It must be mentioned that a transfer function
with
is physically not realisable. The transfer function of an
ideal differentiator is described by 
according to
Eq. (2.6). Any transfer function with can be
decomposed into
 |
and terms in 
with positive powers also occur. Such derivative elements would
deliver for input signals of arbitrary high frequency
corresponding output signals of arbitrary high amplitude, which
are physically not realisable. The
condition of realisability of
the transfer function according to Eq. (3.2) is
as
. A realisable transfer function is called as proper and it always follows
as
. A transfer function with 
is called as strictly proper.
or