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RobG

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Bozeman, MT

Re: miller compensatone
Reply #30 - Jun 21^st, 2011, 7:00am

vivkr wrote on Jun 21^st, 2011, 12:08am:

Rob,

Indeed, the most popular variant of this scheme is to return the feedback to the cascode. However, as you point out, that causes peaking. My experience was that this peaking makes life quite painful, so much so that you do not really reap the full benefits of using this scheme. It turns out to be better to just put the extra stage although you end up burning more power and adding an extra source of noise to your amp.

Vivek

What kind of amps are you building? It usually isn't a problem for me unless I'm driving a large DC current.

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vivkr

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Re: miller compensatone
Reply #31 - Jun 21^st, 2011, 7:55am

well! I am not building any of those fancy amps anymore, but I did spend quite a while playing with all these topologies in an earlier job. There were different kinds of applications there for sensor interfaces.

I didn't use it for driving R-loads though as there were hardly any such applications. I found it quite handy as you can get away with very low compensation cap and still be stable. A very handy advantage if making high-voltage amps by the way as those caps can be rather huge and you want to save every bit of area.

Also tried it once when I had a tricky amp to stabilize but gave up on it because the noise requirements were too tight and it does give you more noise. I think I ended up using the return to the cascode in that one.

Vivek

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sushan Junior Member Offline Posts: 21	Re: miller compensatone Reply #32 - Aug 29^th, 2011, 9:51pm @Loose-electron: Can you please attach the circuit???
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harpoon Senior Member Offline Posts: 112 Hampshire	Re: miller compensatone Reply #33 - Aug 30^th, 2011, 6:03am 100pF is just too wasteful (unless you have lots of silicon space) ... try the following link for ideas :- http://cmosedu.com/cmos1/bad_design/bad_design3/bad_design_3.htm (I think it is similar to what vivek pointed out).
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loose-electron Senior Fellow Offline Best Design Tool = Capable Designers Posts: 1638 San Diego California	Re: miller compensatone Reply #34 - Aug 30^th, 2011, 3:58pm sushan wrote on Aug 29^th, 2011, 9:51pm: @Loose-electron: Can you please attach the circuit??? See attached - Q10 & Q11 buffers the output and drives a feedback capacitor, Cc. Because of the Q10 source follower there is no high frequency feed forward.
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harpoon

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Hampshire

Re: miller compensatone
Reply #35 - Aug 31^st, 2011, 1:28am

loose-electron wrote on Aug 30^th, 2011, 3:58pm:

See attached - Q10 & Q11 buffers the output and drives a feedback capacitor, Cc. Because of the Q10 source follower there is no high frequency feed forward.

This is a classic "indirect feedback" topology that is much better than the standard (and outdated) Miller compensation (but uses a tiny bit more current)

However, I just wish to point out that this is also a classic output stage that is a bit "dangerous" as the output stage current is not defined properly.

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