question about heavy load amplifier compensation structure

[Leo@27]

I'm wondering if there exists a way other than the traditional Miller compensation to compensate a classAB output stage opamp.

I ask this because when driving a large capacitive load, the capacitance used for compensation also increases and is very difficult to integrate it on chip. I need the classAB to provide rail-to-rail output range, and the gain of the second stage op-amp is sufficient for my design, but the capacitive load requirement makes traditional on-chip compensation unusable.

Here's my question: what frequency compensation architecture is typically used when driving an 100s nF- or even uF-rated capacitor? Is it possible that a 3-stage op amp is more viable in this case, as opposed to a 2-stage op amp? Could anyone answer my question please?

 

[nitishn5]

Why can't you make your output (load) pole the dominant pole? I.e. do a Dominant pole compensation

 

[Leo@27]

Why can't you make your output (load) pole the dominant pole? I.e. do a Dominant pole compensation

Hi nitishn5，

thanks for your answer.

Because I also need the rail-to-rail output range, i don't think amplifier's wide output range could coexist with high dc gain.

 

[D.A.(Tony)Stewart]

Can you include current feedback?

--- Updated Jan 17, 2024 ---

2nd stage compensastion

https://vtechworks.lib.vt.edu/server/api/core/bitstreams/f75d6e84-e53f-4cff-aa25-7e52667265f7/content

 

[Leo@27]

Can you include current feedback?

--- Updated Jan 17, 2024 ---

2nd stage compensastion

https://vtechworks.lib.vt.edu/server/api/core/bitstreams/f75d6e84-e53f-4cff-aa25-7e52667265f7/content

Hi Stewart,

No I can't, When using a current buffer, the size of the compensation capacitor is proportional to the root of the load capacitance . This structure is also clearly impractical when driving 10nFs capacitors. Because you may need hundreds of pF compensation capacitor.

 

[mixed_signals]

Hi Stewart,

No I can't, When using a current buffer, the size of the compensation capacitor is proportional to the root of the load capacitance . This structure is also clearly impractical when driving 10nFs capacitors. Because you may need hundreds of pF compensation capacitor.

Hi Leo,

Did you find a compensation method to drive such large capacitive loads with good phase margin?

 

[dick_freebird]

Feedback inside, and load outside a series resistor at the
output is a common way to get high-C-load stability. How
much R and how much output current are questions.

What about external compensation? There have been
some op amps that let you add, I think, though no specific
P/Ns come to mind. Perhaps two pins could break out the
primary internal compensatin cap to let you go bare (up to
some Cload) and add more if need be?

 

[D.A.(Tony)Stewart]

Hi Stewart,

No I can't, When using a current buffer, the size of the compensation capacitor is proportional to the root of the load capacitance . This structure is also clearly impractical when driving 10nFs capacitors. Because you may need hundreds of pF compensation capacitor.

When sensing load current for error feedback, you do not need any extra capacitors . Since a capacitor load using current sensing leads the voltage response, you can add up to 90 deg phase margin.