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

Reference [II.20] is based on the same architecture as the latter filter. However, the
two transconductances of the gyrator are tunable. This second degree of tunability allows the
enhancement of the tuning range. Frequencies from 48 to 780 MHz are now covered at the
cost of reduced linearity and higher noise.
II.4.c.iii Performances Comparison of Gm-C Filters
Many RF bandpass filters, making use of single MOS transistors as transconductors,
were previously reported [II.8, II.21]. Operating in the GHz range, they can reach very high
Q-factors for a very low power consumption, but with signal distortion and noise. In parallel,
the possibility to use an active selectivity for RF front-ends under certain conditions on the
LNA (2dB NF and gain limited to 10dB) was shown in [II.22]. The literature also reports on
more complex Gm-C filters, used at lower frequencies (40-300MHz), which achieve both
interesting RF performances and large tuning range [II.5]. However such filter, with a lowpass
structure, does not have an influence on adjacent channels rejection. Finally, as
previously described, it has been reported a Gm-C bandpass filter for TV tuners that reaches a
good dynamic range while being very low power [II.19]. The performances of these RF filters
are summarized in Table 5.
Table 5. Gm-C & Gyrator-C RF Filters
Ref. [II.8] [II.21] [II.22] [II.5] [II.19] Units
Type BPF BPF BPF LPF BPF -
Mode single single differential differential differential -
Freq. 400-1050 500-1300 1900-3800 50-300 50-300 MHz
Q 2 to 80 60 40 6 -
NF 8.5 18 14 20 dB
IIP3 -15 -26 16 4 dBm
Power 51 274 10.8 72 7.6 mW
Supply 5 1.8 1.8 1.2 V
Techno 350 350 180 180 130 nm
II.4.d Rm-C Filtering
Rm based filters are very similar to the Gm-cells based ones. As explained previously,
a Gm-cell converts a voltage into a current with a certain gain gm whereas a trans-resistance
amplifier converts a current into a voltage with a certain resistive gain Rm, called “transresistance”.
A capacitors associated to this trans-resistance amplifier creates a derivator, as
depicted in Figure 89, which is essential to synthesize a filter transfer function. Indeed, this
leads to a derivative behaviour since:
Vout
= jRmCω
. (III.69)
ΔV
in
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