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Open-loop OTA simulation and PM @ ft or the closed-loop frequency

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Usman Hai

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OTA feedback

Hi all,
For example I have op-amp specs for a 100MHz 10b pipelined ADC. I found out that ft=600MHz. However my feedback factor β=0.33 (say).
During the open-loop ac simulation, do I have to look at the PM @ ft (600MHz) or the closed-loop frequency (ft * β = 200Mhz).
This op-amp does not need to be unity-gain stable. I am confused...please help
 

JoannesPaulus

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OTA feedback

Look at the 200MHz. That's where the opamp is going to be working.
 

    Usman Hai

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Usman Hai

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Re: OTA feedback

Thanks Joannes,
One more thing. Then what about PM@ft? Can the PM be less than 0 at ft, provided we satisfy the condition of PM>70 degree at 200MHz?
 

PaloAlto

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Re: OTA feedback

Usman Hai, what do you mean with ft?

JoannesPaulus, why do you say that the opamp will be working at 200MHz?
 

JoannesPaulus

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OTA feedback

The opamp will be working in closed-loop and the beta of the loop sets where the loop is closed on the ac transfer curve. The stability is determined only by that. Please, correct me if I am wrong.
 

sutapanaki

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Re: OTA feedback

Ok, let's put it straight and clear. You need the loop gain's phase margin, not the open loop gain phase response. Especially when your feedback factor is not 1, but 0.33, as you said. So, simulate the loop gain by appropriately breaking the loop and see what is the phase margin at the place where the loop gain crosses 0dB.
 

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