Re: Matching LDMOS device

geoffrey pike

Hi Clive,
You dont say what the original device was in this case. The VNA reading is what not the old device as its broken? anyway the if its the old device its too high and the sign is probably wrong.
Anyway what to do, i recently did something at 2.4 GHz with a LDMOS device on a blank canvas Rogers 4003 i think
 I used an old DOS program called Puff from CalTech (there must be a modern equivalent)
So you obviously need to have some impedance info from the the data sheet and the substrate Er.
The process i use is to take the the complex input impedance and initially match it to the geometric mean of the source impedance and device impedance, in this case about 6 ohms, then match this to 50 ohms. So both the input and output are transformed in 2 stages and not directly from 0.7 ohms to 50 ohms or 1.6 ohms to 50 ohms.
The Puff program will give you the length of the low Z transmission line (try around 10 to 12 Ohms to give reasonable line dimensions) that you need to take the input or output to this level and real. Then transform to 50 ohms with another line.
All very vague but it will do it. Puff will take the complete s-parameter file and then you can try it over the F range you are interested in, at this point S12 will raise its head and the values you got for the input and output match need to be fine tuned with either flakes or strips previously etched onto the pcb. As S12 is 0.007 you may see only a small effect.
Puff runs under Windows XP but i haven't yet managed to get it to run under Windows 10.

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These 2 quick look examles use a single low Z line to do the transformation and not 2 as mentioned previously, i used the data for 1GHz at 1,3GHz. Worth a try ! ( the output line was shortened to allow a trimmer to finish the transformation)

On Sunday, 14 February 2021, 21:49:42 GMT, 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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