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

Filtering in the mirror, by means of an RC low-pass filter, has also been performed to
prevent bandgap noise amplification. Components values are Clp=4pF and Rlp=30kΩ so that
fc=1.3MHz.
The output follower is required to be very linear and to present a very low output
resistance, to keep a good filter Q-factor. Indeed, when the output resistance increases, the
quality factor of the filter decreases. That is why power consumption is needed.
Figure 155 shows the OIP3 of the filter versus OA follower current at 40MHz with a
Q-factor of 3. This study has been performed using an output follower being a single bipolar
transistor in common-collector configuration with a MOS current source. This graph
demonstrates that lower current consumption through the follower directly results in lower
linearity. Since the Rauch filter takes advantage of a high linearity level, it has been chosen a
12mA current in this stage. Note that Figure 155 was plotted with a differential pair fed by
3mA, and not 5mA as in the final schematic.
Figure 155. Linearity of the Filter versus follower current If
IV.3.b.ii Analysis of the Follower Stage
To enhance linearity, a feedback loop has been added to the single transistor follower,
as depicted in Figure 156.
As detailed in APPENDIX D, this feedback loop increases the small signal current
flowing through the load RL by a factor (β7+1). This allows reducing the distortions of the
emitter-follower, enhancing the linearity of this stage.
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