Tlaloc
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Posts: 81
Dallas, TX
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I personally would believe STB analysis. The problem with the older methods of finding the transfer function involved a number of assumptions that discounted non-idealities. With a giant RC in the feedback (or LC) you have created a pole-zero pair at very low frequencies. This also isolates the real loading created by your input capacitance. It also isolates the feed-forward transmission (going the wrong way) through your feedback path. It does, though, approximate the real transfer function by providing something close to the voltage loop gain (see below).
Better methods include methods proposed by Middlebrook in 1975 that requires you to look at both the voltage (Tv) and current (Tc) transfer functions. This would generally require you to run two different simulations or have two copies of your amp in one simulation. Then, the combined transfer function is 1/(1+T)=1/(1+Tv)+1/(1+Tc). For low frequencies, the Tv term almost always dominates (which is why people continue to use the same method as you did), but at high frequencies, Tc can come into play.
The main problem with the previous approach is that it assumes a zero reverse loop gain. Since then, there has been some advances in the mathematics to account for all possible transmission paths. Middlebrook has published one method using the Dissection Theorem, but the Spectre stb analysis uses a alternate method. The math behind it shows that the answer should be much closer to reality, especially at high frequencies where the non-idealities dominate, than previous methods.
In short, I would trust stb, especially since it is providing the more conservative answer.
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