How Does the Offset Current Affect dVo/dt in an LM353 Op-Amp Circuit?

[swuster]

Homework Statement

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)

Homework Equations

offset current = 50 pA
i+ = i-, ideally
v+ = v-, ideally

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 want to 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.

 

[Redbelly98]

Looks good!

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