differential transistor pair

[mahdi_shamsi]

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?

 

[shrtrnd]

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]

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]

I don't understand why you would divide the differential voltage in half. The situation as described seems to be one in which there is no common mode voltage input. Perhaps the following article on measuring CMRR will suggest a way to clarify the problem.

http://www.eetimes.com/electronics-...-suggest-methods-for-CMRR-measurement--Part-1
http://www.eetimes.com/electronics-...-suggest-methods-for-CMRR-measurement--Part-2

 

[mahdi_shamsi]

thank you.it really helped.