[Op-Amp] current for max. output swing

Discussion in 'Homework Help' started by krlngc, Nov 8, 2015.

  1. krlngc

    Thread Starter New Member

    Dec 9, 2013
    11
    0
    Hi all,

    For the op-amp circuit in attachment i'm asked to find the I_3 current that renders the output swing maximum and the R_E that allows input swing between -1V +1V. The circuit parameters are given as;

    V_CC=5V
    V_EE=-5V
    I_1 =  50uA
    I_2 = 500uA
    R_L = 1K

    I think thatI_3 sets the bias point for Q7 and determines the transconductance. In small signal case, the positive half-wave of the input will cause the i_c to flow over R_L and the negative half-wave will result in a negative current over R_L in order to complete I_3. However I can't go any further. How can I make any comment about the max output swing? What should be my approach on this question?

    Thank you for your help.
     
    Last edited: Nov 8, 2015
  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    What about the situation when Q7 is in cut-off?
     
  3. krlngc

    Thread Starter New Member

    Dec 9, 2013
    11
    0
    I can't really tell what drives Q7 to cut-off. A significant rise in V_E can reverse-bias the BE junction maybe ? Is this my upper limit?
     
  4. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    Q6 can cut-off Q7 transistor. And when Q7 is in cut-off, you will get maximum negative voltage swing.
    But on the other hand when Q6 is cut-off you will get the maximum positive voltage swing.
     
  5. krlngc

    Thread Starter New Member

    Dec 9, 2013
    11
    0
    I see now. But how can Q6 cut-off Q7 ? By stealing all the current that I2 provides?

    Thank you.
     
  6. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    Do you see any other way??
     
Loading...