simple opamp question - input offset voltage

[ssankurathri]

simple op amp

hi

i tied together the op-amp inputs and connected to ground. The output of op-amp is high.(supplied by rail voltages +5V and 0).

next, the inputs are tied together to +5V. The output is still high.

the op-amp is in comparator mode,i.e, no feedback resistor at all.

could someone please tell how the output is coming high all the time.
i read about common mode stuff, but couldn't figure out proper reason.

regards,
skr

 

[saro_k_82]

opamp question

You have to expect this., because every opamp comes with an offset voltage. This offset may or may not change sign with change in input common mode voltage. It is completely normal for the opamp to behave this way.

 

[Kral]

simple opamp

ssankurathri,
The output voltage will be the input offset voltage times the open loop gain of the op-amp. This is usually higher than the output voltage capability of the op-amp.
Regards,
ral

 

[charkyriak]

Re: simple opamp question...

there is no difference in the two topologies that u mention
both times the differential voltage at the input is 0V
now because the op-amp has inherent input offset voltage its output will always be high...if it had the opposite mismatches then its output would always be low when feeded with 0V diff voltage at input

 

[Tahmid]

simple opamp question...

Yes, the reason is the inherent input offset voltage.