Hi. This is not really a homework question, but it sure looks like it...
I'm trying to get the transfer function of the attached circuit. It's a concatenation of two Sallen-Key low pass filters (a 3rd and a 2nd order) which follow a DA-convertor. I know how to solve them separately, and when I multiply the transfer functions, I get a result that looks exactly like the one QUCS produces. But when I try to solve the whole system at once, it produces something else.
The problem is (I think) in the way I represent the opamps. I get the equations for each node (Va, Vb, etc.), and then I add that Vc = Vd and Vf = y (assuming a very large gain factor in both opamps). This works when analyzing each filter separately.
So, my question actually is: is that the right way to analyze the combined circuit (and am I just making an error in solving the equations) or am I overlooking something?
I'm trying to get the transfer function of the attached circuit. It's a concatenation of two Sallen-Key low pass filters (a 3rd and a 2nd order) which follow a DA-convertor. I know how to solve them separately, and when I multiply the transfer functions, I get a result that looks exactly like the one QUCS produces. But when I try to solve the whole system at once, it produces something else.
The problem is (I think) in the way I represent the opamps. I get the equations for each node (Va, Vb, etc.), and then I add that Vc = Vd and Vf = y (assuming a very large gain factor in both opamps). This works when analyzing each filter separately.
So, my question actually is: is that the right way to analyze the combined circuit (and am I just making an error in solving the equations) or am I overlooking something?
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