Op Amps: Unwanted Oscillations

[Raven Luni]

Greetings,

I'm guessing this is a fairly common issue and I've looked up all the info I can on it but I still have questions.

When trying to build a basic electret preamp I have problems with oscillation. Without an oscilloscope I'd say it sounds 'jagged' like a really spiky sawtooth or similar. The solutions I've read about tend to involve putting capacitors in various places such as in the feedback - but I also notice that this dampens the sound and that is a problem since I want it to be sensitive well into the ultrasound range (I'm using TL084s which should work up to about 1MHz)

The only thing I've found that makes a difference is throwing more and more bypass capacitors at it, but we're talking 10s of milifarads for a decent result - that must represent a hell of a current draw??

Anyway - I'd be grateful if anyone could shed a bit more light on what causes these problems and how to deal with them properly

 

[BobK]

I have not had that problem. Can you post the circuit? Normally, a 1uF decoupling capacitor will do.

Bob

 

[KrisBlueNZ]

Post a schematic AND a photo of the board.

 

[Raven Luni]

Sorry for the delay (and thanks for the quick responses).

Anyway, it was some playing around on a bread board so no schematics. It was just the very basic inverting and non inverting designs that you can find anywhere. I was using 68k resistors for the + input divider, and following it with a buffer (output fed back to - input, no other components) and 10uF coupling capacitors at each point (3 in total).

What I did find was that lowering the resistor used to power the electret made a big improvement (was initially using 10K).

 

[KrisBlueNZ]

Well, I guess, if you ever try it again, and have the same problems, post a schematic, a photo of your breadboard layout, and a photo of the oscillation waveform.