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

III.5 Conclusion
Once the Gm-C second order bandpass filter topology adopted, a theoretical study has
been carried out. From this study, it has been demonstrated that linearity, noise and the filter
Q-factor are strongly related. In order to optimize the dynamic range of the Gm-C filter, a
moderate Q has been chosen. A second choice is based on the use of Gm-cells of high
transconductances to decrease the noise as much as possible. However, these high
transconductances have to be linearized so that they can be associated together to obtain a
highly linear Gm-C filter.
Different linearization techniques are proposed in the literature. They have been
assessed in this chapter. Two of them have been used to design filters. These two filters are
based on Gm-cells linearized by means of dynamic source degeneration and by multiple gated
transistors. It comes out that the first filter presents a too limited linearity versus noise tradeoff.
The second filter exhibits a high dynamic range over a more than three octaves tuning
range. However, it exhibits a high sensitivity to process and voltage variations.
Despite its interesting performances in terms of dynamic range and of power
consumption, the gyrator appears to be the main source of degradation and of distortion of the
signal within the Gm-C filter. Furthermore, this solution exhibits strong process and mismatch
dependencies. The sensitivity and the lack of robustness of the MGTR technique lead us to
take the decision not to tape out the filter as a circuit, but to carry on investigations on other
high performance filtering solutions. That is why an operational amplifier based filter is
analyzed in Chapter IV.
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