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  1. #1
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    Question about the beta value in BJT

    hi all,
    I have a question about the beta value of Bipolar transistor.
    Is β a constant value ? If β changes, which factors cause it?

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    bipolar transistor beta

    Beta is a variable quantity within certain limits defined by the manufacturer's data sheet. It is affected by the currents flowing and junction temperature.



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    beta value for bjt

    beta of any transistor depends on the manufacturing material...along with the doping levels....the characteristic resistivity of the materials...the bandgap voltage....many factors....if u still want to know abt the beta dependence of transistor use any Basic Electronics book...it will be very useful..


    1 members found this post helpful.

    •   Alt26th June 2008, 12:38

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    2n3904 beta value

    in the data sheet of any bjt there are 2 β. Hfemin and Hfemax.i want to know that in our calculation which β is used.



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    beta bjt

    in any calculation of the circuits....involving beta....the hfe should be taken as the avg value...as considering the loadline and the beta variation...a safe approach is to use the mid value.....this is what i have practised..



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    bjt beta computation

    Quote Originally Posted by rizwanspirit
    in the data sheet of any bjt there are 2 β. Hfemin and Hfemax.i want to know that in our calculation which β is used.
    If you're designing a circuit with power transitor that should be work on saturated state you have to consider the beta min to sure saturation in every condition.

    Cheers
    F.F. a.k.a. Powermos



    •   Alt27th June 2008, 00:07

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    beta in transistor depend on

    Beta is not used for a saturated transistor. The collector to emitter voltage for the beta spec is much higher than it is for a saturated transistor.

    The minimum beta for a 2N3904 transistor at 1ma and with a vce of 1.0V is 70. But its max saturation voltage of 0.2V is when its base voltage is much higher than 1/70th, it is 1/10th.

    The minimum beta for a 2N3055 power transistor is 5 at 10A and when its Vce is 4.0V. But its max saturation voltage of 3.0V at 10A is when its base current is much higher at 3.3A.



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    forced beta of bjts

    @Audioguru

    I've reported a consideration not a rule, Bmin has to be used when you have to sure to drive in saturation a power BJT (e.g. 2N3055) then in calculation of the driving stage this value is obviously conservative and grant the saturation condition in every working condition.

    Only this...

    Cheers
    F.F. a.k.a. Powermos



    •   Alt27th June 2008, 01:30

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    the used of beta for bjt

    The minimum beta is only used for a linear transistor, not one that is saturated. More base current is needed when a transistor is saturated.



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    calculate saturated ic

    Yes I'm agee with you on this point, this the reason why to design such type of behaviour we usually said that beta forced of the transistor has to be less than minimum beta, this is exactly the same as you said, in fact this require more current into the base than the one that is with the minimum beta (linear and init of quasi saturation region). From this point of view, as a pratical rule, we design saturation in order to have a final forced beta which is less than the minimal value on datasheets. This was when I've started to design switching converter, and also when I've to fight in order to avoid the deep saturation with the Baker clamp way... after that with MOS is another story... :D

    Tnx for reply.

    Cheers
    Pow



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    bjt transistor beta

    collector current is one.



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    how to get beta from a bjt

    ...but there's a "Vce_sat vs Ic graph (at different temperature curves)" in the datasheet with condition hfe=20

    therefore, beta is still used in saturation region... the difference is, beta is small at saturation region compare to beta at active region (usually at 100~400)



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    a in bjt

    hFE is not used when a transistor is saturated. Each transistor is different.
    Some transistors like the 2N3904 and 2N4401 list their max saturation voltage loss when their collector current is 10 times the base current.
    Power transistors like the 2N3055 list their max saturation voltage loss when their collector current is 10A and their base current is 3.3A.

    Curves on datasheets usually show "typical". Maximum amd minimum values could be much worse.



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    beta value for transistor bjt

    ...but how could we calculate how much base current (Ib) needs to make sure that our transistor will fully saturate at a certain collector current (Ic)??
    ... In my case, I usually use the VCEsat vs Ic graph in the datasheet at a certain beta (Ic/Ib), mostly around 10-20... (example: Fig3 page7 http://www.nxp.com/acrobat_download/...C547_SER_7.pdf)...

    ...at a known collector current (Ic) and at beta (Ic/Ib)= 20 (based on the example graph), I can calculate the minimum required base current (Ib) that can surely fully saturate my transistor.. and so far, it always works for me. =)



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    what is beta of bipolar transistor

    Where can you buy "typical " transistors? Sometimes they are not made perfectly and are with minimum specs.
    Maybe a radio manufacturer ordered all the low satutration ones before your order.

    I always design circuits to work perfectly if the transistors are minimum spec. Then ALL my projects work.



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    how to obtain beta of the bjt datasheet

    yeah right... we all know about that... "there's no such thing as ideal in this world"
    ...but my point is when designing a circuit from a scratch, how could we calculate how much base current (Ib) needs to make sure that our transistor will fully saturate at a certain collector current (Ic)??
    ... then, my approach is use the VCEsat vs Ic graph in the datasheet at a certain beta (Ic/Ib)...

    note: beta in saturation is different from the beta in active region.... in saturation, beta is small (around 10~20) and in active, beta is higher (around 100~400).....



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    what is beta in bjt

    The graphs in a datasheet show a "typical" transistor. Many transistors are worse and have a much lower current gain and have a much higher saturation voltage which requires much more base current as shown in the text part of the datasheet.
    Maybe the range is wide because sometimes a semiconductor manufacturer has a poor yield that has many minimum transistors that they still want to sell.



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    veta cuanto vale en bjt

    ... and how could you calculate how much base current (Ib) needs to make sure that our transistor will fully saturate at a certain collector current (Ic)??

    "Trial and error???" I think this is not a good idea and will come up with a very poor design and takes a lot of time..



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    ib and ic relation a bjt in saturation region

    You don't need "trial and error". The datasheet lists the minimum amount of base current for a transistor to saturate with a certain maximum voltage loss.

    Use the minimum and maximum guaranteed values on the datasheet then ALL your circuits will work, not just some of them.



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    beta > 100 for bjt

    Yes of course we can use the datasheet's minimum base current to saturate a transistor, provided that it's available... but some datasheets I encounter (most of them) has no such information.....
    take for example on the datasheet link I've provided before:
    http://www.nxp.com/acrobat_download/...C547_SER_7.pdf



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