# folded cascode opamp slew rate

1. ## folded cascode opamp slew rate

hi

can someone explain to me why slew rate (rising and falling edge)for folded cascode opamp not equal???

thanks

2. ## Re: folded cascode opamp slew rate

This is because of different current source and sink capabilities.

Let's assume NMOS input pair
1)Current sink for differential pair is Iss
2)Current source for both differential pair and cascode branch is I1
3)Current sink for cascode branch is I2

Normally we set I1=I2+0.5*Iss

If all Iss flows through one input NMOS, another one will be turned off
then SR-=I2-max{(I1-Iss), 0}
and SR+=I1-I2

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3. ## Re: folded cascode opamp slew rate

hi bageduke

thanks a lot for your reply...bye the way do u have any notes that i can refer to...

thanks

4. ## Re: folded cascode opamp slew rate

what happend if I1=2*I2?
would SR+=SR- ?

Added after 7 minutes:

And how to caculate SR?
some one suggest
1. 20%~80% rising/falling time
2. 10%~90% rising/falling time
3. Peak slewing.

which one is better?

5. ## folded cascode opamp slew rate

you can find the result in the book of razavi

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6. ## Re: folded cascode opamp slew rate

Originally Posted by a_zai
hi bageduke

thanks a lot for your reply...bye the way do u have any notes that i can refer to...

thanks

Razavi's book is really a help. And you can also refer to Phil Allen's book

Added after 15 minutes:

Originally Posted by st01liyp
what happend if I1=2*I2?
would SR+=SR- ?
Yes. At least this is what I am thinking. But it will consume more power for almost same performance.

And how to caculate SR?
some one suggest
1. 20%~80% rising/falling time
2. 10%~90% rising/falling time
3. Peak slewing.

which one is better?
I prefer the third one. But when I did measurement, I used the first one.

7. ## Re: folded cascode opamp slew rate

hi

if folded cascode 2 stage...can get gain 500 dB???

8. ## Re: folded cascode opamp slew rate

500dB?

Let's see, 500dB=10^25, I don't think anything can generate such a big gain.

If you mean 500=54dB, this could be okay

9. ## folded cascode opamp slew rate

I think slewing phenomena is much more complex than that describes in razavi. It may caused by first stage and second stage later, and miller effect will slow down the slewing in second stage.

10. ## Re: folded cascode opamp slew rate

500dB is terrible

Originally Posted by a_zai
hi

if folded cascode 2 stage...can get gain 500 dB???

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11. ## Re: folded cascode opamp slew rate

Problem with with small output stage current (I0) is when one part of the output stage has no current during the slewing, and you need more time for recovery from off state. This problem could be solved setting slightly higher (1.5*I0) current of the output stage but then dc gain drops (because of ro~1/I).

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