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

III.2.d Dynamic Source Degeneration Technique
In [III.1] and [III.7], the dynamic source degeneration technique, which is an active
source degeneration, is described. The schematic of this linearization technique is depicted in
Figure 113. The red-circled transistors act as equivalent resistors which linearize the
differential pair.
Figure 113. Schematic of the dynamic source degeneration technique
Figure 205 shows the linearization of the transconductance for various transistor sizes.
The degeneration consists in reducing the gm value in order to be more linear. Hence, though
very high linearity level can be reach for small gm values, the method is not appropriate for
10mS or more transconductances values.
Moreover, since this technique is an active degeneration of the MOS differential pair,
noise is increased as described for the resistive MOS common-source circuit.
Figure 114. Transconductance linearization by dynamic source degeneration
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