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current mirror Op-amp fully differential CMFB connection

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Junus2012

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Hello

Could you please help me with this circuit. it is current mirror fully differential amplifier. I would ask your suggestion about the CMFB and how to connect it with this circuit, ist possible to connect the control from the CMFB to IBias ?



please I really nead your help and any suggestion is approciated
 

FvM

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In the present circuit CM output is only ruled my current mirror inbalance, but not influenced by Ib. You would want to add auxilary current sources to control it.

Differential amplifiers with CMFB are discussed in recent analog design text books.
 

Junus2012

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Dear FvM

Thank you for you reply

I have no problem with the CMFB circuit as you said it is available in many text books. consider please I would use the following CMFB, then how the connection will be with my OTA circuit ???

thank you for your reply
 

Junus2012

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I am sorry I didnt attach the image of the CMFB from my previous post , now please refer to it



Now I will use the mirror the current from vcms directly to be the current source (IB)of the circuit. please correct me if I am wrong. the second thing please what the meaning of the auxiliary current source ??
 

FvM

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As said, it doesn't work with the circuit from post #1. You should find out if you analyze it's operation. Ib is mirrored into both low and high side current sources, thus it doesn't change the current difference, at least for ideal behaving sources. You can change the circuit operation by e.g. adding a small positive offset currents to the high side current sources and reducing it's K slightly.
 

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Thank you very much FvM,
it is very clear now for me.
Now I have to find like a CMFB that can do this. I dont know but usually I see from the papers that the CMFB work wit hthis circuit is switched capacitor CMFB and I never seen a C.T CMFB with this topology which I need. if you have any resource about it please tell me

thank you once again
 

Junus2012

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Dear Fvm

I am sorry to disturb you again, I usually see that the CMFB for this kind of an OTA is implemented with SC CMFB not with continous time CMFB as shown with the attached image


where the control transistors of the CMFB are labelled with gate fb.

for me I am not intending to use the SC CMFB so Please do a favour for and help me to implement a CT CMFB for this circuit and again I am sorry to disturb you
Thank you very much
Regards
 

FvM

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Considering the output polarity of your CMFB amplifier, you would place N-channel "FB" current sources at the low side, in parallel to m5a and m5b, controlled by Vcmc. To allow bipolar common mode feedback, you need to add a bias current at the high side, e.g. by adjusting the transistor areas or adding static current.
 
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