op amp question

[Rajinder]

Hi all,
I have a op amp circuit powered from +5V and -5V supply rails.
If the non inverting input is sitting at 1V. If I now put a -2V on the inverting input. Should the output still go negative.
Sorry this has been puzzling me.
Best regards,
Raj

 

[BobK]

When the + input is higher than the - input, the output goes up. So, with 1V on the + input and -2V on the - input, the output will be the max positive output voltage.

However, you should never have a difference in voltage like that unless you are using the opamp as a comparator. For the opamp to operate in the linear region, the + and - input must be only microvolts apart. This is normally achieved by negative feedback.

Bob

 

[Rajinder]

Thanks Bob.

 

[Harald Kapp]

unless you are using the opamp as a comparator.

Using an opamp as a comparator can be done but I do not recommend it. An opamp is not designed for large input voltage differentials and can show some interesting effects if used so. For example saturation with very long recovery time or even a change in polarity of the output signal.
Whenever possible, use a dedicated comparator in such a case for improved performance.