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

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

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<strong>III</strong>.2.e Unbalanced Differential Pairs Technique<br />

Reference [<strong>III</strong>.8] reports the use of unbalanced differential pairs, as illustrated in<br />

Figure 115, in or<strong>de</strong>r to linearize the resulting transconductance. This method is known as<br />

multi-tanh for BiCMOS technologies and may also be used with CMOS technologies.<br />

Figure 115. Unbalanced CMOS differential pairs<br />

For a well-chosen k factor, the resulting current of the two parallel differential pair is<br />

highly linear. Figure 116 shows that the resulting gm is linear on a wi<strong>de</strong>r range than Q1-Q2<br />

and Q3-Q4 differential pairs. However, the final gm is smaller than the sum of all individual<br />

gm.<br />

Figure 116. Linearization of the transconductance by unbalanced differential pairs<br />

Furthermore, it is also possible to add several differential pairs in or<strong>de</strong>r to enhance the<br />

linearization range. This method is <strong>de</strong>scribed in [<strong>III</strong>.9] for BiCMOS technologies.<br />

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