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# all pass filter realization

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#### rambus_ddr

I want to realize the Tranfer function (1+a*s)/(1-a*s), not(1-a*s)/(1+a*s). How can I use one opamp and RC to realize, please give your advice, thanks.

I remember a joke my Filter teacher used to say when he was talking about an 'all pass' filter, he used to give a piece of cable and say that's it!

https://en.wikipedia.org/wiki/All-pass_filter gives an example. The pole and zero locations are at 1/RC and the Rx values are equal to produce a gain of -1.

You will have to consult a book on filter design to get the circuit for two pole versions.

Thanks, flatulent.
I want to realize a rising phase vs frequency, so it need a positive pole, and negative zero. But in many paper and book, there are a negative pole and positive zero.

I want to realize a rising phase vs frequency
I think, it can't be realized as an analog filter.

FvM is correct. Analog methods cannot produce a noncausal system. With rising phase, the output occurs before the input. You will have to use DSP methods.

If you can accept a varying amplitude, you can use a zero on the negative real axis.

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