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Can anybody help me with this opamp?

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force of nature

Newbie level 6
Oct 15, 2007
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Does anybody know what is the exact name of this opamp and how can I see the pole splitting effect caused by the capacitors C1 and C2?

I first guessed that this was a nested miller compensated opamp but now I think it is not because node A also contributes to the output.

I appreciate if anybody could help.

I think this is still a miller compensation structure, which have two part with a level shift stage (gain stage as you say).

More, Gain stage - current gain stage. But In my opinion circuit isn't correct . M3 M4 M5 must be current source, but on this circuits, currents 3,4,5 will be depended from ICMR

The circuit works as buffer that can source and sink currents. It can be designed to provide big driving capability. It has three stages. The second can be regarded as the second differential stage. The third is output stage that provide big driving capability.

this circuit has three high impedance nodes.
so, needs two compensator capacitors.
C1 controls pole of node B and C2 controls pole of node A.
I think its name must be miller, too.

Thank you for your suggestions..

For the last 24 hours I have been thinking about this opamp and I want to share what I found, and also what I could not. First, I think that I can model this opamp by the model given below. Using open circuit time constant method, I found that dominant pole after compensation arises due to C2 and given by 1/[C2.(gmn5.gmn3.rA.rB.rout)]. However, using open circuit time constant method I could not find where the pole the output moved to. this is because, open circuiting all other caps other than Cout gives me the same output pole expression as without compensation. I also tried to derive the whole transfer function (Vout/Vin), but it was too tedious to come up with poles and zeros.

Does anybody have an idea to show that indeed nondominant poles moved to higher frequencies after compensation?

Thank you all..

To talk about compensation, you must describe the feedback struture. Is it unit-gain feedback?

i think it is multistage opamp use nested miller compesation.

Yes, this opamp is used in unity-gain feedback structure.

In this circuit, MN4 is biased by the same gate voltage as MN1 and MN2. This bias current is mirrored back to MN3 via MP4 and MP5 pairs. MN1, MN2, MN3 and MN4 have the same dimension to have same gate source voltages. MN3 converts the voltage at the output of the differential stage to a current and this current is converted back to voltage at node B.

I could not understand what yxo want to tell, if you can be more clear, I will appreciate.

By the way this is a working opamp designed in AMS 0.35 technology.

I think yxo wanted to say that this circuit can't be a rail to rail amp.
I think that you forget the effect of M4 in Nested Miller.gif file.

Is it correct for your small-signal equivalent circuit?
I think OUT should be connected to Vin- for unit-gain feedback. You must employ negative feeback.

Added after 4 hours 14 minutes:

The DC gain is if gmn2=gmn1, gmp5=gmp4, gmn4=gmn1

Added after 11 minutes:

Two zeroes:

Added after 33 minutes:


I think it is nest miller compensation, but the small signal you draw is not exactly the same as your circuit.
There is another forward signal path from gain stage 1 to output node. There is a good reference for you. Please read "Analysis and Design of Analog IC" by Gray and Meyer, Fig. 9.33 equations 9.7x... This may help.

Thank you for your contributions.

I found out that this opamp is a nested miller compensated opamp with feedforward transconductance(NMCF).

The name of the paper that I found useful is "Analysis of Multistage Amplifier-Frequency Compensation" by Leung and Mok.

I want to ask leo_o2 how he found those poles and zeros, because in the paper I found, it says there are 2 nondominant complex poles in addition to the dominant pole. However it says, they used the complex pole approach to find these poles that I do not know what is complex pole approach.

leg1234, thank you for your contribution, it is really useful.

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