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Old 16th September 2003, 08:09 PM   #1
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Default AMP without DC coubling

Hi all,

I'm wondering, why not make an amp where the different stages are seperated by a cap.

The reason I ask is that this would be an easy way controlling the Q-point in the different stages (eg. IN, VAS and OUT) and thereby look closer on effects of different working currents and voltages.

Perhaps this could also be used to try "NO GLOBAL NFB" because it's no longer needed to hold the output DC in place.

Please be gentle, thanks for your time
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Old 16th September 2003, 09:21 PM   #2
sam9 is offline sam9  United States
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Just a couple of thoughts (though negative):

1- There are enough caps in the signal path of amps already. Adding two more sounds a bit dubious.

2-
"Perhaps this could also be used to try "NO GLOBAL NFB" because it's no longer needed to hold the output DC in place."
This is not the sole function of NFB.
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Old 17th September 2003, 06:58 AM   #3
sajti is offline sajti  Hungary
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There was power amplifier with capacitor coupling. But this was 40-50 years before. Good quality capacitor is expensive, ans you need large one to coupling the output devices...

Sajti
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Old 17th September 2003, 07:04 AM   #4
PMA is offline PMA  Europe
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Try it, but keep the cap's and res. low frequency limit (-3dB) of every stage well below 1Hz - just to prevent cap's distortion and changes of phase characteristics in the audio band.
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Old 17th September 2003, 08:35 AM   #5
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Think once again why caps are necessary!

If you can't control the DC-levels between the stages you must use a cap but otherwise not.

If each stage has good DC-precision you won't need any caps.
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Old 17th September 2003, 08:46 AM   #6
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Default The reason

The reason for my question is inspired by this thread:

Unstable VAS current in amp from Slone book

I figured it would be nice to have each stage in a design totally DC-independent of the others because it offers the advantage of fredom of Q-points in each stage.

Maby it's a bad idear, ill toy around in spice and see what I can find out.

Thanks for your time

\Jens
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Old 19th September 2003, 01:26 PM   #7
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Remember that every cap shifts phase and changes resistance due to frequency. Every cap you add compounds this effect. The last amp I built used only two caps on the input to filter DC. The amp I'm building now uses ZERO caps. It will be 100% phase accurate and should be able to amplify a signal from DC upto several hundred Khz
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Old 19th September 2003, 04:38 PM   #8
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Default $$$

That would make you a rich man

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Old 19th September 2003, 10:04 PM   #9
sam9 is offline sam9  United States
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Apart from whatever benefits accrue to being phase accurate, it also makes the feedback loop easier to deal with as each shift brings you closer to 180 deg and to oscillation.
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Old 20th September 2003, 07:55 AM   #10
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Default Phaseshift

Well the idear was to make the phaseshift (cross overfreq) occur long after the openloop gain has fallen to 0 dB.

This way the caps scholud not give any problems with stabiblity

I don't have a working cuircuit yet. I'm busy on an other project also

\Jens
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