Opamp, Mosfet, LED, Voltage Offset issue

[michaeliv]

I found an article from Linear where they tackled the issue by placing a resistor on the output, another one in the inverting input and a cap across the output and the input.

Link to article for anyone that might be interested in knowing more details ?

[uncle_bob]

I found an article from Linear where they tackled the issue by placing a resistor on the output, another one in the inverting input and a cap across the output and the input.

Tried it and it's spot on.

Schematic attached.

Thanks a lot!

Hi

If you check the Linear article against your schematic, there should be cap between the op amp output and the inverting input. Your schematic does not have that cap.

Bob

[swat3d]

Here is the article
http://www.linear.com/designtools/currentsense/19-current_control.pdf

The voltage controlled current source on page 3

Uncle_bob my latest schematic 2 posts above has the cap. I had to use a .2uF. The .1uF didn't cut it.

[uncle_bob]

Here is the article
http://www.linear.com/designtools/currentsense/19-current_control.pdf

The voltage controlled current source on page 3

Uncle_bob my latest schematic 2 posts above has the cap. I had to use a .2uF. The .1uF didn't cut it.

Hi

The schematic *has* the cap shown on it. That is true. The problem is that the cap is shown at the wrong location in the circuit. It connects between op oamp pins. The schematic you show has the cap connecting at the far end of the resistors. Simply put: It goes between pins 1 and 2 on the op amp and not across the MOSFET.

Bob

[swat3d]

Ops, you are totally right! Sorry about that. That may be why I had to increase the capacitance.

[swat3d]

I fixed the issue and actually ended up lowering the value of the opamp output instead of increasing the capacitance which didn't feel right. It works perfectly now.

Here are a few links that anyone running into the same issue might find useful.

http://www.electronicsweekly.com/blogs/engineer-in-wonderland/current-sink-stability-2015-10/
http://electronics.stackexchange.com/questions/69506/stability-problem-in-unity-gain-opamp
http://www.analog.com/library/analogdialogue/archives/38-06/capacitive_loading.pdf

Thanks.