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

III.2.f FDA//PDA technique
Reference [III.10] introduces the Multiple Gated Transistors (MGTR) technique. It
makes use of a fully-differential amplifier (FDA) in parallel of a pseudo-differential amplifier
(PDA), as shown in Figure 117.
Figure 117. FDA // PDA Gm-cell design
The output differential current Imd from a MOS FDA is a function of the bias current I0
and of the differential input voltage Emd, as demonstrated in APPENDIX D:
W
where β = μCox
.
L
1 4I
2
β Emd
(D.35)
2 β
0
I md = Emd
−
Developing this expression with a Taylor series up to the third order leads to:
I ≈ αE −γE
(D.36)
md
md
with α and γ being constant terms. Thus, the current from the FDA has a negative third order
term.
Due to the bias in the sub-threshold region, the differential output current Imd of a
MOS PDA is given by:
λ and ν being two constant terms.
Developing up to the third order gives:
3
md
ν λ md E
I md = e
(D.37)
I md
3
≈ aEmd
+ bEmd
(D.38)
Indeed, since the study is carried out in differential mode, second order terms are neglected.
- 100 -