741 opamp - strange output

[orienteraren]

I must do something really really wrong. I set up a simple 741 CN opamp and feed it with +12V on pin 7, -12 V on pin 4, ground both inputs and measure the voltagge on pin 6 compare to ground and I get -11,4 V! If I leave both inputs (pin 2 and 3) open I get 9,8 V. Shouldn´t I get a 0 out in both cases?

I tried to balance it with a trimming pot between pin 1 and 5 and connecting the - 12 V to the slider. At 10kohm I got the output to about -2,8 V but I think I´m doing something fatally wrong here, this should be straight forward!

Ialso tried different physical compnents of the same type and change the feed to +/- 7,5 V and +/-5 V.

What is the problem?

Best Regards

Fredrik

 

[keith1200rs]

You didn't mention any negative feedback. Without negative feedback the amplifier will amplify any tiny offset by the open loop gain and the output will hit the rails. You cannot leave inputs floating either.

Try connecting the inverting input to the output.

Keith

Added after 1 hours:

I should add - what are you trying to achieve?

Keith

 

[orienteraren]

Hi Keith!

I wanted to use the opamp as a summer but it didn´t work out the way I expeced so I made a setup with just the 741 to see where the problem was. Even if I provide a negative feedback (invering input to output) via a resistor (10 kohm) I do get the same type of problem, a stable dc-value that differs from zero. The only way i get around this is to connect the output and inverting input with a piece of wire ( equals "0" resistance) and by ding that it seems like I´m bypassing the entire opamp, right?

Regards

Fredrik

 

[LvW]

Perhaps an answer from our side is a bit easier if you could show us your circuit.
On the other hand, there are many internet links to learn something about opamp basics. Didn't you try this way?