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!

CIRCUIT ANALYSIS: Non-Ideal OpAmp with 2 resistors - Find Thevenin Equivalent

  1. Feb 5, 2007 #1
    1. The problem statement, all variables and given/known data

    There is a non-inverting op-amp below.

    [​IMG]

    The op-amp is NOT ideal. We assume that [itex]R_i\,=\,\infty[/itex], [itex]R_0\,>\,0[/itex] and A is finite.

    Find the general Thevenin equivalent circuit seen at the terminals.


    2. Relevant equations

    KCL, v = i R


    3. The attempt at a solution

    I changed the diagram to use a model given for a non-ideal op-amp.

    [​IMG]

    Now I add some voltage and current markers.

    [​IMG]

    [tex]V_d\,=\,-V_{IN}[/tex] <-----Right?

    [tex]V_1\,=\,-A\,V_{IN}[/tex]

    Now, the current equations)

    [tex]I_1\,=\,\frac{V_1\,-\,V_0}{R_0}\,=\,\frac{-A\,V_{IN}\,-\,V_0}{R_0}[/tex]

    [tex]I_2\,=\,\frac{V_0\,-\,V_2}{R_2}[/tex]

    [tex]I_3\,=\,\frac{V_2}{R_1}[/tex]

    KCL at [itex]V_0[/itex])

    [tex]I_1\,=\,I_2\,\,\longrightarrow\,\,\frac{-A\,V_{IN}\,-\,V_0}{R_0}\,=\,\frac{V_0\,-\,V_2}{R_2}[/tex]

    Solving for [itex]V_0[/itex])

    [tex]V_0\,=\,\frac{-R_2\,A\,V_{IN}\,+\,R_0\,V_2}{R_0\,+\,R_2}[/tex]

    KCL at [itex]V_2[/itex])

    [tex]I_2\,=\,I_3\,\,\longrightarrow\,\,\frac{V_0\,-\,V_2}{R_2}\,=\,\frac{V_2}{R_1}[/tex]

    Solving that equation for [itex]V_0[/itex])

    [tex]V_0\,=\,\frac{R_2\,V_2\,+\,R_1\,V_2}{R_1}[/tex]

    But which do I use? Are they both right? ONe wrong? Or all wrong?
     
  2. jcsd
  3. Feb 5, 2007 #2
    I think you only did a partial KCL at node 2. Summing currents into the node you should get:

    [tex]\frac{V_d-V_2}{R_i} + \frac{V_0-V_2}{R_2} + \frac{0-V_2}{R_1} = 0 [/tex]

    and for the V0 node:

    [tex]\frac{AV_d - V_0}{R_0} + \frac{V_2 - V_0}{R_2} = 0[/tex]

    Now you can solve for V_0 and use the V_in=V_d constraint, though you seems to think is otherwise. You might have a reason that I don't see, but I think V_d = V_in, and not the negative.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: CIRCUIT ANALYSIS: Non-Ideal OpAmp with 2 resistors - Find Thevenin Equivalent
Loading...