1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Electronic Circuits-Compute voltage gain for OP-amplifiers

  1. Dec 30, 2012 #1
    1. The problem statement, all variables and given/known data

    We got the following circuit:

    http://tinypic.com/view.php?pic=1zl7xg2&s=6

    u_in is input
    u_ut is output



    3. The attempt at a solution

    I assumed that the upper amp has voltage u_in on the positive terminal.
    And that the lower amp has voltage 0 V on the positive terminal.

    then i calculated u_ut but the ratio then becomes: u_in/u_ut = 0

    Because the current from the upper amplifiers output terminal to the output terminal u_ut is

    i=u_in/2R

    which then gives us

    u_ut=u_in -(i*2R) = u_in -u_in

    the right answer should be -1

    :'(

    help please.
     
  2. jcsd
  3. Dec 30, 2012 #2

    NascentOxygen

    User Avatar

    Staff: Mentor

    I would analyze it by saying the voltage on the lower input is vx, and that on the upper input terminal is uin+vx, with vx being the common mode voltage.

    Otherwise you are losing all the information associated with the lower pair of R resistances.
     
  4. Dec 30, 2012 #3

    NascentOxygen

    User Avatar

    Staff: Mentor

    First step: determine, in terms of the input, the voltage at the (+) input of the third op-amp.
     
  5. Dec 31, 2012 #4
    So this is how i did it:

    On the positive input for the 3rd amplifier we have v_x/2.

    And i suppose that the voltage on the output terminal of the upper amplifier is v_x+u_in

    The current through the resistor then becomes:

    [tex]i=\frac{\frac{v_x}{2}-(u_{in} + v_x)}{R}[/tex]

    Then the expression for the output u_o is:

    [tex]u_o=\frac{v_x}{2} - R*i = {\frac{v_x}{2}-\frac{v_x-2u_{in}-2v_x}{2R}*R}[/tex]

    after simplifying we obtain:

    [tex]u_o=-v_x-u_in[/tex]

    u_o/u_in = -1 (if v_x = 0)


    Have i done correct?
     
  6. Jan 1, 2013 #5

    NascentOxygen

    User Avatar

    Staff: Mentor

    If we use your above expression for i, then the "-" here should be "+"
     
  7. Jan 2, 2013 #6
    It is for the resistor after the output of the upper-most amplifier. I thought the current goes out of this amplifier and ends up at the output. So then it should be "-"

    But i re-calculated this task with my method again, and got a quite different result. That turned out to be correct. I calculated without adding v_x.

    The same current: i=(u_in/R)

    u_out=0-Ri= -R(u_in/R)=-u_in

    The gain then becomes -1.

    I think this is sufficient.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Electronic Circuits-Compute voltage gain for OP-amplifiers
  1. Op Amplifier Circuit (Replies: 2)

Loading...