# Understanding Window Comparators

Discussion in 'Homework Help' started by SkiBum326, Jul 15, 2014.

1. ### SkiBum326 Thread Starter Member

May 16, 2014
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Hi Everyone,

I have a question about window comparators, which are discussed on page 862 of Electronic Principles.

I attached an image of the window comparator in question. According to the book, when vin is between 3 and 4 V, the output transistor of each comparator is cut off, so the output is pulled up to +5v? I can't imagine why this would be the case. Wouldn't each comparator just go into saturation?

Thanks in advance for any help,

Austin

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2. ### Jony130 AAC Fanatic!

Feb 17, 2009
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Let as assume that Vin = 3.5V. So for the lower comparator we have:
Vp = 3.5V and Vn = 3V and since (Vp - Vn) > 0 the LM339 output transistor is cut off.
Now for the upper comparator we have:
Vp = 4V and Vn = 3.5V.
And again (Vp - Vn) > 0. And that means when the Vp > Vn, the output will go towards the Vcc line due to the pull up resistor because the output transistor of each comparator is cut off.