Worst CMRR

Discussion in 'Homework Help' started by Alphas, Sep 14, 2007.

  1. Alphas

    Thread Starter New Member

    Sep 14, 2007
    2
    0
    [​IMG]

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

    [​IMG]
    [​IMG]

    What values of the resistor tolerance is chosen?
     
  2. Papabravo

    Expert

    Feb 24, 2006
    10,163
    1,796
    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.
     
  3. techroomt

    Senior Member

    May 19, 2004
    198
    1
    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.
     
  4. Alphas

    Thread Starter New Member

    Sep 14, 2007
    2
    0
    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?:confused:
     
Loading...