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Design >> Analog Design >> [ECE264][Q1] Regarding the parasitic pole location in the Triple-Cascode Opamp?
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Message started by NiceFeng on Apr 12^th, 2009, 12:46am

Title: [ECE264][Q1] Regarding the parasitic pole location in the Triple-Cascode Opamp?
Post by NiceFeng on Apr 12^th, 2009, 12:46am

Dear All,

I am studying the lecture notes of ECE264 from USD by Prof.Song.

In the slide talking about the Triple-Cascode Opamp (TCO), it is interesting to find the impedance of each node is pretty high, ½r_o(g_mr_o)² for the output node, and ½r_o(g_mr_o) for the node where parasitical pole locates.

I am a little bit confusing that the parasitical pole location should be around 1/(½r_o(g_mr_o) C_p). But it is indicated in the slide the pole is locating at g_m/C_p.

Could anyone explain it to me?

Thanks a lot!

Feng

Title: Re: [ECE264][Q1] Regarding the parasitic pole location in the Triple-Cascode Opamp?
Post by nobody on Apr 13^th, 2009, 2:03am

Rp|LF=½ro(gmro), Rp|HF=1/gm
Therefore, the pole is located @ gm/Cp.

Title: Re: [ECE264][Q1] Regarding the parasitic pole location in the Triple-Cascode Opamp?
Post by raja.cedt on Apr 13^th, 2009, 6:02am

hi,
i think the impedance's what you have shown in the slides are at the dc,but as soon as you cross dominant pole output load cap will start dominant and impedances at other nodes are 1/gm, so pole's are at gm/cp.This assumption holds good if cl/cp is very high, otherwise above approximation won't work ..

Thanks,
Rajasekhar.

Title: Re: [ECE264][Q1] Regarding the parasitic pole location in the Triple-Cascode Opamp?
Post by NiceFeng on Apr 18^th, 2009, 9:42am

Hi,Rajasekhar,

Thanks for your explanations.

This question reflects one of my misunderstanding points on pole/zero estimation. I always estimated that based on the DC impedance, which is not always true. Actually the second pole in this op-amp is frequency dependant.

Regards,
Feng