Differential amplifier common mode gain

[paulmdrdo]

Summary:: Differential amplifier common mode gain derivation of forumlas

I'm having a hard time deriving for equations 10-8 -10-9.

I tried adding equation's 18-6 and 18-7 but cannot proceed with the derivation. I need help on this. Thank you!

 

[DaveE]

It's kind of a confusing section, much appears to be left out.

For this to be a differential amp, you would have to set R₂'/R₁' = R₂/R₁. Then the difference mode gain is R₂/R₁ and the common mode gain is 0. If you don't do this, I don't think the premise makes sense.

Then for the sensitivity analysis, I would pick any single resistor and add in an error term eg. R₂ → R₂ + ΔR. Then recalculate the common mode gain. You may find some discrepancies from their result because of how you introduce the component value error term.