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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)


    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


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    Looks good!

    (I'll take your word for it on the 50 pA spec.)
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