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  1. #1
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    Hysteresis in comparator

    What will be the output waveform if the output can switch when
    Vin becomes more than lower threshold
    Vin becomes less than upper threshold

    Consider input Vin rises from 0 to Vdd?

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  2. #2
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    Re: Hysteresis in comparator

    Hi,

    Is that directly copy/pasted from a homework exercise?

    There's no discussion of hysteresis in the question, it only seems to be asking how a comparator works. Also, to be fussy, the question is meaningless and impossible to answer without an accompanying schematic to see which comparator input is on the reference and which is on input signal, and the hysteresis resistors out of curiosity. Comparators are inverters/open drain - either their output pin is tied to the positive rail via a pull-up resistor and the "output" is high, or the output pin is sinking current and then the "output" is low, whether Vin turns on the comparator or off when e.g. Vin is above upper reference voltage depends on inverting input and non-inverting input connections.

    I would hope the output waveform were a squarewave for any comparator used to detect voltages and switch at over- or under-threshold conditions. :)



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  3. #3
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    Re: Hysteresis in comparator

    As described this is the opposite of hysteresis, where
    HL output switching comes from VIN switching follows VIN>VTH.


    You describe switching -within- the deadband. So there
    is none, or less than none (in which case you might self-
    oscillate indefinitely with VTL set by many externalities).


    This is the "why?" of positive hysteresis.


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