volt follower troblem!!!

[Faizan Jawaid]

I need to know what wrong with the following...

The when input voltage is higher than 1.3v the output is exactly equal to input but when the input drops below 1.3v the output remains unchanged...

 

[banjo]

Are you saying that when you set the pot to less than 1.3V, the output voltage remains at 1.3V regardless of how much lower you set the pot? What does the output circuit attached to the OP-AMP look like?
With the given info, I suspect that the load connected to the OP-AMP is sourcing some current that the OP-AMP must shunt to ground. However, the OP-AMP's negative power supply input pin is connected to ground. Therefore, its ability to drive low is limited. I think the output stage of the op-amp cannot pull the pin any lower given the strength of its output driver.
Look at the datasheet of the OP-AMP, it will say how close to the power rails the OP-AMP is able to drive. The solution is to either move the OP-AMP's negative supply terminal to a voltage below ground or change out the OP-AMP for a device that is capable of rail to rail operation.

 

[Faizan Jawaid]

O/P ckt is nothing but a 10K resistance grounded with O/P...And i m measuring O/P with a digital volt meter...n Yes if i set the pot lower than 1.3v the O/P remains unchanged....i.e 1.3v...can u pls explain what are rail-to-rail op amps?

Regards....

 

[XNOX_Rambo]

Read the datasheet for OP07 - especially the parameter "Output Voltage Swing"...

 

[banjo]

Rail to Rail OP-AMPs are able to drive their output pin to both the positive and negative power rail when this is required by the inputs. Most standard OP-AMPs cannot do this. They require some headroom on each side of the power rail.
When using OP-AMPs that cannot drive to the power rail, you have to choose positive and negative power supplies or rails that are above and below the maximum output you want.

 

[Faizan Jawaid]

Thanks for info Banjo....I`ve worked on MAX495 R-to-R Single Supply op-amps...results are quite satisfactory....

REGARDS