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.

 

This isn't about the chips. It's about the layout. Don't use a plug-in board. Stray capacitance all over the place. You do realize that there has to be a capacitance somewhere, don't you? Tear it all down. Wire it "dead bug" style. Keep the bodies of the resistors close to the input terminals, not the output terminal. Think about low impedance at the output, high impedance at the inputs. Change Rtest to 100k (or lower) if you can. Put a .1 cap right across the power pins of the LM 308. Bypass Rtest with a capacitor. Start with 220pf and go more if necessary.

 

The antique LM308 needs an external compensation capacitor to be stable.
Look at its old datasheet to see all about it.

 

Darn! Got another one wrong!

Not that my tips aren't useful, but that old compensation scheme never occured to me.

 

Thanks for all the advice, the compensation caps seem to have solved the problem. I may need to go back and add in some more bypass caps (thanks for that tip), but hopefully it works fine from now on.

Just out of interest what would you suggest to replace the (obsolete if I understand it) LM308?

 

Use the very common LM358 dual op amp--internally compensated
http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00000464.pdf