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Message started by raja.cedt on Sep 25th, 2009, 9:41am

Title: Diffamp
Post by raja.cedt on Sep 25th, 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.

Title: Re: Diffamp
Post by cczhang on Sep 27th, 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

Title: Re: Diffamp
Post by raja.cedt on Sep 27th, 2009, 8:49am
hi,
   could you please explain clearly..

Thanks
Rajasekhar.

Title: Re: Diffamp
Post by thechopper on Sep 27th, 2009, 9:07am
Hi Rajasekhar,

Itīs 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 resistorīs mismatch.
Thus you generated a CM output signal by ONLY applying a differential signal.

Regards
Tosei

Title: Re: Diffamp
Post by raja.cedt on Sep 28th, 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.

Title: Re: Diffamp
Post by cczhang on Sep 29th, 2009, 12:26am

raja.cedt wrote on Sep 28th, 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.

Title: Re: Diffamp
Post by raja.cedt on Sep 29th, 2009, 1:13am
hi cczhang,
                 thank you very much for the paper.

Thanks,
Rajasekhar

Title: Re: Diffamp
Post by thechopper on Sep 29th, 2009, 7:12pm

raja.cedt wrote on Sep 28th, 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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