LNA designing help! - RF Cafe Forums
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Post subject: LNA designing help! Posted: Sun Aug 07, 2005 9:53 pm
Joined: Tue Aug 02, 2005 1:44 am
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 very much.
Post subject: common gatePosted: Mon Aug 15, 2005 1:28 am
Joined: 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 mine.
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.
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