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    UHF Amplifier Matching Woes

    Hi all!

    I'll preface this all by saying while I'm familiar with most of the basics of RF & PCB design, I'm entering new territory here and am definitely well outside my comfort zone.

    I'm working on a design based on the RD15HVF1 transistor from Mitsubishi. It claims 15W of power from VHF through to 500 MHz, and it seems like a perfect choice for my application (small mobile radio amplifier running on 12V).

    Using ADS, I'm trying to get matching set up using the supplied S parameters from Mitsubishi. On my VHF circuit (138-152 MHz) I was able to get a great match with relative ease. I just used the optimization tool and tried for best S11 and S22, and that seemed to do the trick.

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    (plotted results and circuit diagram for VHF section)

    However, trying the same method on my UHF design (430-450 MHz) is causing me headaches. I can't get my optimization to work, and I've tried several different layouts of matching networks including simple LC and Pi/Tee. My lack of experience is showing here, and I don't have the intuition required to "know" what kind of matching circuits would work best here.

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    (not-so-great UHF matching attempt)

    I'd appreciate any suggestions or advice! This is my first amplifier design project and I'd love to be able to get it to work with minimal fuss.

    Thanks!
    Last edited by axel3; 27th June 2019 at 16:18.

  2. #2
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    Re: UHF Amplifier Matching Woes

    You're trying to match this BJT with a simple L-C matching circuit but there is a sample app. circuit in its datasheet and they matched this BJT with TL-C pairs.You matching circuit is relatively narrow-band and you used the same technique for UHF band.I believe you have to inspire by this app. circuit ( Transmission Line -C concept ) to adapt the circuit onto your requirements.In order to realize, you should draw a Q curve on Smith Chart and start from Input/Output Impedance and walk with small steps while staying within Q curve.
    There are app.notes of Motorola that explains this method very well..They are still available..



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    Re: UHF Amplifier Matching Woes

    Quote Originally Posted by BigBoss View Post
    You're trying to match this BJT with a simple L-C matching circuit but there is a sample app. circuit in its datasheet and they matched this BJT with TL-C pairs.You matching circuit is relatively narrow-band and you used the same technique for UHF band.I believe you have to inspire by this app. circuit ( Transmission Line -C concept ) to adapt the circuit onto your requirements.In order to realize, you should draw a Q curve on Smith Chart and start from Input/Output Impedance and walk with small steps while staying within Q curve.
    There are app.notes of Motorola that explains this method very well..They are still available..
    I apologize but I'm not sure what you mean by a Q-curve. Perhaps if you had links to the application notes that would be helpful. I would prefer to stay away from transmission line matching because of the space requirements needed, but if it's the only way to get a good match I suppose I don't have much of a choice. Is there a specific reason why standard LC matching won't work? The only reason I could think of is if it would require impossible component values.

    Like I said before I'm very new to practical RF design like this so any help or directions to good resources would be greatly appreciated. I've done a decent amount of reading on the topic of MOSFET amplifier design but it hasn't managed to make things much clearer.

    Thanks for the advice. I'm going to try and find some more resources but I'm still looking for any help from the community here. I thought low-power RF amp design would be relatively easy; seems that's not at all the case.



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    Re: UHF Amplifier Matching Woes

    Transmission Lines are not necessary, you can also use equivalent L-C components instead of them.But the problem is here that you're trying to match an Power Amplifier for a single frequency even for 175MHz.As you may see, S11 and S22 are not very good, they satisfy the requirements for a narrow-band.I'm saying that use cascaded L-C matching circuits to obtain more consistent result.
    See textbooks/literature for Q curve on Smith Chart...
    You can also use Matching Wizard in ADS by defining s-parameters , ADS will give you better matching circuit.
    Also, Power Amplifiers are not designed for small signal s-parameters only.You must use also nonlinear/large signal model to obtain Optimum Load Impedance and other stuffs ( PAE,Optimum Gain)



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    Re: UHF Amplifier Matching Woes

    I originally tried to use the Matching Wizard in ADS, but it only works for one impedance at a time (as far as I could tell) and for an amplifier both sides of the device need to be tuned at the same time since they interact with each other. I could get each side of the FET matched individually with the wizard, but when you bring them together the impedance is thrown way off.

    I'm doing some research on constant Q lines now and it seems to make sense. I fear I'll run into the same issues the optimizer was - changing one side of the FET affects the other side and you get stuck chasing changes back and forth.

    If ADS can be used better to form the matching networks automatically, that would be great. I haven't found too many resources in my searching so far.

    But the problem is here that you're trying to match an Power Amplifier for a single frequency even for 175MHz.
    Can you explain what you mean here? I think the language barrier is a bit of an issue and I want to make sure we're on the same page as far as understanding what my goals are.

    Thanks!



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    Re: UHF Amplifier Matching Woes

    My sentence structure was a bit lossy above..
    I meant that, S11 and S22 are both not good enough even for 175 MHz.
    I suggest you to use Simultaneously Conjugate Matching technique at the beginning of this design.So, bot Input and Output will be conjugately matched as much as possible.( see ADS Help for that how to do it )
    But then don't forget to consider nonlinear techniques for this PA.



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    Re: UHF Amplifier Matching Woes

    As BigBoss says the old Motorola application notes are still available. They may be old, but should be in your library
    Try searching for
    motorola amplifier application notes
    motorola power amplifier application notes
    constant q amplifier matching

    Some of these might help you
    https://www.highfrequencyelectronics...09_Bichler.pdf
    http://www.bitsavers.org/components/...tions_1994.pdf
    http://mwl.diet.uniroma1.it/people/p...Amplifiers.pdf
    https://cdn.macom.com/applicationnotes/AN721.pdf


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    Re: UHF Amplifier Matching Woes

    Thanks for the advice everyone. What I hadn't even considered was that the device I had chosen isn't really good at my UHF frequencies anyway. Check out the S21 for the 440 band - it's only about 2.5dB or so!

    My VHF design for this chip seems to match up great, so I'll use that for the VHF side, but I'll be looking for a different chip for the UHF side - the AFT09MS015 from NXP looks very promising.



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    Re: UHF Amplifier Matching Woes

    Alright, I'm getting frustrated with myself as I can't seem to wrap my head around what should be pretty basic topics.

    I can get a great match to the input impedance at a give frequency using the ADS optimizer, or just doing it by hand. Same goes for the output impedance. I'm just very confused on how you can apply these circuits together, as when I bring them together into one simulation I can never get either side to stay at a good match. The document I've been referencing the most is the third PDF linked by G4BCH above. The author gives a good overview of one-sided impedance matching but seems to hand-wave over a lot of concepts when discussing how you match both sides of an active device.

    I'm going to keep working at my UHF design using the new AFT09 chip. I've been trying to read over the sources linked, but it's been very overwhelming to take all the information in especially when I've only been exposed to a few RF courses in my undergrad degree.

    I do want to thank you two for your assistance, and I hope that by the end of this design process I'll actually have learned something. For now, though, I'm just frustrated and confused and I hope that you can bear with me.



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    Re: UHF Amplifier Matching Woes

    Success!

    I stumbled across a PDF describing a novel way of matching LDMOS transistors in medium-power applications by W1GHZ: Simple Broadband Power Amplifiers.pdf. I figured "hey, I'm working with an LDMOS transistor!" so I used the full model provided by NXP for the AFT09 FET and built up a full bias & matching circuit using the resistor feedback method. The results speak for themselves:

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    (check out that SWR in the lower right)

    This method worked so well, I might try it out on the VHF design as well just to see how it performs.



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