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The constitutive relation for a non-linear capacitor is I = dQ/dt. Using the backward Euler discretization scheme to discretize dQ/dt one obtains
I = $\displaystyle {\frac{Q - Q_o}{\Delta t}}$ (2.37)
  $\displaystyle \approx$ G . V - $\displaystyle {\frac{C_o \cdot V_o}{\Delta t}}$ (2.38)
G = $\displaystyle {\frac{C}{\Delta t}}$ (2.39)

Equation (2.38) is a commonly used approximation which, however, does not guarantee charge conservation [66,67]. For constant C (2.38) of course simplifies to (2.35). The stamp is given as
yx, y $ \varphi_{1}^{}$ $ \varphi_{2}^{}$ f
n1 G - G - I
n2 - G G I

Tibor Grasser