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differential transistor pair

mahdi_shamsi

Nov 3, 2011
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I have problem with VCM (common mode) in differential BJT transistor pair.when we have VCM or when exactly it happens?if we have same inputs with inverse phase , do we have VCM?
can it be different in the inputs or it should be the same?
 

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shrtrnd

Jan 15, 2010
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I'm still trying to figure-out exactly what you're asking.
I'm thinking you're asking for voltage difference relating to common mode rejection ratio,
in the circuit you showed.
Theoretically, if both transistors are the same, you would have the same CMRR.
In actual practice, because components are not perfect, you would have (hopefully only)
a very small voltage difference. That small voltage difference gives you the CMRR.
Am I talking about what you're talking about, or am I barking up the wrong tree?
 

mahdi_shamsi

Nov 3, 2011
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you are exactly talking about what i am looking for.
CMRR = 20 log(Ad/Ac)
Ac is common mod gain.
can we say that the common voltage is ( Vin1+Vin2 )/2?
So differential voltage is ( Vin1-Vin2 )/2 and we want to amplify this mod?
 

Laplace

Apr 4, 2010
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