# Differential amplifier CMMR, NON-IDEAL Op-Amp used

I have a little trouble with the analysis of a differential amplifier. What I want to do is to determine the CMRR of the whole amplifier taking into consideration the CMRR of the op amp.

This is the circuit:

Let's say that the OpAmp has a CMR of 60 [dB]. What is the CMR of the whole amplifier?

• And also where is your attempt at a solution? – dirac16 Jun 12 at 10:10
• Depends on resistor tolerance to a great extent. See analog.com/media/en/reference-design-documentation/design-notes/… – Peter Smith Jun 12 at 10:15
• The picture you post seems to be aimed at calculating the CMRR of the circuit due to unmatched resistors, not due to the op. amp. – vangelo Jun 12 at 10:31
• Let's say that the OpAmp has a CMR of 60 [dB] You're making your own life more difficult by assuming that. As suggested in the comments, the fact that the resistors aren't perfectly matched means that they deteriorate the CMRR. So assume that the opamp is ideal and then determine the CMRR. – Bimpelrekkie Jun 12 at 10:41
• OK, guys. Sorry for not being explicit. The idea is to see the effect of the CMRR of the OpAmp assuming that resistors are balanced. (The picture is suggestive only). What would be the approach to determine the CMRR of whole circuit? – Andrei Ciobanașu Jun 13 at 15:02