Re: Matching LDMOS device

Robin Szemeti - G1YFG

To stand a realistic chance of a predictive match using a Smith chart you really need all 4 S parameters.  I use SimSmith, as it makes things a bit easier ... but remember the input match "sees" a bit of the output match through S12 so  its not just a question of finding S22 and thats all you need, there is considerable interplay between the input and the output.

You do need the S parameters right at the device terminals though to produce a design from scratch,  VNA's with an S parameter test set come with sets of calibrations that allow you to calibrate and set the reference plane right up to the device terminals, compensating for all the cable lengths.

If you look on Page 7 of the datasheet, you'll find all the S parameters .. you could do worse than use SimSmith with an "S" block to represent the device ... you could simulate the exiting matching in SimSmith and then see what you need to add/take away to sort it out ...

On Sun, 14 Feb 2021 at 21:49, Clive, G3GJA <clive@...> wrote:

Can someone explain how to approach matching an LDMOS PA transistor. I’m repairing a 23cm PA that used a PCB cut from a power amplifier designed around a device internally matched for use around 1485MHz. The PCB had been snowflaked on the output line and a trimmer added to the input matching to get the amp to work at 23cm.


The original device was overdriven resulting in the gate insulation breaking down. The replacement, an MRF184, is not ideal but should still be usable.


I can look at the input with my miniVNA and extract the input impedance at 23cm; it’s 13.3 -j42.4. Is this valid? Is there an easy way to use an online Smith Chart simulator to work out what I need to do to get the input to look like 50R?


I presume that the reading is not much help as it is what is presented to the input socket and not the device itself. The problem isn’t helped by not knowing exactly what frequency the amplifier was originally designed for, so I’ve no idea what the existing matching is doing.


Where do I start?




Clive G3GJA

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Robin Szemeti - G1YFG

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