Post subject: LNA designing help! Posted: Sun Aug 07,
2005 9:53 pm
Joined: Tue Aug 02, 2005
Hi, regarding to several
topologies of designing a LNA? The inductive degenation of common
source should be the most popular one right now, but the limitation
is the high voltage and not so good isolation. The common gate could
be better in applying low voltage and good isolation, as well as
input matching. Why few people use common gate topologies? If I
want to design a low voltage(<1.0v) low power, narrowband RF
LNA (abt 2.45GHz), among all the topologies, which could be more
suitable for me to choose? Can you give me some suggestions? Thanks
Post subject: common
gatePosted: Mon Aug 15, 2005 1:28 am
Mon Aug 15, 2005 12:55 am
My experience with common
gate/base LNAs are that they tend to be more unstable than the standard
common source topology. The biasing bypass on the gate/base is very
sensitive to parasitic inductances which tend to be a pain.
You will have a hard time using a FET based technology for under
1V. A bipolar might have a better chance of working (SiGe).
Post subject: LNAPosted: Mon Aug 15,
2005 11:51 am
The experience reported by "uwavegeek" matches
However, your thoughts about common-gate circuits are
in fact valid at lower frequencies (VHF/UHF), where you can actually
reduce parasitics to a low-enough level.
Uwavegeek is also
right about the 1V issue. It will even be difficult to do a really
good job with SiGe or other bipolar technology.