Ideal 3T opamp saturation voltage always 15V.

[anynick]

I'm a newbie. Using a basic 3T Ideal Opamp as a comparator. The output voltage always seems to be +/-15V. I know I can use a 5T opamp if I want, but I want to understand why it is always +/-15V. If I edit the model to set VPos and VNeg to something else (e.g. 20V, or 12V) it doesn't make any difference to the output.

Is there some way to change the saturation voltage of a 3T ideal opamp?

[anynick]

Ignore this - it's rubbish, I was using the oscilloscope wrongly - the output essentially goes pretty much infinite in both directions, though different for + and -. Presumably some  artificial computational effect.

[_user32]

anynick,

Not a problem.  

Just to clarify for the other users: the virtual three terminal ('3T') opamp model is unbounded from a voltage rail standpoint, so you can happily have infinite gain and Vout > Vsupplies.

On a similar note and a word of caution:  For real parts (by mfg p/n), some of the older 'legacy' models are of the 3T variety (so caution even if they have 5 or more terminal pins), but most of the modern opamp models will have 5 terminals that include modeling for the positive and negative supplies.

The best way to know which model you are using is to check:

1. Double click on the placed symbol and click 'Edit model' -> check the .subckt header (if only 3 pins are noted after the model name, its a 3T opamp.

2. When browsing via the 'Select a Component' dialog, click on the 'View model' buton and check the same.

Here is an example showing why you should be cautious (in v11 for the benefit of both v11 and v12 users).  In this case note the AD380 model was done in v4 of Multisim and is of the 'IIT' variety. 

Symbol pins:8  Subckt pins:3  

- Pat N