Opamp Sawtooth Problem

Hi guys, I'm a beginner in electronics and have a problem I can't solve. I have a 2 stage opamp that I use to amplify a sinusoidal input of 40Hz (1v peak to peak). The opamp amp is designed to returned a non-inverted magnified signal. I have a variable RC filter at the end to filter any high frequencies from the input source. The circuit works well however when the 5k pot is set to low resistance (say 1ohm) the output of the signal becomes a perfect sawtooth (this is when the input signal is constant DC). When the input signal is sinusoidal, the sawtooth is superimposed and looks like noise on the output. Hope this makes sense.

Now here is the strange part. When the pot is set to 20ohm the noise disappears (in other words the sawtooth vanishes from a DC input) and the sinusoidal output looks clean.

Can someone please advise how I can eliminate this problem? I want to set the pot to 1ohm so that all frequencies can pass through and then filter by adjusting the pot.

thanks

Mark.

When you set the output pot to a low resistance you are putting the large capacitor directly on the opamp output. Most opamps don't like that and will become unstable and oscillate.

Put your filter on the input, not the output.

Keith

keith1200rs said:

When you set the output pot to a low resistance you are putting the large capacitor directly on the opamp output. Most opamps don't like that and will become unstable and oscillate.

Put your filter on the input, not the output.

Keith

Click to expand...

Thanks for the reply. I tried your suggestion and I found that the capacitor was filtering high frequencies on the input stage (even when the pot was low). I went back to my original circuit and added a 15ohm resistor before the RC filter. I guess this applies a pre-load on the output. This seem to work and the oscillation went away. I calculated that the 15ohm resistor will still allow 10kHz frequencies to pass through. Any thoughts on this?

thanks

Mark.

I think you should either filter on the INPUT to the opamp or change your values so you have a sensible minimum resistance on the output such as 1k ohms. This would require a lot lower capacitance. The AD822 is "stable" with up to 350pF load - not 1uF!

Keith.

You'll find specifications of minimum series resistor to isolate capacitive loads in many OP datasheets.

In addition, the OP won't be able to drive larger output voltage magnitudes into a 1 uF capacitor. If you intend to handle larger signal frequencies than 40 Hz with non negligible level, you need to increase the filter impedance by several orders of magnitude (smaller capacitor, larger resistor). It may be reasonable to place a buffer amplifier behind the filter.