Op amp power decoupling... again...

[Osvaldo de Banfield]

The local decoupling capacitors are for give quick current variations produced by an amplifier device (IC, tube, transistor, FET) and preventing them to interact with other active devices though a common supply impedance, not for decoupling the PS itself.

If you like to listen the power supply, then wire a transformer directly to your loudspeakers :-D

 

[jcx]

at the lower frequencies electrolytics are effective the op amp output current loops for single ended consumer home audio signals usually return through the signal ground - the electrolytics should be from each PS rail to that ground return - not rail-to rail

 

[Pafi]

There is another technical reason to use the nearest voltage value. As the capacitor volume is increased (the case of higher capacity and or voltage values), the internal parasitic inductance also increases. Then the frequency of resonance (of the self inductance and capacitance) of it decreases. If you use this cap. above this parallel resonant, the capacitor no longer behaves as a cap, it does as an inductance, increasing their reactance with frequency. Then, higher cap insulation voltage, worse high frequency performance.

Clear like water.

Please include ESR in your analysis! You will find that for electrolitics ESR is much higher than Xc and Xl at 'resonance' frequency. So actually they don't resonate with their self inductance at all. Not even polymer electrolyte caps. Ceramic caps do this with the inductance of the electrolyte cap.

 

[Osvaldo de Banfield]

ESR is in the order of tens milliohms normally. Completely negligible at this currents.

 

[Pafi]

ESR is in the order of tens milliohms normally. Completely negligible at this currents.

Current? We are talking about impedances. The value of current is irrelevant if you are talking about resonance, it is a linear network.

ESR can be low, but Xc and Xl are even lower around resonance. Lets take some good Low ESR example:
https://industrial.panasonic.com/cdbs/www-data/pdf/RDF0000/ABA0000C63.pdf
http://industrial.panasonic.com/cdbs/www-data/pdf/AAB8000/AAB8000C193.pdf

If ESR of n*10 mohm was negligible, then what do you say about Xl= n*1 mOhm?

In audio band inductance never dominates in case of local decoupling.

 

[Osvaldo de Banfield]

The negligible AC current flowing in decoupling cap of an small signal op amp, through the negligible ESR of electrolytic capacitor at power lines, give negligible (squared) AC RMS voltages.

 

[Pafi]

If it was negligible then why bother inductance? It causes even lower voltage! If voltage on ESR was negligible, then voltage on the whole part would be negligible and nothing would left for talking about.

Independently from your self-contradictional reasoning the fact is still true: ESR is much higher then Xl. You can not ignore the highest additional factor in a calculation.

Weather a voltage can be negligible or not depends on the circumstances (amplitude, waveform, frequency, amplitude of the useful signal, PSRR of OPA, performance expectations).