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LNA broadband matching - RF Cafe Forums

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Post subject: LNA broadband matching Posted: Wed Jan 31, 2007 4:39 am


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



Post subject: LNA output matchPosted: Wed Jan 31, 2007 3:48 pm


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.



Post subject: MatchingPosted: Thu Feb 01, 2007 12:46 pm


Joined: Wed Feb 22, 2006 3:51 pm

Posts: 104


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.



Post subject: Posted: Wed Jul 11, 2007 12:29 pm


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.


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