# window comparator using a single op-amp

Discussion in 'Electronic Basics' started by Lee, May 8, 2005.

1. ### LeeGuest

does anyone know a circuit of a window comparator using only a single
op-amp...i need a shematic for this...tnx

2. ### Bob MonsenGuest

Hmmm. There was a posting on sci.electronics.design recently asking for
the same thing. Go look there.

same user

4. ### Rich WebbGuest

Same homework question.

5. ### Bob MonsenGuest

In that case...

10V--------o--------.
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[220k] [100k]
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.->|--o-------(--- Inverting Input
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Vin-o | |
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'----(---|<----o-- Noninverting Input
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[100k] [220k]
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Gnd--------o--------'

The output of the comparator will go high when Vin is between 2.5V and
7.5V. The things in [] are resistors. The ->|- are diodes. It should be
viewed using a fixed space font, like courier. Otherwise, nothing lines up.

You get to set the window by changing the resistor values. The low
window limit is set by the left two resistors, whereas the high window
limit is set by the right two resistors.

As the input increases from 0, the noninverting input first follows the
input at 0.7V above it. The inverting input is fixed at 3.125V. Once the
input gets to be 2.5V, the output of the comparator will go high, since
the non-inverting input will then exceed the inverting input (which is
still fixed at 3.125V.)

As the input continues to increase, once it gets to 3.125 + 0.7V, the
inverting input will start to rise. It's still 1.4V lower than the
non-inverting input, so the output is still high.

When the input gets to 6.875-0.7V, the non-inverting input will stop
increasing. The inverting input will continue to climb as it has been
doing. Once the input gets to be 6.875+0.7V, the inverting input will be
larger than the now fixed non-inverting input, and the output will go low.

Thus, the output is high when 2.5V < Vin < 7.5V.

6. ### Bob MonsenGuest

I don't have a problem with folks asking homework questions. Electronics
is part theory, part recipies, like all engineering. Sometimes, the best
way to proceed is to ask.

7. ### Rich WebbGuest

I don't have a problem with it, either, providing that the poster
demonstrates that he has invested some effort into solving the problem
himself. The goal isn't the answer to the question, it's the method of
solving it.