2-stage folded cascode opamp design

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sudhakarj.tech

Joined: 27 Feb 2008
Posts: 17

29 Feb 2008 10:10 folded cascode opamp

how to calculate the compensation capacitor and how to calculate current (esp. in the 2nd stage) of a 2-stage folded cascode opamp [i think there is seperate kind of approach is there for 2-stage folded cascode opamp compared to standard 2-stage opamp compensation? Especially i need to know about 2-stage folded cascode opamp design and i seen all books or papers contains single stage folded cascode opamp.
So, where can i find the design procedure for 2-stage folded cascode opamp?

PaloAlto

Joined: 08 Oct 2007
Posts: 188
Helped: 27
Location: Sevilla, Spain

01 Mar 2008 19:58 folded cascode op amp

I'm not sure I understand what you mean by 2-stage folded cascode. Is it like 1st stage->folded cascode->2nd stage or is is something else?

If you mean the 1st one is a pretty common one, I could tell you how to select the currents.

sudhakarj.tech

Joined: 27 Feb 2008
Posts: 17

03 Mar 2008 6:11 folded cascode design

hi PaloAlto,

Ya First stage is a common folded cascode stage and the second stage is the common-source amplifier (standard opamp). but i am facing problem how to calculate
compensation capacitance and currents for the second stage

PaloAlto

Joined: 08 Oct 2007
Posts: 188
Helped: 27
Location: Sevilla, Spain

03 Mar 2008 9:04 folded cascode op amp design

To decide the sizing of the output transistors you may want to look at minimum current limitations (also SR) From that current you can calculate / simulate the minimum W/L that you need.

Now, there are many ways to bias those two transistors. The one I like the most is a class-AB using translinear loops. Try to find papers from Hogervorst, he explains the biasing very well.

Briefly you need two loops, one for the PMOS and another for the NMOS. Every one of them will have e.g. the output transistor, two other NMOS in series in diode-like connection and another small NMOS. The current of the diodes squared should be equal to the product of the small NMOS current and the output one.

Well, it's much simpler if you can read it on a paper... Rolling Eyes

See if you can reach something from that guy...

sudhakarj.tech

Joined: 27 Feb 2008
Posts: 17

03 Mar 2008 10:51 folded cascode opamp design

hi PaloAlto,

Can you send me the link about biasing of "Hogervorst", so that i can learn more from it and him.......

Thanks

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PaloAlto

Joined: 08 Oct 2007
Posts: 188
Helped: 27
Location: Sevilla, Spain

03 Mar 2008 11:40 folded cascode amplifier design

OK, here's the paper:

http://www.edaboard.com/viewtopic.php?t=113509&highlight=operational+amplifier+vlsi+cell+libraries

I believe it's explained in that paper. Otherwise follow the thread of those guys, in some of their papers should be explained.

In any case, it's what I told you before, look for the translinear loop and write the current equations

sudhakarj.tech

Joined: 27 Feb 2008
Posts: 17

03 Mar 2008 12:43 design of folded cascode opamp

hi PaloAlto,

Thanks for nice contribution, when i go for searching for that paper, it is showing that it is removed from the link. I think it was not there.

If you have that paper means, would you not mind to send that paper to my email:

sudhakarj.tech(at)gmail.com

Anyhow thanks for your encouragement.

jnuhope

Joined: 10 Apr 2007
Posts: 53
Helped: 4

05 Mar 2008 6:00 folded cascode compensation

First you have to decide where/what's the requirement for your closed loop BW. (= f*gm1/Cc)
2. Decide where you want to put your 2nd pole at. (Wp2=gm2/CL) based on loop BW and phase margin.
3. With CL known, you can determine gm2 and the bias current ->W/L
4. The hardest part is to determine your gm1 and Cc assuming f is known. I've seen papers using Cc as 1.2x CL, but I guess that's also determined on your KT/C requirement.

Let me know what you think and hope this helps.

sudhakarj.tech

Joined: 27 Feb 2008
Posts: 17

05 Mar 2008 11:06 two stage folded cascode

hi jnuhope,

As per your information, its fine for the First stage design, but when i go for designing
the second stage, How can i calculate the currents in the 2nd stage. And how can i correlate Tail current with the Second stage current so that i can get the better compensation.

jnuhope

Joined: 10 Apr 2007
Posts: 53
Helped: 4

05 Mar 2008 18:19 two-stage-op-amp cascode

so you already designed your first stage and knows the closed loop BW is. The second stage current mainly servese you two things:
1. Provide gm2 to set your 2nd pole.
2. Provide charginig/discharging current. (Slew Rate concern)

Based on the above two critireas, you can claculate your 2nd stage current.

Tail current means the first stage current? If so, that is determined by your first stage gm1 which is in turn determined by your required closed BW.

Let me know if that's clear. Don't heisitate to post up your questions.