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invert digital signal: BJT or MOSFET

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by earckens, Mar 4, 2018.

  1. earckens

    earckens Member

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    Not necessarily for TTL, yet for a digital signal: how would you invert the signal: using a BJT or a MOSFET?
     
  2. Grossel

    Grossel Member

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    I find it most easy using pull-up resistors and bjt's, mosfet works as well. For TTL's be aware that many of the old circuits have open collector output and input will be regarded as high if not connected (not recommended).
     
  3. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    Need more information:

    What is the amplitude of the input signal?
    Where do you want the trip point to be? ~Half of the input amplitude?
    How much delay can you tolerate?
    Does the delay from rising edge of input to falling output have to be the same as going the other way?
    What does the output have to drive? Source current same as Sink current?
    What is the power supply voltage used on the output side?
    Are you simultaneously require level shifting from input to output?

    I could come up with about 20 more...
     
  4. dave miyares

    Dave New Member

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  5. earckens

    earckens Member

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    Answers written behind your questions.

    Go ahead ;)
     
  6. earckens

    earckens Member

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    The reason I prefer MOSFET is that it is easier to drive
     
  7. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    Harkens back to day of RTL.

    About your current sink, current source requirement? Do you mean you want a single circuit that can both source 12V/100Ω = 120ma and sink 120mA. Or do you mean that sometimes you would like to source 120mA into a grounded load (using circuit A) and other times you would like the circuit to sink 120mA from a load returned to +12V, and you would be willing to change (to circuit B) depending on the nature of the load?
     
    Last edited: Mar 4, 2018
  8. dave miyares

    Dave New Member

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  9. dknguyen

    dknguyen Well-Known Member

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    I do too because no gate resistor needed like the base resistor in a BJT.
     
  10. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    Hardly puts the trip point halfway between 0V and 12V so has crummy noise immunity. With an NPN, it starts turning on at about 1V unless you create a two resistor voltage divider. Same with an NFET except that it begins to turn on at Vth and the resistors can have higher resistance...

    If you turn it upside down and use PNP or PFET, the thresholds move to ~11V and (12-Vth) respectively.
     
  11. dknguyen

    dknguyen Well-Known Member

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    Sorry, that post seemed a little incomprehensible to me. Sort of like it was typed on a phone.
     
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  12. AnalogKid

    AnalogKid Well-Known Member

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    To invert a 12 V signal and produce another 12 V signal, you will get more consistant performance with a BJT and a zener diode than with a MOSFET, meaning a more predictable trip point with less drift due to temperature changes and better repeatability from one component to another.

    ak
     
  13. earckens

    earckens Member

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    It should not be that complicated? Isn't it possible just to list "pro" and "con" in either general terms for each approach?
     
  14. earckens

    earckens Member

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    thanks, that I do understand.
     
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  15. earckens

    earckens Member

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    And what if the trip point does not matter, as in a simple low frequency circuit (less than 200Hz)?
     
  16. dknguyen

    dknguyen Well-Known Member

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    I wonder how that compares to a MOSFET or BJT totem pole. where the N-device is on the high-side and P-device is on the low-side.

    EDIT: Wait, NVM, that doesn't invert the signal. Having the N-device on the low-side and P-device on the high-side would...but then there's some risk of shootthrough-like behaviour if the devices aren't matched properly.
     
    Last edited: Mar 4, 2018
  17. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    That's ok. English is not my native language...

    Here is a picture that has serious problems. V(out1) vs V(in) at three different values of load resistance.

    Note the skewed threshold.

    Note the output voltage when the load is 100Ω.

    Note the power dissipation in R1.

    69.png
     
  18. earckens

    earckens Member

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    The load (in my case here) is high impedance.
     
  19. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    That is not what you said here:
    source = sink, impedance about 100Ohm
     
  20. dknguyen

    dknguyen Well-Known Member

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    Ah, I see. THat clears things up.
     
  21. earckens

    earckens Member

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    My apologies! I re-read that post, the other answers are ok, sorry, impedance should be high
     
  22. ChrisP58

    ChrisP58 Well-Known Member

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    One of my favorite little utility components is the pre-biased BJT transistor, sometimes listed as a digital transistor. Here's one example. https://www.diodes.com/assets/Datasheets/ds30384.pdf

    Very nice for inverting signals and for LED current buffers.

    There are many more from multiple vendors with different resistor ratios, collector current ratings, and in both polarities.
     
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