I have learned the Butterworth filter, normally it is used for low pass design. And the amplitude response is:


mentioned in this Butterworth Introduction. But as I know, the Butterworth can be designed as a high pass as well. Then what is the amplitude response for high pass? Is it simply like this?

$$|H_{hp}| = \frac{1}{|H(jw)|}$$


If $H_{LP}(j\omega)$ is the frequency response of a Butterworth low-pass filter, then its inverse would go to infinity for $\omega\rightarrow\infty$ (because $H_{LP}(j\omega)$ goes to zero for high frequencies). So that's not what you want.

The correct high-pass frequency response is obtained by replacing $\omega/\omega_c$ by $-\omega_c/\omega$, resulting in


which approaches unity with increasing frequency, as should be the case for a high-pass filter.

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