SaM - Definition of CMRR (Common Mode Rejection Ratio)

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• The sign is always positive.
• And obviously ==for an ideal differential amplifier, we have a common mode rejection ratio, which is infinite==, since ideally $A_C=0$.
• Where: $A_1$ and $A_2$ are the terminals of a differential amplifier.
  Here is a simple schematic:
  
  • Idelly: $R_D=0$ and $R_{C1}=R_{C2}=\infty$.
    Such that: $V_o^{\prime }=A_d{V}_d$.

We have also seen an example, where we calculate the CMRR for an Instrumentational Amplifier.IMPORTANTE

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Formula: $\text{CMRR}=\frac{|A_d|}{|A_C|}=\frac{A_2-A_1}{2(A_1+A_2)}$ What you need to remember:

• $V_0=V_C{A}_C+V_d{A}_d$
• $V_d=V_2-V_1$ & $V_C=\frac{V_2+V_1}{2}$
• $A_d=\frac{A_2-A_1}{2}$ & $A_C=A_2+A_1$
• Where:
  • $A$ is the gain.
  • ${}_d$ means “differential”.
  • ${}_C$ means “common mode”.

Professor’s notes: