Doubt about opamp

[premkumar9]

Hi,
I tried an opamp (TL082) as a buffer stage (output pin connected to negative input ). When I gave +15 and -15 supply and varied the positive input from 0 to 10 V, the output was not going above 9.4V. But when I connected GND to - supply point (instead of -15V) I got 10V output for 10V input. Why does this happen?

 

[crutschow]

Don't know. It should give 10V output in either case.
What's the op amp load?
Post your circuit diagram.

 

[premkumar9]

Thank you for the reply. Circuit diagram is attached. Load is 20mA.

 

[crutschow]

You circuit is very simple and I see no reason for the anomaly you observed.
Double check all the wiring and voltages.

 

[premkumar9]

Here +15V and -12V supplies are given to the OpAmp. The source has 0-10V variation with respect to its GND. This GND is not directly connected to any of the OpAmp pins as there is no GND pin for it. Is this OK?

 

[JimB]

Just to remove any ambiguity, make sure that it is connected like this:



JimB

Afterthought:
Make sure that the supplies are well decoupled to ensure that the opamp is not oscillating.

 

[premkumar9]

Thank you for the reply. I think, oscillation is the reason. I can see oscillation in the output beyond certain input level (checked with oscilloscope). Where can I get more information on the probable reason and solution for such problems?

 

[atferrari]

Is that opamp requested to supply 20 mA to the load?

I would check the datasheet but it seems too much for that opamp. I could be wrong.

 

[JimB]

The TL082 is dual op amp, what is the other channel doing?
Is it just sitting there with floating inputs? That could be a source of problems.
Look here for how and why to deal with an un used op-amp:


Also, what about the supplies?
Are they good and "stiff", proper power supplies or new batteries, not some unknown wall wart or a half flat battery.

JimB

 

[crutschow]

Do you have decoupling capacitors directly from the op amp power pins to ground?

 

[premkumar9]

Do you have decoupling capacitors directly from the op amp power pins to ground?

Yes.

 

[premkumar9]

Is that opamp requested to supply 20 mA to the load?

I would check the datasheet but it seems too much for that opamp. I could be wrong.

Yes. In actual use we need to source 20mA. Is it a problem?

 

[premkumar9]

The TL082 is dual op amp, what is the other channel doing?
Is it just sitting there with floating inputs? That could be a source of problems.
Look here for how and why to deal with an un used op-amp:


Also, what about the supplies?
Are they good and "stiff", proper power supplies or new batteries, not some unknown wall wart or a half flat battery.

JimB

Thank you for the information.