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biasing currents for mirrors in folded OpAmp

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Junus2012

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Hello dear friends,

I attached you two images of the same circuit folded cascode opamp, one from Jakb Baker and the other from Huihsing

in jakob he is bising the current source of the upper transistor using the half value of the tail current of the differential pair,
in Huijsing he is biasing using the same amount of the biasing current,

As a result the currents as you can see will be different at the down mirror .

which one of them do you thing should be better ?

fol_Huij.jpg

fol_Jak.jpg

Thank you in advance
 

This will impact the slewing characteristics.
Refer to Razavi's book on slewing and you will see the difference between the two choices.
 
Apart from slewing it also affects noise performance. The PMOS current sources at the output will generate more output noise for higher currents.
 
thank you for your answer, but now which one of the two choices should be better, :)
 

Better in what aspect. One is better for slewing, the other maybe for noise. You have your specs and you have to choose the circuit and biasing that is the better fit.
 
You do not have to arrange the current exactly as the book. It totally depends on your requirement.
 
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