Operational Amplifier Circuit

1. Mar 9, 2009

swuster

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?

 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. Mar 10, 2009

Redbelly98

Staff Emeritus
Looks good!

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