The Laws of Circuit - you can learn and practice by just reading
Copyright. Charles Kim 2006
left of the
figure. The current through through the
capacitor is, of course, I = dV/dt. No second thought.
Now look at the capacitor in the op amp circuit in the right part of the
figure. Applying node voltage method at node n,
(Vn-10)/1.25=(0-10)/1.25= -10/1.26 (since Vp=0=Vn) is the
current flowing out from node n to the left. Then, how do
you write your equation for the current flowing through
the capacitor? Just dV/dt? What V is this then? Remember
in node voltage method, all voltages are node voltages.
In other words, the voltages in equation must be
reference to ground. The voltage (V) in dV/dt must be the
voltage across the capacitor (Hail Mary Pass!). Then what
is the voltage across? Since current is assumed to flow
from left to right, applying passive convention, the
voltage across is (left side of voltage) minus (right
side of voltage). That means, V = Vn-Vo. Therefore,
finally, the current flowing from node n toward node o
is: d(Vn - Vo)/dt = - dVo/dt, since Vn=0.