# Wideband matching network design

I have read about matching networks, authors usually will describe a target impedance to be presented, to say, the output of an amplifier, and then they design the matching network. To widen the bandwith they usually create a matching network with more sections, thus, lowering the general Q and presenting the same impedance to a wider range of frequencies.

But my question is this: What's the point on presenting the same impedance over a wide range of frequencies if the S-parameters (and thus the required load/source to be presented to the amp) change with frequency? For example the optimum source impedance for LNA design is not the same at different frequencies, in a power amplifier the optimum load varies with frequency, so whats the point on designing a matching network that presents the same load over a wide range of frequencies?, instead, shouldnt a network be designed to "track" those impedance changes, thus presenting the ideal source/load required for each frequency?, how is a network like that designed?

• I'm not familiar with the issue you're asking about, but I suspect a matter of scale. Say you have a system operating around 100 MHz. A simple matching circuit might give a good match from 98 to 102 MHz. A better matching circuit might give a good match from 90 to 110 MHz. If you tried to operate at 50 or 150 MHz, then the amplifier characteristics might change enough that you'd need a different matching circuit. Nov 7, 2017 at 3:31
• If you gave some concrete examples of the amplifiers you're thinking of, and the matching circuits you've seen proposed to use with them, it might make it more clear. Nov 7, 2017 at 3:32
• Thats actually my question, if I want to match over a wide range say between 100MHz to 800MHz, how does one design a matching network since the S parameters are different over frequency?
– S.s.
Nov 7, 2017 at 17:53
• Can you give an example of the amplifiers you're looking at? Nov 7, 2017 at 18:00
• FWIW, my field is high speed digital rather than classical rf, but we often want to amplify very broadband signals (like 100 kHz to 10 GHz). Our solution is "choose an amplifier that's well-enough-matched over the required band" rather than "design a matching network". Nov 7, 2017 at 18:42

I've developed a tool (based on the algorithms described in [2]) for matching a frequency dependent complex source to a frequency dependent complex load impedance. It is still WIP, but I think it works a treat. You'll have to compile that from the source and provide the impedances to match as s1p files.

https://github.com/Qucs/qucs/pull/555

On the other hand, Keysight ADS comes with a similar tool based on the RFT and the analytic methods.

Some references:

[1] Wideband circuit design. H.J. Carlin. 1998. CRC Press

Hope it helps

• I've read the work of Carlin and Yarman aswell both are incredibly complex texts, if im not mistaken they use the real frequency technique, I always though making a code using their algorithms was too laborious and eventually I gave up. AFAIK ADS doesnt have a tool which allows you to match a frequency dependent load. I appreciate your tool, but I would like to know more about the nuts and bolts on how to make them by myself.
– S.s.
Nov 9, 2017 at 1:47