common mode signal parameters not included in closed loop op-amp

[anubhavsingh244]

Consider an open loop opamp circuit:

Now consider the second circuit in a closed-loop configuration (negative feedback):

Why are internal common mode signal parameters not included in the internal gain equation of the op-amp? Is my interpretation of all these concepts correct?

References:
Wikipedia
**broken link removed**
Sedra Smith Specific Pages
Gayakwad Specific Pages

 

[LvW]

The answer is simple: For practical purposes it makes no sense to include common mode parameters in the closed-loop expressions because theit influence will certainly be less than the errors caused by other uncertainties (tolerances of opamp parameters and passive parts). That`s good engineerig practice.

 

[anubhavsingh244]

The answer is simple: For practical purposes it makes no sense to include common mode parameters in the closed-loop expressions because theit influence will certainly be less than the errors caused by other uncertainties (tolerances of opamp parameters and passive parts). That`s good engineerig practice.

But the common mode signal parameters are indeed included in the closed loop configuration of op-amp circuit. Refer to think link: Sedra Smith

What I don't understand is why common mode signal parameters of op-amp not included like I have shown in the last equation in above post (unlike it happens in open loop configuration). There's CMRR, differential gain and differential voltage , common mode gain and common voltage voltage of the circuit, and then there's CMRR, differential gain and differential voltage , common mode gain and common voltage voltage of the op-amp. In the closed loop config, common mode signal parameters of the op-amp are not included like they are included in open loop configuration.

 

[Easy peasy]

usually it is considered that the input signals are within the stipulated CM range for the opamp, AND, that there is zero CM ac signal to contend with.

It is a whole different design matter if there is significant AC CM signal on the inputs, then, formulae for CM gain/attenuation need to be considered in determining what the real output will be,

as in many things the CMRR rolls off with increasing freq, hence more of any CM signal reaches Vout as the freq goes up ....

 

[FvM]

Take it as granted that the basic analysis of closed loop gain abstracts from common mode parameters.

 

[dick_freebird]

Take it as granted that the basic analysis of closed loop gain abstracts from common mode parameters.

Yes, every theory, formula, macromodel has embedded
some assumptions and simplifications to make it useful /
tractable. You may disagree with this until creating one
such, is on you. For teaching the basics, focus on the
most significant to what's being taught.

Then at some point beginner lessons fail to satisfy, and
now you want something more comprehensive. Maybe
full-transistor is for you. I knew a guy who would solve
a hundred-transistor op amp (for which he was locally
infamous) for AVOL using h-parameters.

I ain't him.