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Hysteresis in comparator

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deveshkm

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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?
 

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. :)
 

As described this is the opposite of hysteresis, where
HL output switching comes from VIN<VTL and LH output
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<VIN<VTH, at a frequency
set by many externalities).


This is the "why?" of positive hysteresis.
 
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