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3rd August 2007, 12:05 #1
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compensation in folded cascode OPAMP
Hi all
Can anybody please help me in the compensation of Folded_cascode OPAMP.I am getting 48' PM.I want it to be 50 degrees

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3rd August 2007, 12:21 #2
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compensation in folded cascode OPAMP
adjust your compensation capacitor value such that to cancel the second pole with zero.

3rd August 2007, 18:54 #3
compensation in folded cascode OPAMP
Add your output cap

4th August 2007, 09:50 #4
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Re: compensation in folded cascode OPAMP
Folded cascode is actually the OTA amplifier. So, there is only one dominant pole at the output node. The load capacitor itself behave as a compensation caps in the loop. Vary your capacitor until you achieved a sufficient phase margin.

4th August 2007, 11:42 #5
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compensation in folded cascode OPAMP
increase your output cap since this is nothing but the dominant pole of your opamp, so moving it inward will decrease/increase gain of the opamp.
amarnath

4th August 2007, 12:20 #6
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Re: compensation in folded cascode OPAMP
I guess in your circuit, the bandwidth(UGF) is large, and load cap you set is small, you should decrease the input transistor's transconductance to, and
reduce the parasitic cap in your folded point to put the nondominant pole higher.

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5th August 2007, 12:28 #7
Re: compensation in folded cascode OPAMP
Something important, the number 45 might not be all that bad, it depends on your code. I once designed a circuit whose phase margin was 60, but the output from the simulator was 110 (I am not sure about the number though), check your code.

6th August 2007, 04:55 #8
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compensation in folded cascode OPAMP
second pole limit you PM

6th August 2007, 07:43 #9
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Re: compensation in folded cascode OPAMP
Yes: Folded cascode have dominat pole at the output because it is a voltage to current building block.
But: The intermediate stage in the folded cascode create a secondary pole, sometimes called parasitic pole. It is made by the diffusion diode caps found at the current summing node and the gatesource cap of the final current source, the cascode device and the transconductance of that device.
The trick: If the diffusion diodes are small in comparison to the gatesource cap of the cascode device you can make a technology match of the zero/pole compensation. Use a gateoxide cap and a series resistance of a MOS equal to the cascode device. If you setup the equaltions for the second pole and the zero you will see the match. in practice you can cancel up to 20%. The secondpole/zero match is the factor which allows you to extend the phase margin in the presence of the process corners.
Enjoy the space of creativity, design and analysis.

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6th August 2007, 14:41 #10
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Re: compensation in folded cascode OPAMP
Hello, rfsystem
I am interested in your points. Can you give me more details or materials about your trick ? As I know, the 2nd pole is around gm(cascode)/Cgs(cascode).
Usually, set the cascode devices smaller to get a good AC reponse.
Thanks

6th March 2008, 06:16 #11
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Re: compensation in folded cascode OPAMP
what can be done is putting a capacitor between the folding node and the output node. This will basically by the miller multiplication effect seperate the first two poles of the folded cascode topology.
1 members found this post helpful.

6th March 2008, 07:02 #12
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compensation in folded cascode OPAMP
1. increase output cap
2. lower low impedence nodes R & C

6th March 2008, 12:11 #13
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Re: compensation in folded cascode OPAMP
try and keep the w and l of the input MOS low.
this will reduce the cap at the folding node and this move further away from origin.
i tried this and obtained a PM of 90.

6th March 2008, 14:24 #14
Re: compensation in folded cascode OPAMP
look at the stage with the highest gain. put a capacitor in PARALLEL with it, so Miller effect will help you a bit.
Cheers

28th June 2011, 02:39 #15
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29th June 2011, 14:07 #16
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Re: compensation in folded cascode OPAMP
hi arsenic I am interested in your method of compensation I even tried it it gave me a high PM but what I can't understand is that
when I varied the value of this capacitance from 0.25p to 1p the value of the bw decreased only about 20hz and the phase margin increased to 80degrees
so what is the explanation of this??

30th June 2011, 03:13 #17
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Re: compensation in folded cascode OPAMP
This method cause miller multiplication is increased by the cascode transistor gain.

30th June 2011, 12:10 #18
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1st July 2011, 02:26 #19
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Re: compensation in folded cascode OPAMP
In fact, you can analyze it by yourself.
Please derive smallsignal transfer function for it.
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