I want to find taylor series expansion of a gate charge function. The equation is g(x)= 4.5e-12 - Qdepl - (beta*((Vds - Vgs + Vt)^3 + (Vgs - Vt)^3))/((Vds - Vgs + Vt)^2 - (Vgs - Vt)^2);
The problem is that there are two variables in this equation Vgs and Vds. I am confused how to expand in Taylor series around some operating point (Vgso,Vdso). For first order and second order terms we need to calculate Jacobian and Hessian matrices respectively. However, in its general form of taylor expansion e.g. g_taylor(x)= g(xo) + Go(x-xo) + 1/2Ho(x-xo)^2. Here g(xo) is the constant value of g(x) at operating point xo=Vgso,Vdso. Simliarly Go is the Jaobian calculated by taking first order partial derivative of g(x) w.r.t Vgs and Vds. The confusion here is that i dont know for (x-xo) term, what will be x? and what is xo? As xo is just two values of operating point where as x is equation of g(x) (that what i am thinking) . So how can i subtract a equation from two values? then how can i multiply that value with a jacobian? Similar problem is with the next ter,. Please clearify my concepts.