Complex Filter Design

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MichaelHelal

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Question: Why is there a peaking in the complex bandpass filter response?

Hello everyone
I am designing 5th order butterworth Complex Band Pass filter, I finished the design of the 2 stage miller opamp.
After running the simulation to check the filter response and by comparison with the ideal one, I find a peaking in the response of the actual designed one,, does anyone know why this peak happen?

Note: Opamp GBW is around 300 MHz and the cut-off frequency of the filter is 1.5 MHz
Note2: in the red curve, I use actual resistors and capacitors while in the green one I use ideal res&caps
 

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The simplest (conceptually) way to check it is to perform PZ analysis. The real passive elements has ca 10% parasitic capacitance to ground which modifies transfer function. There is also a question about your opamp PZ location. By closing the loop, any opamp zero is forming directly a pole.
Start with model of opamp and PZ analysis. Maybe split filter to low pass and high pass circuits can help in debugging.

//edit
Please notice your peak has only 0.5dB, it is not too much
 
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Dear Dominik, thank you for your sharing
PZ analysis may be too complicated to locate the pole or the zero as it's a huge circuit.
I tried to replace the designed opamp with a macro-model and started tuning gain and BW and the response improved by increasing the BW.
 

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I didn't get what you said about any zero is forming a pole in the closed-loop, can you explain this?
 

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If G(s) was a zero you can see that it would form a pole in the denominator
after you do the analysis -




Regards, Dana.
 

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