An op amp differential amplifier as shown is built using four
identical resistors, each having a tolerance of ± 5 %. Calculate the worst possible
CMRR.

What values of the resistor tolerance is chosen?

 

It's given at 5%. For each of four resistors you choose either +5% or -5% and you see which combinations produce the worst result. It seems intuitive that all four resistors at exactly the nominal value produces the best result.

 

won't cmrr be the least (worst) when the differential gain is less, and common mode gain is most. simply apply the appropraite min/max resistance values to the dc and ac analysis formulaes.

 

can anyone substitute the value, Rx=(1±0.05)R


my teacher say its a positive 10,R4’ = 1.05R , R3’ = 0.95R, R3 = 1.05R, and R4 = 0.95R

but I could get negative 10, and this should be the answer as the CMRR drops 10 units when R4’ = 0.95R , R3’ = 1.05R, R3 =0.95R, and R4 = 1.05R


so who is right?