"General solution" to an output stage

Discussion in 'General Electronics Chat' started by SPQR, Dec 8, 2012.

  1. SPQR

    Thread Starter Member

    Nov 4, 2011
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    Me again,

    If you have a digital logic circuit that has a single output (0-5V, 1's and 0's only), and you aren't sure there is enough current and perhaps there's a bit of noise.
    [​IMG]

    Do you have a general solution that you turn to in most cases?

    Transistor? OR gate? AND gate? Buffer? Something else?

    Thanks
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,431
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    The devil is in the details.

    How much current do you want to source and sink?
     
  3. SPQR

    Thread Starter Member

    Nov 4, 2011
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    Oh yeah!!! I forgot about that!:D

    I'm thinking of something "simple".

    It would drive another digital circuit, an LED, a MOSFET - not "high current" requirements, but rather "digital level" requirements.

    And another piece of info, when I build things, I tend to "overdo it".
    If I only need to drive a 555, I would add something that would drive a MOSFET:)
     
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    I don't have a general answer as it is not a general question. Another digital circuit may just need a piece of wire, or it may need a complex level shifter.

    I may use a MOSFET to drive a LED but I would choose one with a "logic level" gate so the logic drives it by itself, such as a BSS123.
     
  5. SPQR

    Thread Starter Member

    Nov 4, 2011
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    Hmmm...Ok, so it looks like there is no "general answer" and so each individual problem needs to be address...how would I say...individually....:)


    Thanks to all!
     
  6. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    I would say each situation needs it's own answer, and any situation may have many answers.
     
  7. davebee

    Well-Known Member

    Oct 22, 2008
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    The easiest solution might be to see if your existing circuit can do the job without needing additional parts.

    If not, sometimes you can parallel output stages for more current capability - try to find the data sheet for your exact part number and see what it says about driving loads.

    If those won't work, the next easiest thing may be to use another logic chip that has been designed for stronger output; bus driver chips often can drive quite a lot of current.

    If those failed, then you could start to look at discrete transistors.

    As for noise, once you let it in you'll have a hard time getting rid of it. Better to try to keep it out in the first place.
     
  8. SPQR

    Thread Starter Member

    Nov 4, 2011
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    THAT I am learning!:)
    And thanks for the BSS123 - I've looked at it and it seems to be an interesting MOSFET I should learn about. I'll buy a few and start experimenting.

    I think you hit the nail on the head.
    When I've played with the 555, it seems that most of the time I don't need any "help" with the output - and I've read you can arc weld with it.:D

    But I'm working with 74HC4040s and some similar ICs and the power to drive LEDs doesn't seem uniform. Thus my thoughts on additional circuitry after the IC doing the "thinking".

    So I'll play a bit with buffers, transistors, MOSFETS and, now that you mention buss drivers, I'll find some of them and see how they work.

    Perhaps I shouldn't have used the word "noise", but rather "clean-up" - something like a Schmitt trigger does. No output (0) until a certain level, then a fixed output (xV) determined by Vcc.

    Thanks again for the excellent comments!
     
    Last edited: Dec 23, 2012
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