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

IV. Rauch Filtering
IV.1 Sallen-Key versus Rauch Filters
IV.1.a Towards an Operational Amplifier Based Filter
To circumvent the limitations of the Gm-C filters, operational amplifier based filters
have been studied. Indeed it has been demonstrated that Gm-C filters are limited by strong
degradation and distortions of the RF signal due to the gyrator. The idea behind using an OA
based topology is to take advantage of both a high loop gain and a certain filtering of the
signal at the input node VA of the active component, due to the RC network. This is illustrated
in Figure 138. The final purpose is to obtain a highly linear filter not compromising the noise.
Figure 138. Principle of an OA based filter
Furthermore, to reduce the number of active and passive components in the circuit, it
has been chosen to study a second order bandpass filter [IV.1]. As discussed in the
introduction, a second order bandpass filter allows reaching the required specification on the
RF filter stage.
Previous literature considers two main types of second order RC bandpass filters using
a single wideband amplifier: Sallen-Key filters and Rauch filters. Rauch filters are also
referred as multiple feedback structures [IV.2].
Following filters are studied with two capacitors set to a common value C, as shown in
Figure 139, Figure 140 and Figure 142. This simplifies computations and shows interesting
properties for a frequency tunable filter as it will be detailed later.
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