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

IV.3.a.iii Interests of a BiCMOS Technology
A BiCMOS technology is able to offer a higher transconductance in the differential
pair stage for a same current, as well as a high voltage gain, due to a higher gm/I than in
CMOS technologies. Hence, it may be possible to obtain the required 30dB voltage gain using
a simple OA topology.
Another interest in using NXP BiCMOS technology relies in the possibility to use
MIM (Metal-Insulator-Metal) capacitors. These capacitors, which use metal levels 5 and 6
separated by a high-κ material, exhibit several advantages such as a high Q-factor and a good
linearity. This is very interesting to obtain high quality capacitors banks which do not degrade
or limit the linearity of the filter. Indeed, the 65nm TSMC CMOS technology available at
NXP only allows the use of MOS capacitors characterized by a very high density of
integration but also a higher sensitivity to bias and process variations.
IV.3.b Operational Amplifier Design in 0.25µm BiCMOS
IV.3.b.i Initial Design
OA design in BiCMOS is also based on a two stages topology, as depicted in Figure
154. As said, a high gain-bandwidth product is required, as well as a high linearity level.
Current in the differential pair is set to 5mA. For the initial OA design, the follower is built by
means of a single transistor.
It is worth adding that for decoupling, only one Cdc capacitor connected to Vin- is
required since Cf, located in the feedback, also acts as a decoupling capacitor for Vin+.
Vin+
Vmirror
T3
T1
Rlp
VDD
bias bias
Clp
T0
5mA
T4
T2
- 138 -
Cdc
Vin-
VDD
Figure 154. Initial schematic of the OA in BiCMOS
Cc
T5
T6
If
Vout