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Old 24th March 2002, 09:04 AM   #1
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Question Power supply decoupling capacitors (beginner question)

I'm trying to learn a bit more about audio eletronics and have found a good book which i'm working my way through. It is primarily based around op-amp circuits powered from 9-volt batteries. In virtually every circuit, the +9V supply rails have two decoupling capacitors: one 100uF electrolytic and one 0.1uF bipolar. I understand that the electrolytic acts as a smoothing cap and would really only be necessary in a transformer derived power supply (am I correct?). But what does the additional 0.1uF bipolar do and why is it necessary?

(If anyone is wondering, the book is "Practical Audio Amplifier Circuit Projects", written by Andrew Singmin and published by Newnes, 2000)
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Old 24th March 2002, 09:57 AM   #2
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Hello bm_mode ,

The 0.1 uF caps are non polarised film caps, chosen for low ESR.
Typical electrolytic capacitors, exhibit significant internal series inductance and series resistance, which affects high frequency charge/discharge performance, and also actual uF value at high frequencies.
The bypass film caps help to reduce this total effective ESR.
Always use modern switch mode power supply electrolytics, for series connection and bypass.
These capacitors are designed and constructed to give low internal inductance.
Higher voltage rating will also give lower ESR - for example 100uF/63v will give less ESR than 100uF/16v.
You are in New Zealand, Yeah ?
http://www.wescomponents.com/ has just about every cap and semiconductor that you could ever want.

Hope this helps,
Regards, Eric.
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Old 24th March 2002, 11:36 AM   #3
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A battery supply needs bypass caps because the chemical reaction that provides the current takes time.If you use anything faster than about 20V/S you may want to add a 0.01F with 10 ohms in series with it too.
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Old 25th March 2002, 02:18 AM   #4
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Thanks for your help Eric and djk.

Actually, I just got the WES components catalogue sent to me about two weeks ago. Haven't ordered anything yet but I will be using them - it's hard to source parts within NZ at realistic prices.

djk - when you talk about using "anything faster than 20V/uS" are you refering to fast transient inputs for the amp to amplify? What does the 10 ohm resistor do? I'm not certain if you mean for the resistor to be in series on the power supply rail between the battery and the amp or in series with the cap between the supply rail and ground?

Thanks,
joseph
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Old 25th March 2002, 07:42 AM   #5
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When I said 20V/S I mean an op-amp like an OPA2604. Really fast stuff, say a 400V/S AD842, need very careful layout. The 10 ohm resistor was to go in series with the 0.01F cap, and the combination in parallel with the +/- supplies at the load. The reason for this is the 0.01F may resonate with the inductance of the traces and the resistor helps kill the Q. Audio isn't that fast, 2V/S per peak volt of output is plenty fast. Damm fast op-amps can induce all sorts of noise from overshoot and ringing that doesn't show up otherwise.
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Old 26th March 2002, 01:29 AM   #6
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Hello bm_mode,
In the WES catalogue, go for the EXR range of caps.
They have black casing with gold lettering, and have nicely improved everything that I have put them into.

Regards, Eric.
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