LNA Broadband Matching - RF Cafe Forums
Post subject: LNA broadband matching Posted: Wed Jan 31,
2007 4:39 am
Joined: Tue May 02, 2006
I have question regarding LNA matching.
I need to match the output LNA to conjugate matching for best
output return loss ( S22).
The frequencies BW is very wide
and it is difficult for me to match to all BW.
Does anyone know
any good techniques for broadband matching?
I am using smith
chart utility as part of ADS.
Thanks in advance
Post subject: LNA output matchPosted:
Wed Jan 31, 2007 3:48 pm
Joined: Fri Feb
17, 2006 6:25 pm
You didn't mention
what your frequency, bandwidth, or matching requirements are.
If you want a perfect 50 ohm match (1.0:1 VSWR) you must conjugate
match at one frequency. This type of match will have good match
over a small percentage bandwidth. Increasing the number of stubs
or matching elements will extend the bandwidth (20 - 30%max).
Using two amplifiers between 90 degree hybrids in a balanced
configuration gives good match over a 3:1 bandwidth.
have gain to spare, a lossy match on the output can greatly increase
The simplest method is to put an attenuator on
the output. The return loss will improve by twice the value of the
attenuator and the gain will be reduced by the value of the attenuator.
Post subject: MatchingPosted: Thu
Feb 01, 2007 12:46 pm
Joined: Wed Feb
22, 2006 3:51 pm
Just a reminder - there
are theoretical limits to the bandwidth that can be covered for
a given return loss with non-lossy components. That's caused by
the reactance (usually capacitive) that is part of (one of) the
impedances you're trying to match.
Herbert Carlin was the
engineer who worked a lot of that out - he's written a couple of
books that include that information. If memory serves, one is titled
So if you have extra gain or power available,
Joe's answer - a pad on the output - is good. Otherwise, you have
your work cut out for you.
Software is available - Peter
Abrie (via Artech House) and Thomas Cuthbert (via his website) come
Post subject: Posted:
Wed Jul 11, 2007 12:29 pm
Jun 26, 2007 10:27 am
Location: Dallas, TX
This is just an addition to the previous posts. Those
guys have made great points which are really good.
technique (which may just be an addition or adjustment to the techniques
already discussed) is to basically design a filter using passive
components as the output stage. What I mean is, you know what your
optimum LNA load impedance is and you can use this to design the
first stage of your broadband filter. Then, based on gain requirements
and what your actual load impedance is, you can calculate the other
component values based on required/allowed ripple, stop band rejection,
corner frequencies, etc. This technique has been used in some designs
for broadband input matching, but it can be used for output matching
as well. The big drawback to this is the area you will consume from
using large passive devices.