Transistor Formula

Discussion in 'General Electronics Chat' started by MusicTech, Sep 13, 2008.

  1. MusicTech

    Thread Starter Active Member

    Apr 4, 2008
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    Does anybody have a list or a good site for formulas for formulas that demonstrate ratios between the base voltage and current that travels from collector to emitter for the different transistor types like NPN, PNP bipolar, etc.?
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    It's related to voltage, but current is the standard method.

    Each transistor has a unique response. That's the problem with BJT's.

    Download some datasheets. You'll find that there is a minimum specification for hFE, which is the base current required for a collector current response; ie: current gain.

    You will see that manufacturers only guarantee minimum HFE specifications; seldom do they specify maximums or typicals. That is because BJT's all exhibit unique qualities.

    It's also a big reason why MOSFETS are becoming all the rage nowadays. They're either ON, or OFF. If the frequency is low enough, you can more or less simulate BJT response by replacing it with a MOSFET that's switched on and off very quickly, and a cap to charge/discharge.

    Basically, with a MOSFET, you know that if you can supply the gate with enough current to charge it up, it will be ON in a certain time period. Or, you can discharge it to be OFF in a certain time period.

    But for what you're asking - I don't know of a site that simple.

    You could try Fairchild's website - they have a pretty good parametric search for gain, voltage, etc. Mouser's site is really good too.

    But you need to know your parameters ahead of time.
     
  3. MusicTech

    Thread Starter Active Member

    Apr 4, 2008
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    OK, cool thanks, I will look into that. I gotta get some mosfets I don't have any handy
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    You will be far better off if you post what your requirements are.

    MOSFETs are great, but so are BJTs. Electronics is not a "one size fits all" universe.

    Clearly stated objectives will get you far better results than random stabs in the dark.

    It will get easier as you gain experience. Think of MOSFETs as on/off switches, and BJTs as linear devices; although there is certainly a great deal of crossover.

    It is far easier to set up BJTs to operate in the linear region.

    If you try to use MOSFETs in a linear region, you will find that they are very non-linear. Better to use them as switches.

    Download datasheets from the manufacturers' sites and study them. It will take quite a bit of work and study before you will really gain an understanding.

    Simulate circuits. See what they will really do. It isn't easy - but the positive results are worthwhile.
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    I spent several months studying BJTs long after I was out of school, and working as a lead tech. The book I used was Malvino's Transistors (I think that is the name, I currently have a female niece sleeping in the room where my old bookcase is so I can't look it up). Then I sat down and derived all the formula's from scratch for a TRS-80 program, that is long gone along with the computer.

    I can say I know BJT's well, but the math is convoluted, and there are a lot of variations depending on how their configured. When I get a chance to look up the book I'll post it, I highly recommend it.
     
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