Common Mode Rejection (CMR)

Discussion in 'Homework Help' started by magnet, May 12, 2005.

  1. magnet

    Thread Starter Member

    Apr 25, 2005
    16
    1
    [attachmentid=639]


    Please refer to the attached diagram

    I am in need of help in calculating the change in decibels of the CMR if impedance falls by a factor of 500.

    Thanks
     
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    I may be wrong but the schematic you have posted looks a bit odd.

    The two resistors labeled Re are in parallel and both ends of these two resistors are tied to ground. That pretty much means they have nothing to contribute to the circuit.

    Also the amplifier looks like it might be an opamp. If that is the case, then it would be helpful to indicate which input is the negative one and which is the positive one.


    hgmjr
     
  3. magnet

    Thread Starter Member

    Apr 25, 2005
    16
    1
    Sorry, please refer to the new attachment.
    [attachmentid=642]
     
  4. mozikluv

    AAC Fanatic!

    Jan 22, 2004
    1,437
    1
    hi

    where is the feedback network?
     
  5. Brandon

    Senior Member

    Dec 14, 2004
    306
    0
    There is no need for a feedback network. This is just a typical difference amplifier application but done with an op-amp. If you remember, the op-amp's input is 2 Darlingtons connected in via differential input.

    Telling you how to find the CMR is some technical data that got pushed out by other crap. Maybe if you attempted to redraw it into a format like what ever you covered in class for CMR.
     
  6. mozikluv

    AAC Fanatic!

    Jan 22, 2004
    1,437
    1
    hi

    i have already gotten how a classroom looks. :D :D :D :D :D :p
     
  7. magnet

    Thread Starter Member

    Apr 25, 2005
    16
    1
    This question is on a distance learning course and is only covered by a description within the unit.

    I you know how to help or where I may find information on this, especially where calculations are involved, it would be greatly appreciated.

    Thanks
    Magnet
     
  8. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    Your CMR will always be infinite with a perfect device - considering the op amp input as a differece amp. Without Rs, this would be the case for your example - both Re's are identical, so an common voltage applied to the inputs will leave the differential input balanced.

    The voltage drop across Rs will unbalance the voltages felt by the input resistors, and cause a shift in the output. If you think about it, lowering Rs by 500 ohms will decrease the voltage across it, and thus improve CMR.
     
  9. Brandon

    Senior Member

    Dec 14, 2004
    306
    0
    Mm.....

    Everything I have is for the transistor level and without knowing Betas/gm of the xtrs I couldn't give u my equasions to solve for it.

    You could possible take a different approach to solving this tho. CMR is just the diff mode gane / common mode gain. You can simulate both of these situations and divide the resulting gains and get an approximate answer. Just put a differential input, say 5v and 0v, and then a common mode input, 4v and 4v or so.
     
  10. magnet

    Thread Starter Member

    Apr 25, 2005
    16
    1
    This question is a bit puzzling because all references about CMR equations are to do with the Voltage. The hint within the question was to add minimal resistor values!!
     
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