opamp gives a signal out, even VCC+- = 0V

[hokilo]

Hi,

how is it possible that some opamps work properly that means in terms of VCC+ and VCC-, for example when VCC+ is 10V and VCC- is -10 V the possible maximum Output is +-10V. And this works for some. But for a lot of op amp types like for example the opamp LF356N, the opamp amplifies and gives a signal out even if VCC+ and VCC- is 0. How is this possible, I used Mulitisim 10 for this, is this an error in the program? And the offset inputs have also no effects on the output voltage. Does anyone else have the same problem?

Thanks
Hoki

[I.Foldvari]

In Multisim, components can have more that one SPICE model assigned to it.

My guess would be that you have selected model "Motorola/LF356" for the LF356N, which is a the 3-pin model (only inputs +/- and output) and therefore doesn't simulate VCC+- .

Try to place the LF356N with the model "National/LF356_4". This is a 5-pin model (* nodes: 3=+ 2=- 1=out 5=V+ 4=V-) that will simulate VCC+-.

There are two ways you can change the model for a component

• Select the model in the component browser dialog when placing it
• Select "Edit component in DB" from the value tab of the component's property dialog

[hokilo]

Yes thanks now the VCCs are working.

But why are the Balance Pins 1 and 5 never working, they are not connected and I cant use them.

[DanielHermyt]

Hi Hoki,

the model doesn't support these pins. This is the only reason why you can't use them. Compared to the model that is presented at http://www.national.com/pf/LF/LF356.html, it's the same behavior.

If you need the inputs, you have to modify the model itself.