NonLinear source – Why is the sources output zero (or crazy) in AC analysis only?
In AC analysis, Spice linearizes all the nonlinear circuit equations including your formula. It does this by taking partial derivatives at the circuit’s DC OPERATING POINT. Spice uses the derivative it computes from the formula as the proportionality constant or “gain” of the NonLinear source during AC analysis. When calculating the derivative, the sources X and Y inputs (X and Y in the formula) are set to the dc voltage determined by the dc operating point of the circuit. This can lead to surprises when X or Y are used in the formula and one or both of their dc operating points is zero! Formula Examples: 1. Fxy = 1.0, the derivative of a constant is always zero. So the output is always zero in AC analysis. 2. Fxy = 1.0*X, the derivative = 1.0 as expected. No surprise here. 3. Fxy = X*X and the dc voltage of X is zero. The derivative is zero: in the limit as X goes to zero, for a very small change in X around zero, the output is zero. Note the output is going to zero much faster than t
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