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CMFB (Read 1676 times)
raja.cedt
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CMFB
Sep 11th, 2008, 11:02pm
 
hi  
I have two questions based on  stability
  1.How to decide CMFB ugb?is there any relation ship between main amplifier ugb and CMFB ugb?I think CMFB ugb should be high
2. In gain boosting techniques whats the relation between main amplifier  ugb and boosting amplifier ugb .My guess is boosting amplifier ugb has to be high because if it is fast then it won't add much phase so that we can get higher phase margin(correct me if i am wrong)
3.I have seen many cmfb architectures(resistive divider,switch cap ,differential pair circuit,parallel triode pmos)in this which one is better from stability point of view?

THNAKS
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Tlaloc
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Re: CMFB
Reply #1 - Sep 12th, 2008, 9:33am
 
1.  The CM UGBW should be comparable to the differential signal path.  Otherwise power supply noise could be amplified to the output.  Most of the time, though, you don't have a lot of choice since it can be difficult to set the CM UGBW independently of the differential UGBW.

2.  The boosting amp should be at least 3τ faster.  With a single pole model for the boosting amp, you introduce a pole-zero doublet near the boosting amp's UGBW, which can create problems with the main amp's phase margin.

3.  They all behave fairly similarly in terms of stability.  The active device architectures can greatly limit your output swing, so that turns out to be one of the major factors in deciding the architecture.
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