Hi All, I'd very much appreciate some help with the following problem. The circuit I have is attached. Vin is 5 V DC from a LM7805 regulator. Supply voltage to the op-amp is ± 9 V (dual supply). The op-amp is an LM308. Rtest is 1 MΩ, Rsample is either 200 kΩ or infinite, Rsample changes with time, but it is either << Rtest or >> Rtest. When Rsample << Rtest everything is fine and the output is at or near the supply voltage, as expected. My problem occurs when Rsample >> Rtest, then the output is not a steady 5 V DC as I would like, instead it oscillates almost rail to rail with a frequency of around 910 Hz. I would love to know what causes this problem and how I can fix it so I get a nice even DC signal at the om-amp output (doesn't have to necessarily be 5 V) when Rsample >> Rtest. Thankyou very much for the help.