# how to determine the bandwidth of cmfb

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#### betty1007

##### Member level 1
hi friends,
when i design a full differential amp, how to determine the bandwidth of cmfb?

#### surianova

betty1007 said:
hi friends,
when i design a full differential amp, how to determine the bandwidth of cmfb?

i think u can measure in closed loop, u put ac=1 at the reference of opamp and measure the closed loop gain at the output refer to the reference, the -3dB of the gain is the bandwidth of the cmfb.

#### betty1007

##### Member level 1
to surianova
what i want to know is how large the bandwidth of the cmfb is needed,when i design a full differential amp with cmfb.

#### surianova

betty1007 said:
to surianova
what i want to know is how large the bandwidth of the cmfb is needed,when i design a full differential amp with cmfb.

the bandwidth of your opamp should less much than the bandwidth of your differential amp. (maybe less than 1 Mhz, i am not sure). bcoz we want the ouput of cmfb opamp to be almost DC, this only can acheive with low bandwidth of cmfb opamp. If the output of the cmfb opamp nosiy, it will cause the output of the diff amp noisy as well bcoz the output of cmfb opamp is connected to the tail of diff amp. If the bandwidth of opamp is low, definitely the slew rate is low as well, so it will longer time for the system to stable when your power supply is ramping up. So, everything is very much depend on the system.

#### betty1007

##### Member level 1
surianova said:
betty1007 said:
to surianova
what i want to know is how large the bandwidth of the cmfb is needed,when i design a full differential amp with cmfb.

the bandwidth of your opamp should less much than the bandwidth of your differential amp. (maybe less than 1 Mhz, i am not sure). bcoz we want the ouput of cmfb opamp to be almost DC, this only can acheive with low bandwidth of cmfb opamp. If the output of the cmfb opamp nosiy, it will cause the output of the diff amp noisy as well bcoz the output of cmfb opamp is connected to the tail of diff amp. If the bandwidth of opamp is low, definitely the slew rate is low as well, so it will longer time for the system to stable when your power supply is ramping up. So, everything is very much depend on the system.
I think in differential amp , common mode noise introduced by output of cmfb opamp will be concel by itself. Is there any other considerations in cmfb bandwidth.

##### Member level 1
In P.R.Gray's book:
make the unity-gain frequency of the DM and CMFB loops about equal.
in pp 857

garm

### garm

points: 2

#### paley

##### Member level 4
In P.R.Gray's book:
make the unity-gain frequency of the DM and CMFB loops about equal.
in pp 857
But it's difficult

#### avt

##### Member level 5

imo there is only the specified function/damping of the cmfb as long as you have enough amplification of the amplifier in the cmfb loop. but this will get down beyoond the 3dB-point - so you will have no full damping between 3dB and ugbw - this might be quite some frequency range - for 40 dB of the c,fb amplifier for example 2 decades - in frequency change og the amp in the feedback loop might add on, doesn't it ...

so I would assume - there is not the assumed function of the cmfb beyoond 3 dB frequency ...

so this would mean the 3dB-frequency of the cmfb loop have to be in the range of the ugbw of the amplifier - or the other poster who argued that you just want to set up a dc cm-voltage is right - and the cmfb-amplifier can have a quite decent bandwidth ....

maybe I'm wrong - but let me know then why ...

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