Opamp pull up resistors?

Discussion in 'Analog & Mixed-Signal Design' started by Leo Silver, Dec 7, 2017.

  1. Leo Silver

    Thread Starter Member

    Apr 27, 2016
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    0
    Dear all,

    I have tried to replicate the circuit that I found online:
    https://electronics.stackexchange.c...king-of-a-comparator-based-oscillator-circuit

    As the last person there states in real life the output doesn't oscillate whereas if you put the pull-up resistors it will oscillate.
    Could someone explain the role of these additional resistors and the power sources he used?

    Thank would help a lot.
    Thanks in advance.

    I think the answer may lie in here: https://www.allaboutcircuits.com/textbook/semiconductors/chpt-8/positive-feedback/

    So it makes sense now about the positive feedback loop however I still would like to know about the source added to the inverting input.
    If someone can come up with a simple explanation I would appreciate it though.
     
    Last edited: Dec 7, 2017
  2. DickCappels

    Moderator

    Aug 21, 2008
    4,258
    1,230
    The second circuit which is the one with a pull-up resistor on the output uses an LM339, which is not an opamp -it is a comparator with an open collector output.

    It can be used like an opamp is a pull-up is added but they don't work very well from what I have read.
     
  3. Leo Silver

    Thread Starter Member

    Apr 27, 2016
    46
    0
    Thank you very much for your fast reply. Indeed this is a quad comparator as the person stated and thanks for pointing this out.

    In my case I replaced it with a typical LM741. I am just wondering how the pull-up resistor with the power source at the output will affect the charging and discharging of the capacitor and what role it plays to the circuit.
     
  4. DickCappels

    Moderator

    Aug 21, 2008
    4,258
    1,230
    If the LM741 has the correct power supplies (typically ±12V) the resistor should not have any effect on the circuit's operation. There are times when a load resistor can help reduce crossover distortion but distortion is not really a problem in Schmitt trigger outputs :)
     
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