# Comparator query.

#### Shakespeare

Joined Sep 12, 2022
16
Why in comparators |+Vsat| and |-Vsat| are not exactly equal

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#### Papabravo

Joined Feb 24, 2006
19,603
Why in comparators |+Vsat| and |-Vsat| are not exactly equal
It depends on what kind of comparator you are talking about.
If we are talking about using a comparator with an open collector output, you would never expect that to happen.
If we are talking about using an opamp for a comparator, then you have to look at the output stage of the device to get the answer. Most operational amplifiers cannot get equally close to both of the power rails.

#### WBahn

Joined Mar 31, 2012
27,900
Why in comparators |+Vsat| and |-Vsat| are not exactly equal
This sounds like a homework question. Is it?

#### DickCappels

Joined Aug 21, 2008
9,513
Is this homework?
Is there any particular comparator you have in mind?

#### MisterBill2

Joined Jan 23, 2018
13,798
why would you expect the outputs from two different portions of a circuit to be the same?? I would not expect that, especially when the output is digital, either a "1" or a "0", often 0 volts or 5 volts.

#### Papabravo

Joined Feb 24, 2006
19,603
why would you expect the outputs from two different portions of a circuit to be the same?? I would not expect that, especially when the output is digital, either a "1" or a "0", often 0 volts or 5 volts.
As I noted in post #2, he "might" be using an opamp as a comparator. It is not usually done for reasons that become manifestly obvious in practice, but if it can be done, some curious soul will try to do it.

The extremely high open loop gain and the low frequency pole come to mind.

#### MisterBill2

Joined Jan 23, 2018
13,798
Even using an op-amp as a comparator, the saturation levels are not specified as equal, maybe not even specified at all.
Besides that, the range of a logic one and a logic zero are not the same, usually. In normal comparator use it does not matter.

#### Papabravo

Joined Feb 24, 2006
19,603
Even using an op-amp as a comparator, the saturation levels are not specified as equal, maybe not even specified at all.
Besides that, the range of a logic one and a logic zero are not the same, usually. In normal comparator use it does not matter.
I agree

#### Ian0

Joined Aug 7, 2020
6,711
Why in comparators |+Vsat| and |-Vsat| are not exactly equal
Just to make the hysteresis calculations trickier than they need to be!
But compare it with TTL logic - logic 1 output can be anywhere between 2.4V and 5V. Logic 0 is 0V to 0.8V.