Maker Pro
Maker Pro

Op Amps: Unwanted Oscillations

Raven Luni

Oct 15, 2011
798
Joined
Oct 15, 2011
Messages
798
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

Jan 5, 2010
7,682
Joined
Jan 5, 2010
Messages
7,682
I have not had that problem. Can you post the circuit? Normally, a 1uF decoupling capacitor will do.

Bob
 

KrisBlueNZ

Sadly passed away in 2015
Nov 28, 2011
8,393
Joined
Nov 28, 2011
Messages
8,393
Post a schematic AND a photo of the board.
 

Raven Luni

Oct 15, 2011
798
Joined
Oct 15, 2011
Messages
798
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

Sadly passed away in 2015
Nov 28, 2011
8,393
Joined
Nov 28, 2011
Messages
8,393
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.
 
Top