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Design >> Analog Design >> How much capacitance as the load for this closed loop Opamp?
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Message started by Jacki on Jul 3^rd, 2012, 10:09am

Title: How much capacitance as the load for this closed loop Opamp?
Post by Jacki on Jul 3^rd, 2012, 10:09am

Hello,

I have a question about the driving ability as shown in the figure below,

Capacitor Ca is the negative feedback loop and capacitor Cx is the positive feedback loop. When I design the OTA, how many capacitors I should take as the load, is it only Ca, or it should be Ca+Cx, or it is something else? Could anybody give me some comments?

Thanks.

Title: Re: How much capacitance as the load for this closed loop Opamp?
Post by aaron_do on Jul 3^rd, 2012, 5:59pm

For common-mode, the capacitances add, but for differential mode they subtract.

Title: Re: How much capacitance as the load for this closed loop Opamp?
Post by Jacki on Jul 4^th, 2012, 1:04am

Hello aaron_do,

Thank you very much!

Jacki

Title: Re: How much capacitance as the load for this closed loop Opamp?
Post by Jacki on Jul 4^th, 2012, 4:58am

Hello,

I have another question, how about the noise analysis for Ca and Cx. Consider it operates at a sampling and hold system. At the sampling phase, both Ca and Cx are sampled, and at the hold phase, the connection for Ca and Cx are shown in the figure above.

Thanks.

Jacki

Title: Re: How much capacitance as the load for this closed loop Opamp?
Post by BackerShu on Jul 18^th, 2012, 9:23pm

An interesting question.

It seems to me that for noise analysis input should be assumed to be AC ground. In this way, it should be Ca+Cx.

But is the input of the whole stage the same as the input of the OTA?