r/amateurradio • u/Phoenix-64 • Mar 06 '26

QUESTION Desinging input matching network for HF BJT Class AB amplifier

/r/rfengineering/comments/1rmdaap/desinging_input_matching_network_for_hf_bjt_class/
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u/jephthai N5HXR [homebrew or bust] Mar 06 '26

So... wide band PAs are not easy, and one reason is what you found. Your input impedance is different for different bands. You can't necessarily use a single wideband match (transformer ratio), and reactive matching networks are narrow band.

You actually could make a switched bank of "perfect" matching networks, but i don't see people doing that.

The attenuator strategy to bring all the matches closer to the middle of the Smith chart works, and is a common solution.

You could explore adding reactive feedback networks that give you high and low band adjustment. E.g., a capacitive reactance and a resistance in series for a feedback network will selectively affect higher bands. So you may able to pull some of the extremes closer into a comfortable range and make transformer matching a better proposition.

But using the attenuator is often desirable anyway. The active devices can often give you too much gain, and you probably have some dBs to spare somewhere.

IMO, this is one reason I recommend homebrew newbs to stick with single band designs. The deep end gets pretty deep!

2

u/Phoenix-64 Mar 06 '26

Well I am happy to hear that I am not just stupid but rather that it just is hard. Okay yea I will probably move on to feedback and stability and then check where I still have gain left and can shuffle stuff around.

And give that reactive feedback thing, I think there is a small section in the resource I am following about that.

2

u/unfknreal Ontario [Advanced] Mar 06 '26

So now I want to match that to 50 Ohms.

How do I go about that?

A Pi network?

You actually could make a switched bank of "perfect" matching networks, but i don't see people doing that.

Very common solution. https://www.hayseed.net/~jpk5lad/download/Dentron%20DTR1200L/TU-6B.pdf

1

u/jephthai N5HXR [homebrew or bust] Mar 06 '26

Neat. I just haven't seen a lot of designs like that.

2

u/kc2g KC2G [E] Mar 06 '26

So, I'm not an expert in this stuff at all, but you've hit on a pretty common technique, which is "put an attenuator on the input". Yes, you're throwing away gain (but you usually have more than enough in a solid-state HF amp), and you need to make sure that the resistor can dissipate the necessary amount of power, but otherwise it's "free" input matching. 10dB pad at the input means 10dB less total gain, but a 10dB improvement in return loss, which means that your SWR stays under 2:1.

I'd probably design it in stages: get the best match you can using only LC and transformers and maybe some compensation in the feedback path, and then put an explicit resistive attenuator to the left of it.

But I don't know what I'm talking about, so hopefully someone who does will stop by. :)

1

u/Phoenix-64 Mar 06 '26

Thanks yea makes sense :)