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Window comparator waveform

  1. Dec 23, 2016 #1
    While studying opamps I came across comparator and window comparator.
    I had trouble understanding case of Vin > 2/3 Vcc.

    As per waveform, comparator 1 is functional and o/p goes negative.
    But isn't now Vin also more than 1/3 Vcc, so shouldn't comparator 2 make o/p positive.

    Also when Vin < 1/3 Vcc, then as per waveform, comparator 2 is active and o/p goes negative,
    but now isn't comparator 1 also active as Vin is also less than 2/3 Vcc, so o/p can also go positive.

    How does the circuit know which opamp to follow?

    Attached Files:

  2. jcsd
  3. Dec 23, 2016 #2


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    It seems that comparators A1 and A2 have outputs which when LOW can sink current (to ground) but when HIGH do not source current, but merely present an open circuit (high impedance or off) at the output. In TTL logic this was known as an open-collector output.

    The practice of connecting together multiple outputs is generally not encouraged. Here, it could be termed a "wired-AND" gate, meaning their common output is HIGH only if all the comparators are driven HIGH. If even one output is LOW then the wired connection will go LOW.

    Last edited: Dec 23, 2016
  4. Dec 23, 2016 #3
    Last edited by a moderator: May 8, 2017
  5. Dec 24, 2016 #4

    jim hardy

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    Look on datasheets for terms "Open Collector" and "Wired Or" . It's not uncommon, and that's why they make "open collector" logic ICs.


    observe Output pin can only accept(sink) current.
    A 555 timer's Discharge pin is open collector. A single 555 package could do the job of your schematic in post 1.
  6. Dec 24, 2016 #5
    Thanks for the reply. I'm confused about source and sink currents. In your picture how can output Y be sink current. Collector current will flow from Y ti the BJT. It can't flow in reverse direction for npn transistor.
    i got this diagram from the internet for source and sink currents.

    Link of pictures.

    Attached Files:

  7. Dec 24, 2016 #6


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    The output Y is able to conduct current (conventional flow) into the collector of the transistor and through the transistor to ground. The transistor when turned on is a switch to ground. It cannot source current. There is no switch to Vcc. The diagrams you posted are confusing you as they talk about currents prior to the output transistor which is not relevant to this discussion. The link posted by jim hardy in post #4 does not have any way to connect to anything that is anywhere connected to an output except Y. So why concern yourself with any currents in any transistor other than at the terminal Y?
  8. Dec 24, 2016 #7

    jim hardy

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    Vocabulary question not physics question. Your link answered it, even if not succinctly.

    As Averagesupernova said,
    once you accept that current is positive charge in motion it's obvious -
    Sourced current comes OUT of a terminal, just like water from a spigot,
    and Sunk current flows INTO a terminal just like water down the drain.

    A Source delivers current,
    a Sink accepts it.

    look up 555 datasheet - the OUT pin can both source and sink, DIS pin only sinks.
    Last edited: Dec 24, 2016
  9. Dec 24, 2016 #8


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    Y can conduct current to ground—but only if the load is an external impedance connecting to +Vcc , similar to the arrangement in post #1.

    If you were to connect a load between Y and ground, you can see that current could never flow through the load: when Y is HIGH or open-circuit there would [obviously] be zero load current, and when Y is LOW there would be zero load current, so it is useless to connect an external load between Y and ground. Y cannot supply current to a load, Y can only absorb current from the load.

  10. Dec 25, 2016 #9
    Last edited by a moderator: May 8, 2017
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