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BJTs and MOSFETs

  1. Jan 21, 2008 #1
    I hate to admit it, but one of my weaknesses is electronics, specifically semiconductors. I have been trying to analyze a circuit for about 3 hours, and I have been reading my electronics books (sedra and smith, and some general EE book) and my head is spinning. There are too many theoretical equations and circumstances on the operation of transistors, but not enough information on using them and designing with them. I get the idea of saturation and cutoff as switching, and active regions as small signal amplification, that is all easy, but when it comes to making the circuits and such, I get confused very fast.

    Does anyone know of a good guide to using NPN and PNP transistors? How about MOSFETs too? I really want to master these devices, but I feel like I should have been taught more basics before being flung into semiconductor physics and circuit models of the internal workings. I passed my microelectronics courses but I can't do much with the knowledge I gained from them. Even some very basic but complete guidelines on how to work with these devices would be more than helpful. Why do I sometimes see .4V drop and sometimes .2V drop? I just need a smaller beginning step to get good at this. Does anyone have some advice?
     
  2. jcsd
  3. Jan 21, 2008 #2

    NoTime

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    At a basic level all of these devices are just rheostats (variable resistors).
    Instead of using your hand to twist the knob you have a control signal and a transfer function that says how that signal affects the resistance.

    Real world devices have annoying characteristics like PNP, NPN or whatever.
    Selection of one device over another involves picking those devices where the annoying characteristics have a minimal impact on the end result.

    Does that help?
     
  4. Jan 21, 2008 #3
    Well, I understand that but its a little more basic than what I'm looking for. Basically I want to be able to do what experienced guys do and look at a BJT circuit and say oh ya its doing so and so at about x voltage. I'd also want to be able to know some basic rules and guidelines for setting up a circuit in design. . like if I want to design a switching transistor circuit, what are the steps it takes, and what order, and what assumptions can always be made and which ones can be made only sometimes, because even in my textbook with tons of semiconductor physics equations, it often makes assumptions, but its just hard for me to keep track of them and know when they apply and when they don't.

    The examples in my books always require me to find currents and voltages dependent on these elaborate equations with junction width, length, temperature etc. but I am not even provided these parameters in the datasheets of real transistors, and I think its just an exercise in the book rather than real design practice. You just have to look at the equations the book gives and solve . . no design effort at all. Even when I do them the books doesn't tell me what the circuit does.

    I hear BJTs are current devices, but datasheets and junction voltage drops are telling me its saturation point is dependent on a voltage, and then if this is the case, why does a lower beta ensure overdrive when thats current and the datasheet is telling me its on/off ability is determined by a voltage.
     
  5. Jan 21, 2008 #4
    Also, I think I designed my original circuit complete(a PNP BJT and a P-channel FET that switch off after the voltage reaches a certain level) and I'm going to simulate it in spice, but I still am curious about good information on these devices.
     
  6. Jan 21, 2008 #5

    NoTime

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    Well, if you are designing transistors then you need elaborate equations with junction width, length, temperature etc. If you are designing circuits then you need what's put on the datasheet.

    The whole business is about learning to exclude extraneous information.
    Something like the math word problems "If a train leaves station A at 30mph ..... where will they meet". The real world answer is "Hopefully, never", because that's a train wreck.

    I don't know if I can come up with a specific answer other than experience will help.
     
  7. Jan 21, 2008 #6
    My recommendation Electronic Design 4th ed Discovery Press, maybe more along the lines what you are looking for. BTW, I agree with NoTime's sentiment!!!
     
  8. Jan 21, 2008 #7

    dlgoff

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    I got a little confidence by experimentation. Back in my day, they didn't have such things as Spice to simulate circuits. I just had to sit down with a bread-board and test for myself how to design a circuit.

    As NoTime says, "I don't know if I can come up with a specific answer other than experience will help."
     
  9. Jan 21, 2008 #8

    marcusl

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    Look at Horowitz and Hill, The Art of Electronics. It still has some equations but is very oriented towards practical applications and results.
     
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