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how to design a cmfb(two stage fully differential amplifier)

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zhutiancheng

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2 stage differential amplifier

I want design a two stage fully differential amplifier which is applied to SC circuit. But I found that the circuit didn't work when i use one common cmfb circuit. I don't know how to design a cmfb to fit it. if who know some structures can solve the promble, please give me some advise. thanks!
 

noiseless

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two stage differential amplifier

if u design for two stage OTA, you need try to use two cmfb for each stage
 

sixth

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two stage cmfb

zhutiancheng said:
I want design a two stage fully differential amplifier which is applied to SC circuit. But I found that the circuit didn't work when i use one common cmfb circuit. I don't know how to design a cmfb to fit it. if who know some structures can solve the promble, please give me some advise. thanks!
Try to use continuous time CMFB firstly, if the cointinuous time CMFB is correct then replace the resistor with switched capacitor. It is to be noted that the feedback must be negative. Don't connect it as a positive feedback!
sixth
 

zuzuzu

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cmfb design

It is recomanded to apply Switch-Cap CMFB instead of C-T CMFB. Ken Martin's book should be a good reference.
 

mists

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differential amplifiers 2-stage

no, only one cmfb circuit can serve stable common voltage for two stage opamp.

noiseless said:
if u design for two stage OTA, you need try to use two cmfb for each stage
 

zhutiancheng

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two stage opamp cmfb

I know that two cmfb can solve the problem.but in that case, i must add a transistor in the second stage. i want to use one cmfb, but don't know how to turn the feekback to negative one.
 

selina_wch

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2 stages differential amplifier

well,
negative feedback means that if the OP output voltage is larger, the CMFB circuit will increase OP's current to make OP's output decrease. So, first of all, you must confirm this point.
The way of Mr./Ms sixth is right , and another way is writing an expression of controlled voltage witch is helpful for your simulation.
 

mists

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differential amplifier twostage

the attached file show you full different two satge opamp use only one cmfb, it from gray's book.

 

safwatonline

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mists

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two stage differential amplifiers

no, vb1 is just a active load bias, no need to cmfb
the paper your refer need second stage cmfb bacause it is class AB output, if only class A output as this schematic, it need not second cmfb, also you cna refer chapter 12 of gray's book.
 

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