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Design >> Analog Design >> Fully Diff Amplifier
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Message started by HeavyDesign on Nov 19^th, 2012, 1:24am

Title: Fully Diff Amplifier
Post by HeavyDesign on Nov 19^th, 2012, 1:24am

Hi Guys,
I'm working on a fully differential amplifier with his CMFB and I have some issues to simulate his AC gain. Is anybody has an idea how to simulate the gain of the amplifier and the gain of the CMFB. There is 2 loops in 1 loop and I didn't find any books/documents talking about how to do. I have the feeling that it's like a grandma recipe! no one wants to share it.
Anyway, Thanks for ur help...

Title: Re: Fully Diff Amplifier
Post by raja.cedt on Nov 19^th, 2012, 2:20am

hello,
Why two loops, are they one CMFB and one differential loop or any thing else?
If one CMFB and one differential loop you have spectre use CMDM probe. That's one option.

Otherwise for finding differential mode just apply differential ac input and dc common mode then simulate you will get only differential mode gain (i assume you have good CMRR) similarly apply only common mode ac voltage around a bias then you get common mode gain.

Thanks,
Raj.

Title: Re: Fully Diff Amplifier
Post by buddypoor on Nov 19^th, 2012, 2:59am

HeavyDesign wrote on Nov 19^th, 2012, 1:24am:

From your description I don`t know which gains you are interested in:
* AC gain of the whole amplifier?
* Loop gain of the amplifier (you speak about 2 loops)? Whic loops?
* What is "gain of the CMFB"? Gain of the CMFB loop or the feedback block only?

The best idea would be to show us the circuit diagram.

Title: Re: Fully Diff Amplifier
Post by HeavyDesign on Nov 19^th, 2012, 5:52am

From your description I don`t know which gains you are interested in:
* AC gain of the whole amplifier?
=> the AC gain of the Amplifier alone to study the stability and then the hole loop CMFB with the amplifier
* Loop gain of the amplifier (you speak about 2 loops)? Whic loops?
=>for me the amplifier is 1 loop and the CMFB (a basic amplifier) is the other loop. If we assume that A1 is the AC gain of the 1st stage (the fully diff amplifier and A2 is the AC gain of the CMFB, the total gain will be A1*A2?! So I'd like to have my A1/A2 and my A1*A2

* What is "gain of the CMFB"? Gain of the CMFB loop or the feedback block only?
=> Gain of the CMFB amplifier (for me CMFB loop an fdback block is the same? Am I wrong??)

Anyway thanks for helping

Title: Re: Fully Diff Amplifier
Post by threepwood on Mar 23^rd, 2013, 6:56am

oups