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

A parallel self resonating circuit is then obtained considering a parallel capacitor Cp to
the gyrator. This leads to a resonant frequency given by:
filter.
g m1g
m2
ω . (III.37)
CC
0 =
p
A filter using a gyrator as a resonant element is called in the literature a Gyrator-C
Figure 72 depicts a fourth order high-pass filter. The equivalent gyrator-C filter is
obtained transforming the inductors of the circuit into gyrators.
Figure 72. Synthesis of a filter using gyrators
II.4.a.ii Impact of Real Elements
Though previous transconductances were considered as ideal, imperfections appear
when implementing the design. It can be either an input capacitance (for instance the Cgs of a
MOS realizing the gm) or a non-infinite output resistance. This leads to the schematic shown
in Figure 73, where C2 takes into account the real model of the transconductance as well as
the internal node capacitance of the gyrator.
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