telescopic opamp input level

Discussion in 'General Electronics Chat' started by manissri, Jan 27, 2006.

  1. manissri

    Thread Starter Member

    Jan 23, 2006
    hi all
    why telescopin opamp has the input common mode level and out put common mode level is not same.
    suppose the supply range is 0- 5 volts then the input common mode can not be 2.5v. and the output common mode level is at 2.5v.
    where as in folded cascode the input common mode level and output common mode level can be at 2.5 volt for both input and output.
    plz explain.
  2. chesart1

    Senior Member

    Jan 23, 2006
  3. chesart1

    Senior Member

    Jan 23, 2006

    In the ideal world, an op amp with two inputs v1 and v2 and a differential gain ad will have a output signal vo as follows:
    vo = ad(v1-v2)

    However in the practical world the output is not only dependent on the difference between v1 and v2. The output is also dependent on the common mode voltage. The common mode voltage vc is calculated as follows.

    vc = 0.5(v1+v2)

    For example, if v1 = 1 uvolt and v2 = -1 uvolt, the output will not be the exact same as if v1 = 100 uvolts and v2 = 200 uvolts even though the difference voltage is still 2 uvolts.

    The common mode rejection ratio cmrr is:
    cmrr = ad/ac
    where ac is the common mode gain and ad is the differential gain.

    The differential voltage vd is
    vd = v1-v2

    The actual v1 is
    v1 = vc + 0.5*vd

    and v2 is
    v2 = vc - 0.5*vd

    The output voltage vo is
    vo = ad*vd + ac*vc

    The gain a1 is the gain from the v1 input with the v2 input grounded.
    The gain a2 is the gain from the v2 input with the v1 input grounded.

    The differential gain can be expressed as
    ad = 0.5*(a1-a2)

    And the common gain can be expressed as
    ac = a1 + a2

    I hope this helps you.