III. Gm-C Filtering - Epublications - Université de Limoges

Topologies comparison
for a same Q-factor and f0
Figure 53. Comparison of fourth order bandpass filters
Figure 54. H3 and N+5 rejections according to the fourth order topology
Despite a higher selectivity than second order bandpass, these filters are constituted
with four or more reactive elements, leading to a more complex circuit. In particular, the
frequency tuning requires keeping the sets of coefficient of the transfer function constant, as
said for low-pass high order filters. As explained, to minimize the number of reactive
elements in the circuit, a second order bandpass filter is preferred to higher order bandpass
filters.
Furthermore, Figure 55 depicts the use of a second order bandpass filters tuned in
frequency at constant Q-factor or at a constant-BW, and the achieved rejections are in line
with the order of magnitude of the required specifications.
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