I am working on solving a problem, namely I need to add input and output matching to my circuit and I am not sure where I should put them. Additionally, in the circuit I want to use a bandpass filter at 2.4 GHz - should it go before the output matching or after it? (Here is the aplication note btw: https://www.analog.com/media/en/technical-documentation/data-sheets/hmc414.pdf )

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I understand that the input impedance matching must be at the input of the circuit (as the name would suggest, but maye it's not that clear). In this case, where should the output impedance matching be placed?

And again - in the case of using a bandpass filter, should it be after TL3 and C3 lines? What about the matching at the output in this case? Should it be after this filter or before? I'm afraid that I will do something wrong.

  • 1
    \$\begingroup\$ Your question makes no sense. You talk about input impedance matching then ask if it (the BPF) should be after the TL3 and C3 lines and, of course, those components are related to the output. I'm confused. Then you don't say why you are trying to match and to what it is that you wish to match. More confusion. \$\endgroup\$
    – Andy aka
    Jan 12 at 21:09
  • \$\begingroup\$ @Andyaka I am writing about impedance matching at both the input and output, using an LC circuit. \$\endgroup\$
    – MagicMan
    Jan 12 at 21:13
  • \$\begingroup\$ Just re-read what you wrote and see how confusing it is. \$\endgroup\$
    – Andy aka
    Jan 12 at 21:14
  • \$\begingroup\$ I added BPF issue to the question name \$\endgroup\$
    – MagicMan
    Jan 12 at 21:14
  • \$\begingroup\$ Your final paragraph still reads wrong \$\endgroup\$
    – Andy aka
    Jan 12 at 21:15

2 Answers 2


The question seems to conflate and confuse multiple parts of the signal chain. They should really be considered one at a time where feasible.

At the input: the datasheet makes it clear that RFIN is already matched to 50 ohms and doesn't require additional matching:

This pin is AC coupled and matched to 50 Ohms.

Between amplifier and filter: On the amplifier output, TL1/TL2/TL3 are the 50-ohm matching + bias-tee as discussed on one of your past questions.

If your bandpass filter has 50 ohm input impedance, then you just put it downstream of C2. If the filter needs a different impedance or special matching, then you'll need to design a matching network that matches the amplifier's output to the input of the filter, perhaps performing calculations with the help of tools such as ADS or a Smith Chart.

After the filter: If the output of the filter is not already matched according to the filter datasheet, you'll need to design a similar matching network to match its output impedance to whatever load you plan to connect (likely 50-ohms if it's coax or a 50-ohm antenna). If the filter already has a 50-ohm output impedance, then you're done - connect it to your coax/50 ohm microstrip/load.


And again - in the case of using a bandpass filter, should it be after TL3 and C3 lines? What about the matching at the output in this case? Should it be after this filter or before?

As asked, this question is unanswerable because we would need to know everything about your design -- and then, we'd complain that you just want us to do your design for you.

The meta-question that goes with this is "how do I answer this question for myself?" The answer to that question is that you think up two or three ways that you might do it, you do "strawman" designs, and you evaluate each one. Then you choose the one that's best for your particular design effort. Next month you may design something at the same power level with the same part that uses a different scheme, because that's what makes sense for next month's design.

Note that this isn't an "advise to a beginner" thing -- this is what experienced designers do all the time. We do it with electronics, software, mechanical devices, etc. You name it -- if you can draw it down and analyze it for cost and fitness, you'll see engineers at every experience level doing exactly that to find the best approach.

I'm afraid that I will do something wrong.

If it works, it's right. If it works and doesn't blow your budget for parts, it's really right. If you build one and it blows up, you've just had a concrete learning experience*, so there's value to be had even if it doesn't work.

I suggest you try three things:

  • A matching network to the nominal impedance of your output, followed by a bandpass filter.
  • A bandpass filter, followed by a matching network.
  • A bandpass filter that also matches impedance with the output (this is a thing; it's especially easy if your bandpass filter is fairly narrow, and it reduces component count at the cost of making the design harder to understand).

* Or at least a very smoky learning experience.


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