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!

Operational Amplifier Circuit

  1. Mar 9, 2009 #1
    1. The problem statement, all variables and given/known data
    Look up the offset current for the LM353 op-amp and predict the time rate of change of the output for the circuit below after the switch opens. (Find dVo/dt)

    opamp.jpg

    2. Relevant equations
    offset current = 50 pA
    i+ = i-, ideally
    v+ = v-, ideally


    3. The attempt at a solution
    I really don't know where to begin. Vo = V- in this case, so I think the 50 pA are flowing through the other branch. When the switch is flipped, the capacitor discharges, losing voltage. But how can I derive dV/dt when ideal op-amps have infinite gain?

    [edit] Had some more insight on this, but still wanna check to see if it's right:
    If the gain of the op amp is A:

    I = C dV(+)/dt
    dV+/dt = 5 V/s

    Vo = A(v+ - v-) = A(v+ - Vo)
    Vo = v+ * A / (1 + A)
    dV0/dt = dv+/dt * A / (1 + A)

    If A is large then A / (1 + A) = 1 so dV/dt = 5 V/s.
     
    Last edited: Mar 9, 2009
  2. jcsd
  3. Mar 10, 2009 #2

    Redbelly98

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Looks good!

    (I'll take your word for it on the 50 pA spec.)
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Operational Amplifier Circuit
  1. Operational amplifier (Replies: 2)

  2. Operational amplifier (Replies: 13)

Loading...