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

Now let’s consider a second structure [II.17], depicted on Figure 86:
Figure 86. Constant-Q frequency tunable bandpass filter
In this structure, assuming gm1 = gm2 = gm leads to the following quality factor, the
central frequency being the same as before.
Q
1
g R
2
= (III.68)
m3
C
C
A constant-Q move is achievable thanks to gm tuning or keeping C1 = C2. However,
the 1/R dependency of Q makes this structure noisy but also being strongly subject to
temperature and process variation whereas a robust implementation is required for
industrialization.
II.4.c.ii Study of a Gm-C Bandpass Filter for TV Tuners
Now let’s describe more accurately a Gm-C bandpass filter designed for TV tuners
[II.18, II.19 and II.20]. Y. Sun et al have proposed a second order bandpass filter architecture,
also called biquad in the following, which is then cascaded to obtain a higher order filtering.
The second order Gm-C filter, the basic biquad, is the differential structure depicted in
Figure 87. This is the architecture which is chosen and implemented in next chapter. It was
chosen to tune only capacitors so as to keep a constant gm value which is linearized.
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