.
     
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  4. atferrari

    AAC Fanatic!

    Jan 6, 2004
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    Yes, it makes sense. Never thought of that before.
     
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  5. xxxyyyba

    Thread Starter Member

    Aug 7, 2012
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    This is TTL NAND gate. Here is where I get value for reverse beta:

    rev beta.jpg

    But I'm still confused why they call it "maximum" when it can be exceeded. It would be great if Multisim has some type of mechanism which will inform user when some component (in this case BJT) work beyond it's capabilities. In practice it would be damaged but here one can think that everything is fine :)
     
    Last edited: Feb 22, 2015
  6. GopherT

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    Nov 23, 2012
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    It depends. It depends on definitions.

    If it is "Reverse" Beta, then the actual value of maximum reverse beta may be "-0.1", which is bigger than your measured value of "-0.2504"
     
  7. WBahn

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    Mar 31, 2012
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    But you don't know that it IS being exceeded (and I don't know that it's not). The beta is just ONE of many parameters that determines the collector current. One of the hallmarks of the Gummel-Poon model was its ability to better model the variation of beta with collector current and other parameters.

    Take a look at the model and you'll see that it is quite complicated (and it is the "simple" model, after the Ebers-Moll).

    http://onlinelibrary.wiley.com/doi/10.1002/0471715832.app1/pdf

    What is the reverse Early voltage in your model?
     
  8. WBahn

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    Mar 31, 2012
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    It's still positive because the model uses a separate dependent current source from emitter to collector for reverse mode operation.
     
  9. xxxyyyba

    Thread Starter Member

    Aug 7, 2012
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    Thanks for help WBahn. Reverse Early voltage for this model is 1.92063V
     
  10. WBahn

    Moderator

    Mar 31, 2012
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    With an Early voltage that low, you are going to get a lot of additional collector current in addition to that due to the beta of the transistor.

    Essentially (and it's an oversimplification) you have a dependent current source that is controlled by the beta of the transistor. Given a constant base current, this would result in a constant collector current regardless of the collector-emitter voltage and hence would get a flat Ic vs. Vce curve. But in reality if you increase the Vce you will increase the collector current resulting in a sloped Ic vs. Vce curve. The slopes for these curves for different values of Ib tend to extrapolate back to a point on the negative Vce axis, -V_A, where V_A is called the Early Voltage. In small signal, this is modeled as a resistor, called the output resistance, in parallel with the dependent current source. The key point is the beta is only involved in one part of the collector current.
     
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