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-   -   Stability of an amplifier under capacitive loading (https://www.diyaudio.com/forums/solid-state/359704-stability-amplifier-capacitive-loading.html)

PRR 6th September 2020 06:52 AM

Quote:

Originally Posted by steveu (https://www.diyaudio.com/forums/solid-state/359704-stability-amplifier-capacitive-loading-post6331147.html#post6331147)
....My guess is that the output transformer phase was wired wrong....

That would tend to oscillate in the *middle* of the audio band.

There's an infinite number of ways an amp can squeal supersonics.

chip_mk 8th September 2020 08:51 AM

I would argue it's not the inductor that guards the amp from capacitive load but the R we put in parallel. The role of the inductor is to bypass the R at low frequencies and avoid power losses on the serial resistor plus keep the dumping factor low.
For example, putting a 2-4 ohm resistor in series to output would make every amp completely immune to capacitive load. With obvious side effects.

chip_mk 8th September 2020 02:26 PM

Correction:
every amp -> any amp

knutn 8th September 2020 07:43 PM

Use common emitter or common source (a la Pass F5) output stage and the problem with capacitive loading vanishes...

Bonsai 8th September 2020 09:28 PM

Not true I’m afraid.

spladski 9th September 2020 12:13 AM

Expanding Bonsai's reply - The output stage cannot be separated from the rest of the topology for a stability result. If just one element could give control, then amp design would be easy.

knutn 9th September 2020 09:12 AM

Quote:

Originally Posted by spladski (https://www.diyaudio.com/forums/solid-state/359704-stability-amplifier-capacitive-loading-post6335111.html#post6335111)
Expanding Bonsai's reply - The output stage cannot be separated from the rest of the topology for a stability result. If just one element could give control, then amp design would be easy.

I only partly agree. If you have designed the amplifier with a proper phase/amplitude margin, the stability of the amplifier is not worsened by capacitive loading with a common emitter/source output stage, in opposite to a common collector/drain output stage.

duncan2 9th September 2020 09:24 AM

Exactly Spadski -- that's why you have compensation capacitors ( in most designs ) that all parts will work in harmony at low distortion of any sort /stability /active parts taken into account in relation to all parts of the circuit and so on.


People buy books by D.Self/JLH to help with their own designs do they not realize months of thought on designs went into them?
When you read the whole design philosophy that goes into them each section having to be smoothly connected to the next you will see its no "5 minute job " .


Tear down an audio IC or a power amp IC and read up on the long lines of maths that have gone into them to make them stable and work together .
I think shops like Radio Shack etc have made people think -- I will go and buy an input section (block ) -- a VAS section -a driver section and with my basket as I pick them off the shelves an output section , now which one will I choose - BJT/Mosfet /Vacuum Tube -- gee ! I like that red colored block it will go well with my wallpaper.

Bonsai 9th September 2020 09:53 AM

The stability margin is always affected by a capacitive load if you do not provide some way of isolating the amplifier from the capacitive Load. Very easy to see this in a sim as I explained in my earlier post.

You can of course comp the amp so that it tolerates capacitive loads without the inductor, or, so that it instead relies on A bit of the cable inductance to do the job. But, if you do this, you are giving away loop gain which is a valuable tool for reducing distortion in my view.

knutn 9th September 2020 10:18 AM

Quote:

Originally Posted by Bonsai (https://www.diyaudio.com/forums/solid-state/359704-stability-amplifier-capacitive-loading-post6335525.html#post6335525)
The stability margin is always affected by a capacitive load if you do not provide some way of isolating the amplifier from the capacitive Load. Very easy to see this in a sim as I explained in my earlier post.

You can of course comp the amp so that it tolerates capacitive loads without the inductor, or, so that it instead relies on A bit of the cable inductance to do the job. But, if you do this, you are giving away loop gain which is a valuable tool for reducing distortion in my view.

I still don't agree. My point is: With a higher capacitive loading, the loop gain drops, so the phase margin is not worsened.

This can be seen clearly if you apply a square wave to the input of the amplifier and look at the amplifier response.


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