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

II.4 Active Filtering Solutions
From the literature, it comes out that Gyrator-C and Gm-C are the most used
techniques to handle fully active RF filtering. Since they are continuous-time filters with no
discretisation of the signal, they are well matched for current NXP tuner architectures. This
chapter aims at describing their principle and the state-of-the-art of these techniques. Other
solutions, which are emerging techniques, are studied later in this manuscript.
II.4.a Gyrator-C Filtering
II.4.a.i Gyrator General Principle
The gyrator is the most popular structure to handle active RF bandpass filtering. Its
principle consists in synthesizing an inductive behaviour by means of two transconductors in
the configuration depicted in Figure 71.
Figure 71. Gyrator configuration
To find the equivalent impedance of this load, first steps of computation give:
g
I
m1
in
V
in
= g
= jCωV
m2
From these two equations, one gets:
V
out
out
- 58 -
(III.33)
(III.34)
Vin
⎛ C ⎞
Z = = ⎜
⎟
in jω
. (III.35)
I in ⎝ g m1g
m2
⎠
Hence, the input impedance Zin of this load shows an inductive behaviour, with an
effective inductance Leff given by:
C
L eff = . (III.36)
g g
m1
m2