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LM339 doesn't go high

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Siddu_N

New Member
Hi everyone,

Can someone please explain why the LM339 comparator's output doesn't go high, even when the positive input is more than the negative input.

No matter what i try, the output goes only to a few picofarads. when the negative input is higher than the positive input, then the comparator works fine and the output goes to - V saturation. Can someone shed some light on this?

Thanks in advance.
 

ericgibbs

Well-Known Member
Most Helpful Member
Hi everyone,

Can someone please explain why the LM339 comparator's output doesn't go high, even when the positive input is more than the negative input.

No matter what i try, the output goes only to a few picofarads. when the negative input is higher than the positive input, then the comparator works fine and the output goes to - V saturation. Can someone shed some light on this?

Thanks in advance.
hi,
Its an open output, it requires a pullup resistor to the V+ supply, about 3.9K.:)
 

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Siddu_N

New Member
Thanks for the reply.

I tried adding the resistor but it still doesn't go high. Where exactly do I add the resistor? Between the comparator and the positive input? or between the comparator and the positive supply? I tried both but still the output stays at a few picovolts..
 

Hayato

Member
Picovolts? So you are trying to simulate the circuit?

As Eric said it is an open collector output. So you add the resistor between the supply and the output.
 

Siddu_N

New Member
Thanks a lot both of you.. Yeah i was simulating it before getting to build the actual circuit. Using it to build a programmable gain amplifier.
 

sheldonstv

New Member
post yr schematic so that we might see .....
 
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