Differentiating impulse response

On 15 Mai, 13:27, "gkn" <gei...@ii.uib.no> wrote:

> Hi again. You said that the discretized filter becomes unstable when using
> the bilinear transform on the new zero (by the differentiation). What I
> then find confusing is that there are many analog prototypes which have a
> zero like this. And the BLT of a stable filter is also a stable filter, so
> what's wrong here?

Your last statement is wrong. You have the counter-example.

To be more specific: the BLT maps the left-half complex plane
(excluding the imaginary line) into the unit circle. The pole of the
continuous time differntiator at infinity gets mapped onto the Nyquist
frequency. For analog designs, the pole at infinity is usually
countered by some kind of (possibly system inherent) lowpass.

Regards,
Andor