need help about stability of two-thomas filter

[sharpsheep]

I have designed a two-thomas filter with a bandpass ouput. The architecture of the filter and opamp can be found in the attachment. The problem is when I simulate the loopgain and phase of the first loop as the way in the attachment, the initial phase of is 0 rather than 180. It seams that the loop is a positive feedback. I was confused by this problem seriously.

Pls help me and give me some advice on sovling the problem.

 

[LvW]

I have designed a two-thomas filter with a bandpass ouput.
.........................
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

 

[sharpsheep]

Thanks!

But according to the circuit, the output of the second stage has been already changed. This topology is some type of modified two-thomas filter.

I have designed a two-thomas filter with a bandpass ouput.
.........................
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

 

[FvM]

I don't see a positive loop gain in the schematic.

But it's impossible to see from your posting, if both amplifiers are biased correct in the loop gain measurement or what's exactly the loop gain definition. Also the amplifier properties are unknown. Looking at the partial gain contributions and bias points may reveal the problem.

 

[LvW]

Thanks!
But according to the circuit, the output of the second stage has been already changed. This topology is some type of modified two-thomas filter.

Sorry, you are right. I didn´t check your circuit carefully. There is, indeed, a phase reversal between output and input.