LM339 doesn't go high

[Siddu_N]

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]

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.

 

[Siddu_N]

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]

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]

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]

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

 

[Siddu_N]

Thats not necessary anymore. I got it working now. Thanks a lot guys.