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

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

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and<br />

Loop gain LG and phase LP can be computed with the following formulas [IV.6]:<br />

( ) ⎟ ⎛ S 21 − S12<br />

⎞<br />

LG = 20 log<br />

⎜<br />

(IV.26)<br />

⎝1<br />

− S11.<br />

S 22 − S 21.<br />

S12<br />

− 2.<br />

S12<br />

⎠<br />

( ) ⎟ ⎛ S 21 − S12<br />

⎞<br />

LP = phase<br />

⎜<br />

. (IV.27)<br />

⎝1<br />

− S11.<br />

S 22 − S 21.<br />

S12<br />

− 2.<br />

S12<br />

⎠<br />

In the following, gain and phase margins (respectively GM and PM) are then <strong>de</strong>duced<br />

from these two expressions [IV.7] since:<br />

and<br />

GM (IV.28)<br />

= 0dB − LGLP<br />

= −180°<br />

PM −180<br />

+ LPLG<br />

= 0dB<br />

= . (IV.29)<br />

To ensure the stability of the system, an open-loop study has been carried out, taking<br />

into account parasitics effects. Large margins have been taken in nominal case, so that<br />

stability margins stay acceptable in worst cases (45MHz, -20°C, 3V supply and process<br />

dispersion). These margins, specified in Table 24, are crucial to ensure a robust<br />

industrialization.<br />

Table 24. Stability margins specifications<br />

Filter Specifications Nominal case Worst case Unit<br />

PM 50 40 <strong>de</strong>g<br />

GM 15 10 dB<br />

IV.2.b.iii Operational Amplifier Specifications<br />

Table 25 summarizes the specifications set on the OA <strong>de</strong>sign. As said, GBW has to be<br />

as high as possible to ensure that the transfer function second peaking takes place at very high<br />

frequency. A specification of 4GHz has been set.<br />

Furthermore, an OA voltage gain higher than 30dB is required so that gain K remains<br />

stable all over the frequency tuning range, as it has been observed previously.<br />

The output impedance Zout is very important as well since it <strong>de</strong>gra<strong>de</strong>s the filter quality<br />

factor when non zero. The specification on this parameter has been set to 2.5Ω.<br />

- 134 -

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