Parasitic Oscillations: Explanations & Circuit Models

[apra143]

Was hoping someone could sum up what parasitic oscillations are (perhaps in terms of some simple circuit equations/models). Currently studying basic Op Amp circuits and during the lab the instructors mentioned this term and everyone seemed to magically know what it refers to

Also, in the labs we are looking at feedback with closed looped Op Amp circuits (where Gain = Rf / Ri), just having troubles with the terms, "closed loop gain" and "open loop gain".

Cheers for any explanations.

 

[berkeman]

Welcome to the PF, apra143.

Open loop gain refers to the high gain versus frequency of the opamp when you do not close any feedback around the amp. So ground the - input and drive a signal into the + input, and watch the output. Trouble is, the open loop gain at normal (audio) frequencies of most opamps is so high (check a typical datasheet), that the output will just bang between the power supply rails. With a gain of 10^5 or whatever, you will not be able to put in a small enough input signal to be able to get a 1Vpp output signal or anything reasonable that you can measure.

Closed loop gain is just the Vo/Vi gain that you measure when you close the feedback loop with some sort of components between the output and the - input of the opamp.

Parasitic oscillations can happen if the opamp feedback is not designed correctly for the gain-phase characteristics of the opamp (and can be aggrevated by poor layout on the PCB or prototype board). Have you learned about phase margin and gain margin yet? That's the key to understanding parasitic oscillations.

 

[apra143]

Have you learned about phase margin and gain margin yet? That's the key to understanding parasitic oscillations.

Thanks for the reply.

Haven't gone over phase and gain margins so probably why. It just seems odd when words are mentioned at early stages without a hard definition. When it comes to electronics subjects, this situation seems to be the norm. Not so with others subjects (e.g. computing, physics, chemistry, math) I find.

Thank goodness for Sedra & Smith, whatever detailed information it can provide; but again, there's always a lot of:
"We'll deal with this concept, in detail, in a later chapter."