Single-pole filters are rather limited (6 dB/octave slope).
For better band-pass and band-reject filters we require more poles and
zeros and thus more reactive circuit elements.
A simple solution is to connect two or more single-pole RC filters
in sequence.
If filter 
draws no current from filter 
, the
transfer function for the combined filter is
One way to do this is to choose a large impedance for .
Hence 
has more poles and zeros than or .
The corner frequency for a high-pass filter is 
and
the transfer function may be written as
For a low-pass filter the corner frequency is 
and
We may build a two-section low-pass filter by requiring 
and 
, as shown in figure 3.7, so that
Figure 3.7: Two-section low-pass filter.
A special case occurs when 
and we obtain one corner frequency but the slope of
the filter is 
.
The results are similarly for a two-section high-pass filter.
A band-pass filter can be built from one low-pass filter and one high-pass filter, as shown in figure 3.8. The order of the filter sections does not matter as long as the impedance rule is obeyed.
If 
we have only two straight regions and the
band-pass frequency range degenerates to zero.
Example: Show that the magnitude of the transfer function
falls off -6 dB/octave at both the low- and high-frequency extremes.
For small
(ie.
and
.
Thus
For large
and
.
And thus
