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how build second stage CMFB in op with Class-ab output st

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sethtalk

sethtalk

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Added after 10 minutes:

supplement:
fig.jpg is a two-stage hybrid cascode compensated ota with class-ab output stage
which is propose by Mohammad Yavari,2003;

the figure shows that the first stage CMFB control the M13 drain current,
but, i dont know how to buid second stage CMFB ,what device should be controled?
can anyoun tell me or give me a simplified schematic ?
 

Re: how build second stage CMFB in op with Class-ab output s

like this:
 
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    sethtalk

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    dtfan

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Re: how build second stage CMFB in op with Class-ab output s

oh, forgot say, the net connect PMOS and NMOS are connect to VoutP and VoutN respectively.
 

hi,caosl
thanks for your reply, but i think you mis-understand my question,
i know the switched-capacitor CMFB architecture and how it work,
but my doubt is what device should be controled in second stage CMFB circuit,
take first stage CMFB for example,the first stage tail current is controlled
by first stage CMFB (gate node named cmfb1),but what device in second stage
should be controlled? or, add extra device? how ?
 

Hi
as I know his circuit is similar to simple two stage.
so we didn't need two CMFB.
where is connected outn and outp?
 

    sethtalk

    sethtalk

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Re: how build second stage CMFB in op with Class-ab output s

hi,hr_rezaee,
thanks for your sincerely reply,in fact,the output stage need cmfb circuit,don't forget the output node is high impedence node,you can refer to the paper
"A 1.8V Digital-Audio Sigma-Delta Modulator in 0.8u CMOS--B.P.Brandt ,1997"
,the author use similar op architecture in his delta-sigma design,in the paragraph he mentioned why the op need 2 cmfb ckt

[ <-- paper

sethtalk
 

hey, sethtalk , can you be kindly to tell me which software do you use to draw the transistors in the showed figures above^^
 

Re: how build second stage CMFB in op with Class-ab output s

sethtalk said:
hi,caosl
thanks for your reply, but i think you mis-understand my question,
i know the switched-capacitor CMFB architecture and how it work,
but my doubt is what device should be controled in second stage CMFB circuit,
take first stage CMFB for example,the first stage tail current is controlled
by first stage CMFB (gate node named cmfb1),but what device in second stage
should be controlled? or, add extra device? how ?
Click to expand...

hi, you mis-understood the schematic i drawn too. The two PMOS and NMOS are added extra devices, not the output stage transistors in your schematic.
 

Re: how build second stage CMFB in op with Class-ab output s

sethtalk said:
hi,hr_rezaee,
thanks for your sincerely reply,in fact,the output stage need cmfb circuit,don't forget the output node is high impedence node,you can refer to the paper
"A 1.8V Digital-Audio Sigma-Delta Modulator in 0.8u CMOS--B.P.Brandt ,1997"
,the author use similar op architecture in his delta-sigma design,in the paragraph he mentioned why the op need 2 cmfb ckt

[ <-- paper

sethtalk
Click to expand...

yes, I saw this paper.
If outn is connected to left N1 and outp to right N1, yes we need two CMFBs.
But I don't know where must be connected outp and outn is this schematic.
regards
 

Re: how build second stage CMFB in op with Class-ab output s

coasl,
the schematic (fig_cmfb.jpg)is your suggestion?
it seems a feasible method to control output stage CM voltage,
although the extra pmos/nmos add extra load ,
i will do some work on it
thanks!
 

Re: how build second stage CMFB in op with Class-ab output s

sethtalk said:
coasl,
the schematic (fig_cmfb.jpg)is your suggestion?
it seems a feasible method to control output stage CM voltage,
although the extra pmos/nmos add extra load ,
i will do some work on it
thanks!
Click to expand...

yes, it's just about what i mean.

Added after 3 minutes:

By the way, the first stage's cmfb could be implemented by another simple method, it is just needed to add two transistors. For more details, you can refer to this paper:
"Very low-voltage digital-audio ΔΣ modulator with 88-dB dynamic range using local switch bootstrapping", IEEE Journal of Solid-State Circuits, vol. 36, pp. 349 - 355, March 2001

Goodluck!
--san
 

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    sethtalk

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