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

D.2 MOS Degenerated Common-Source Circuit
Figure 206 depicts the schematic of a degenerated MOS transistor. The equivalent
transconductance of this stage is given by [D.1]:
gm
1
gm,
eq = ≈ if Zdeg is large enough
⎛ 1 ⎞ Z
(D.24)
deg
1+
Z deg ⎜ gm
+
r ⎟
⎝ 0 ⎠
Figure 206. Degenerated MOS transistor
The equivalent input noise power of such a circuit can be computed. First let’s
compute the output noise current, which is:
gmVn
I n,
out = (D.25)
1+ g Z
m
deg
Now, assuming only thermal noise, one can obtain :
2 2
Vn , in = 4kT 3g
( 1+
gm
Z deg )
(D.26)
Hence, noise increases as Zdeg increases.
m
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