sharpsheep said:
I have designed a two-thomas filter with a bandpass ouput.
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It seams that the loop is a positive feedback. I was confused by this problem seriously.
Yes, indeed - there is positive feedback loop.
I´m sure, the circuit didn´t work, did it ?
But that´s no surprise as there is no phase reversal within the loop.
You either have to interchange the inputs of the second stage or to introduce an additional inverting stage. That´s normal TOW-THOMAS design.
Regards
LvW