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raja.cedt Senior Fellow Offline Posts: 1516 Germany	Diffamp Sep 25^th, 2009, 9:41am hi, while reading differential amplifiers i found 4 gains (differential to differential, common mode to differential, common mode to common mode,differential to common mode)..i didn't understand theoretically how differential mode will be converted into common mode Thanks, Rajasekhar.
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cczhang New Member Offline Posts: 9	Re: Diffamp Reply #1 - Sep 27^th, 2009, 2:02am HI，generally speaking, differential opamp has common mode feedback circuit, and there will be mismatch between mosfets, so there will be four gains
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raja.cedt Senior Fellow Offline Posts: 1516 Germany	Re: Diffamp Reply #2 - Sep 27^th, 2009, 8:49am hi, could you please explain clearly.. Thanks Rajasekhar.
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HdrChopper

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Re: Diffamp
Reply #3 - Sep 27^th, 2009, 9:07am

Hi Rajasekhar,

It�s simple: just imagine a resistive loaded diff pair with a purely differential signal, and assume a mismatch on the load resistor.
The single ended output signals will not be perfectly centered around the nominal CM since both are different (non-symmetrical) due to the resistor�s mismatch.
Thus you generated a CM output signal by ONLY applying a differential signal.

Regards
Tosei

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Keep it simple

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raja.cedt Senior Fellow Offline Posts: 1516 Germany	Re: Diffamp Reply #4 - Sep 28^th, 2009, 9:49pm hi, thanks for your ans, i have understand your ans like the following. Due to mismatch in resistor output voltages are not perfect differential, so you can divide them into differential and common mode. Correct me if am wrong. Thanks, rajasekhar.
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cczhang

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Re: Diffamp
Reply #5 - Sep 29^th, 2009, 12:26am

raja.cedt wrote on Sep 28^th, 2009, 9:49pm:

hi,
� �thanks for your ans, i have understand your ans like the following. Due to mismatch in resistor output voltages are not perfect differential, so you can divide them into differential and common mode. Correct me if am wrong.

Thanks,
rajasekhar.

Hi,maybe this paper will help you understand the gains.

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general_discription_cmfb_theory.pdf

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raja.cedt Senior Fellow Offline Posts: 1516 Germany	Re: Diffamp Reply #6 - Sep 29^th, 2009, 1:13am hi cczhang, thank you very much for the paper. Thanks, Rajasekhar
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HdrChopper

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Re: Diffamp
Reply #7 - Sep 29^th, 2009, 7:12pm

raja.cedt wrote on Sep 28^th, 2009, 9:49pm:

hi,
� �thanks for your ans, i have understand your ans like the following. Due to mismatch in resistor output voltages are not perfect differential, so you can divide them into differential and common mode. Correct me if am wrong.

Thanks,
rajasekhar.

That's correct Rajasekhar.

Regards
Tosei

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Keep it simple

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