III. Gm-C Filtering - Epublications - Université de Limoges
III. Gm-C Filtering - Epublications - Université de Limoges
III. Gm-C Filtering - Epublications - Université de Limoges
You also want an ePaper? Increase the reach of your titles
YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.
IV. Rauch <strong>Filtering</strong><br />
IV.1 Sallen-Key versus Rauch Filters<br />
IV.1.a Towards an Operational Amplifier Based Filter<br />
To circumvent the limitations of the <strong>Gm</strong>-C filters, operational amplifier based filters<br />
have been studied. In<strong>de</strong>ed it has been <strong>de</strong>monstrated that <strong>Gm</strong>-C filters are limited by strong<br />
<strong>de</strong>gradation and distortions of the RF signal due to the gyrator. The i<strong>de</strong>a behind using an OA<br />
based topology is to take advantage of both a high loop gain and a certain filtering of the<br />
signal at the input no<strong>de</strong> VA of the active component, due to the RC network. This is illustrated<br />
in Figure 138. The final purpose is to obtain a highly linear filter not compromising the noise.<br />
Figure 138. Principle of an OA based filter<br />
Furthermore, to reduce the number of active and passive components in the circuit, it<br />
has been chosen to study a second or<strong>de</strong>r bandpass filter [IV.1]. As discussed in the<br />
introduction, a second or<strong>de</strong>r bandpass filter allows reaching the required specification on the<br />
RF filter stage.<br />
Previous literature consi<strong>de</strong>rs two main types of second or<strong>de</strong>r RC bandpass filters using<br />
a single wi<strong>de</strong>band amplifier: Sallen-Key filters and Rauch filters. Rauch filters are also<br />
referred as multiple feedback structures [IV.2].<br />
Following filters are studied with two capacitors set to a common value C, as shown in<br />
Figure 139, Figure 140 and Figure 142. This simplifies computations and shows interesting<br />
properties for a frequency tunable filter as it will be <strong>de</strong>tailed later.<br />
- 119 -