Simple CMFB using two transistors only

[Junus2012]

Dear friends,

I see some people are using the CMFB presented in the circuit below (transistor M3 and M4 with the VCM connection), it looks very simple but I don't know why it is not very known like other standard circuit.
Can you explain me please the work princible of it ?

thanks

 

[frankrose]

It seems extremely mismatch sensitive. The size or threshold difference generated error of the added devices is amplified by the folded cascode stage, I think it is not a practically viable solution.

 

[Junus2012]

It seems extremely mismatch sensitive. The size or threshold difference generated error of the added devices is amplified by the folded cascode stage, I think it is not a practically viable solution.

Me also I think is not practical solution otherwise people would follow it as it looks very simple,,, but still some of them are following like in book of operational ampliifer theory and design by Johan Huijsing

 

[Junus2012]

by the way I found the explanation from the boook as shown below with the circuit,

He put a condition to use this circuit if the amplifier has DC positive CM feedback, this status I didn't understand, how can I know it from my amplifier ?