III. Gm-C Filtering - Epublications - Université de Limoges
III. Gm-C Filtering - Epublications - Université de Limoges
III. Gm-C Filtering - Epublications - Université de Limoges
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Idiff (A)<br />
levels reached with this technique are still poor, as it may be observed in Table 8, and very<br />
high currents are nee<strong>de</strong>d to reach high gm values.<br />
0,0015<br />
0,001<br />
0,0005<br />
0<br />
-0,0005<br />
-0,001<br />
-0,0015<br />
-0,5 -0,4 -0,3 -0,2 -0,1 0 0,1 0,2 0,3 0,4 0,5<br />
Vin (V)<br />
0,5mA<br />
1mA<br />
1,5mA<br />
gm_diff (A/V)<br />
- 97 -<br />
0,004<br />
0,003<br />
0,002<br />
0,001<br />
0<br />
0,5mA<br />
1mA<br />
1,5mA<br />
-0,5 -0,4 -0,3 -0,2 -0,1 0 0,1 0,2 0,3 0,4 0,5<br />
Vin (V)<br />
Figure 111. Linearization of the transconductance by current increase<br />
<strong>III</strong>.2.c Active and Passive Source Degeneration Technique<br />
The principle of active and passive source <strong>de</strong>generation (or emitter when <strong>de</strong>aling with<br />
BiCMOS technologies) is to use an active or a passive <strong>de</strong>vice, connected to the source of the<br />
MOS transistor to linearize the transconductance gain. In this configuration, illustrated in<br />
Figure 112, the impedance Z<strong>de</strong>g acts as a local feedback.<br />
Vin<br />
Iout<br />
gm<br />
Z<strong>de</strong>g<br />
Figure 112. Source <strong>de</strong>generation<br />
Using feedback theory, the equivalent transconductance of this stage can be computed.<br />
It gives:<br />
g m<br />
g m _ <strong>de</strong>g = . (<strong>III</strong>.33)<br />
1+ g Z<br />
m<br />
<strong>de</strong>g<br />
Hence, for gm.Z<strong>de</strong>g>>1, it can be approximated to<br />
1<br />
g m _ <strong>de</strong>g ≈<br />
Z<br />
, (<strong>III</strong>.34)<br />
<strong>de</strong>g<br />
which is very interesting since the transconductance can then be set by a single impedance.<br />
Resistors are usually used but they increase noise (as <strong>de</strong>monstrated in APPENDIX D). In<strong>de</strong>ed,<br />
the noise power is proportional to Z<strong>de</strong>g. That is why inductances are preferred. Feedbacks<br />
making use of transistors are used as well.