how to understand the slew rate of full differential folded

• Apr 6, 2007

• #1

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sissi

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how to understand the slew rate of full differential folded cascode op amp?SR=Iss/CL or SR=ID4/2CL?

 

• Apr 6, 2007

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lladnar23

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I think it is SR=ID4/CL, assuming CL is connected between Vout- and Vout+.
When the input is railed in one direction, you get the full drain current (ID4) going through either Q5 or Q6. When the input swings to the other rail, the same current switches to the other (Q6 or Q5). So it seems like the current available to charge/discharge CL is simply ID4

 

• Apr 9, 2007

• #3

eelinker

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Re: how to understand the slew rate of full differential fol

the maximum current which can charge the output cap is simply Id4 when a step is applied at input so S.R>=id4/cl

 

• Apr 9, 2007

• #4

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sissi

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hi,
when a step is applied at input ,but a part of ID4 can't change?because Q8 and Q10 operate in saturation.

 

• Apr 9, 2007

• #5

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pankajdudulwar

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SL= I/CL.where CL is load capacitance. Now using this concept u can analyze the circuit and will get the above equations. In the differential amplifier the current get switch from one branch to the other so consider this also n u will get ur answer

 

• Apr 9, 2007

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sissi

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hi,I see
when ID11>ID4, 5、6 turn off,SR=ID4/CL
when ID11<ID4,SR=Is/CL
so we usually choose ID11<=ID4

 

• Apr 9, 2007

• #7

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yschuang

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Re: how to understand the slew rate of full differential fol

So, for example.

when ID11>ID4, 5、6 turn off,SR=ID4/CL
Given Id4=0.9Iss ,Id11=Iss ,
therefore SR=0.9Iss/CL

when ID11<ID4,SR=Is/CL
Given Id4=1.2Iss ,Id11=Iss ,
therefore SR=Iss/CL

However, I do not think so.

 

• Apr 9, 2007

• #8

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sissi

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What do you think?

 

• Apr 9, 2007

• #9

hr_rezaee

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Hi
normal state:
I4=I2+I8 (1)

large and fast input:
Q2 is turn off.
then
Iout-=I4-I8
from (1) =>
Iout=I2=0.5*I11

sr-=0.5*I11/Cl

simar above
sr+=-0.5*I11/Cl

=> SR=I11/Cl

 

• Apr 10, 2007

• #10

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yschuang

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Re: how to understand the slew rate of full differential fol

I agree with hr_rezaee.

 

• Apr 10, 2007

• #11

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sissi

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hi,
if ID11>ID4, how is it going on?

 

• Apr 10, 2007

• #12

hr_rezaee

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Re: how to understand the slew rate of full differential fol

sissi said:

hi,
if ID11>ID4, how is it going on?

Click to expand...

Hi
in my equations for any state that I4=I2+I8
answer is I11/Cl
so for I11>I4, it will be I11/Cl.
regards

 

• Apr 10, 2007

• #13

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sissi

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hi, hr_rezaee
i have gotten the answer.I11>I4 should be avoided when we desing.