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Subject: two mosfet added to counter slew rate

Date: 08/31/03 at 1:28 AM
Posted by: Jason
E-mail: jason_class[AT55]yahoo.com (replace [AT55] with @ to reply)
Message Posted:

Hi Russ,

From book, it is said that Q12 and Q13 is added to counter the slew rate for this folded cascode amplifier. Kindly see the folowing page

http://www.geocities.com/jason_class/ad_folded_cascode.gif


When Q1 turn on while Q2 turn off, the foloowing is true.
ID1= 2IB
ID11 =Ibias1 + ID12 =ID3
I do not know how do we get ID11=Ibias1 + ID12.
Can you help me to understand this? Can you give me an idea how to analyse the current at drain node of Q3(considering Q11 , Q12 , Q3 and Q1 current)? I am confused with the addition of Q12 with its gate has the same bias with Q3 and Q4, why this need to be added to counter slew rate and how it manage to do so?

Then ID4=ID11=Ibias1 + ID12
Why ID4 =ID11?
WHy not ID4 = Ibias1 + ID13
Can you guide me how to analyse the current at drain of Q4(considering current from Q11, Q13, Q4 ad Q2)?
Thank you, Russ.

best regards
Jason


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