# Common mode gain simulation for differential amplifier using balun

1. ## Common mode gain simulation for differential amplifier using balun

Dear friends,

So far one of our mentors, Panch told me about excellent source to simulate the fully differential amplifier using Balun, I am using now this instance and giving me good shorcut. One issue I have with simulating the common mode gain (ACM),

attached you see the setup according to the source explanation

Under the AC simulation we probe the VOC which represent the ACM (because the input is 1 V).

My question that generally ACM = VOD/VID where VID are two inphase signal, how this equation is achieved from the proposed setup ?

to have a detail about the setup please read section 3.1 from the below document (suggested by Pancho)

https://designers-guide.org/analysis/diff.pdf

https://designers-guide.org/analysis/diff.pdf

Thank you very much in advance

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2. ## Re: Common mode gain simulation for differential amplifier using balun

I think the second paragraph of 3.1 Gain in the a.m. PDF should read (only for the Balun nomenclature of your a.m. picture!):

To measure the common-mode gain, set the AC magnitude on Vic to 1 V and on all other sources to 0. The common-mode voltage gain is then identical to the voltage at the c terminal of B1 and The common-to-differential-mode conversion is identical to the voltage at the d terminal of B1.

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3. ## Re: Common mode gain simulation for differential amplifier using balun

Dear Erikl,

I think I did two mistakes, first in my plot, the output balun should be Bo and the the input balun is B1.

The second mistake is about the difinition of the Common mode gain in fully differential amplifier, I stated it as differential output voltage / common mode input ..... ACM=(VO1-VO)/Vic

while it should be ACM= Voc/Vic = [(Vo1+Vo2)/2]/Vic

isnt it ?

Thank you again

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4. ## Re: Common mode gain simulation for differential amplifier using balun

Originally Posted by Junus2012
... first in my plot, the output balun should be Bo and the the input balun is B1.
In order to stay consistent with Ken Kundert's Test Bench description, you should name the input balun Bi (i for input) and the output balun Bo (o for output).

Originally Posted by Junus2012
... it should be ACM= Voc/Vic = [(Vo1+Vo2)/2]/Vic
isnt it ?
Yes, I think so!

1 members found this post helpful.

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