problem while Floating Input pin of op-amp.

hello,

I am using op-amp comparator for comparing input value to the reference voltage. While input pins of the opamp are not connected(floating) still i get output of 4v at the output pin due to that in by default mode LED on output pin turn on.
here i attached circuit. please find it and provide solution.

Here as shown in circuit while i am giving 5V supply to the opamp my led suddenly turn on even i have not connected input pins to the reference supply and input supply.

LM339 has PNP input transistors, so the inputs will definitely float "high". Place a pull-down resistor that is able to sink the comparator input current of maximal 250 nA. 1 MOhm would be a good starting point.

You can never leave inputs to op amps or comparators open. They all need a path for the input bias current, otherwise they will "float" to an undefined state.

hello crutschow,

Thank you for quick replay. yeah i konw that we can never leave inputs to the floating but while testing that board i found that while i am giving supply to op-amp and i have not started input pins supply(ref supply and input supply) then led turns on. so i am confuse that have i take this as a issue or that is abnormal test condition ??

So from your replay i conclude that that is abnormal test condition. there is no issue with this circuit. It will work properly. is it right ???

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hello FVM,

As you said that Place a pull-down resistor that is able to sink the comparator input current of maximal 250 nA.But from my circuit i think that pull up resistor (at output of opamp) make the output 4V while inputs are not given. Am I right ??

jay1691 said:

.....But from my circuit i think that pull up resistor (at output of opamp) make the output 4V while inputs are not given. Am I right ??

Click to expand...

no. The comparator has an input offset voltage that could be positive, negative or zero so the output could be high or low or oscillating.

Dear Audioguru sir,

Thank you. As per your suggestion i have checked input offset voltage of LM339ADR. it is 4mV (@ VCC = 5 V to 30 V,VO = 1.4 V) so the output rail swing to high due to positive input offset voltage.