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How to Improve the IM3 - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues. Original posts:

Amateur Radio | Antennas | Circuits & Components | Systems | Test & Measurement

Post subject: how to improve the IM3 Posted: Wed Jun 10, 2009 2:57 am


Joined: Sun Jun 07, 2009 9:14 am
Posts: 4
I am designing an amplifier
Pout:>25W(CW) @IM3<-25dBc
The circuit and picture as following:
the Mosfets are two MRF9045.
the amplifier's IM3=-26dBc(at)270Mhz(at)25W(CW)
but the amplifier's IM3=-18dBc(at)400Mhz(at)25W(CW)
How can I improve the IM3?


Post subject: Re: how to improve the IM3Posted: Sun Jun 14, 2009 11:40 pm


Joined: Sun Jan 11, 2004 4:44 pm
Posts: 8
Hey Tiany,

That's a nice project you have going there! I'm not the world's foremost expert on amplifier design, so I'm just shooting from the hip here. Of course one way to increase IP3 performance is to increase the current, but I assume you've already considered that. The next thing is to look for lossy components that are wasting the current you have available. I cringe just a bit at those coax transmission lines having such small radius bends when passing through the ferrites. Have you confirmed that they are not causing reflection problems? That could be a huge source of performance loss if they have really bad S11 or S21 because of that radius. Just a though - you've probably already thought of that too, but just figured I'd throw it out there.

Like I said, nice job!


Post subject: Re: how to improve the IM3Posted: Mon Jun 15, 2009 9:40 am


Joined: Wed Apr 08, 2009 11:07 am
Posts: 33
That is the subject of very many recent research papers!

Obviously, the bias point of the devices is critical. You can try varying the bias to get some improvement.

Output match is important, so try varying that. 2nd, and even 3rd harmonic match canbe important also, although your broadbandwidth will make that interesting.

Your choice of devices matters, try a different device. Some are more linear than others.

Then there are a raft of more complex methods, such as Doherty amplifiers, feedforward amplifiers, or predistorters. I would be tempted to try a predistorer (you can make one up using some diodes and a transformer).


Maguffin Microwave Consulting


Post subject: Re: how to improve the IM3Posted: Wed Jul 29, 2009 11:22 am


Joined: Wed Jul 29, 2009 11:11 am
Posts: 1
hello all,

i am susan here, i am interested in this post, but i too have the problem to improve the im3 so please any let me know about my problem.

thanks alot



Post subject: Re: how to improve the IM3Posted: Thu Aug 06, 2009 2:09 pm


Joined: Wed Apr 08, 2009 7:53 am
Posts: 12

I think the problem is hat you are having a small P1dB.. In the P1dB, the IM3 is -30dBc.. and you are having this just at 30W, where ideally you should have it at 90W. This cold be for many reasons, but perhaps the most important ones are: bias pioint and output matching.
First, i´ll try to find the best bias point for the implemented matching, if this doesn´t solve the problem, then i think you should review the output matching. Are you sure you are matching the output for the highest P1db?

Take care


Posted  11/12/2012

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