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LNA broadband matching - RF Cafe Forums
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Noise
Post subject: LNA broadband matching Posted: Wed Jan 31, 2007
4:39 am
Captain
Joined: Tue May 02, 2006 4:59 am
Posts: 15
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
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Joe
Post subject: LNA output matchPosted: Wed Jan 31, 2007 3:48 pm
Captain
Joined: Fri Feb 17, 2006 6:25 pm
Posts: 6
Location:
US
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.
If you have
gain to spare, a lossy match on the output can greatly increase bandwidth.
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.
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fred47
Post subject: MatchingPosted: Thu Feb 01, 2007 12:46
pm
General
Joined: Wed Feb 22, 2006 3:51 pm
Posts:
104
Hi!
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 Broadband Matching.
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
to mind.
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jaslovkel
Post subject: Posted:
Wed Jul 11, 2007 12:29 pm
Captain
Joined: Tue Jun
26, 2007 10:27 am
Posts: 11
Location: Dallas, TX
Hi Noise,
This is just an addition to the previous posts. Those guys have
made great points which are really good.
Another 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.
-J
Posted 11/12/2012
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