Matching circuit MOSFET

[ronald001]

I know that mosfets have a giving input and output resistance - and that it has to be matched to 50R - for example with coax or striplines - but how do you calculate these coaxes? ( resistance and lenght )
Is there a relatively simple calculation for it?
Also, is it possible to verify the calculated lenght with for example a NanoVNA?

I already tried to measure a output matching circuit on one of my test boards ( 2 pcs of 25R coax / 13CM each ) but did not seem to get logical results..

[Bton-FM]

http://k6jca.blogspot.com/2019/07/14-tr ... -coax.html

I read this yesterday rondald it was quite helpful.

[Bton-FM]

Also it’s impedance when talking about AC.

[ronald001]

Also it’s impedance when talking about AC.

Thanks for the link
And you are correct, made a translating error

[Bton-FM]

Also it’s impedance when talking about AC.

Thanks for the link
And you are correct, made a translating error

That's what I thought It was because i remembered you are dutch

[nrgkits.nz]

I use SimSmith do all the calculations for me, rather than messing around with complex mathematical equations involving reactive impedance.

You can get SimSimth here https://www.w0qe.com/SimSmith.html

It's very useful for simulating matching networks involving lumped elements and coax of any impedance etc... and its also free software.

Calculating the coax lengths and impedance for push pull amplifiers is very easy with SimSmith. You first need to draw out the circuit which I've already done - see the attachment.

Next you need to know the output impedance of the dual LDMOS device you're using (as seen from drain to drain - balanced configuration) - you can refer to the datasheet for this, or if its not there you can calculate it. To calculate it, take one half of the dual LDMOS device and use the formula which Albert has posted on the forums previously. Output impedance = VCC * VCC / 2 * Power out. Also since we are using one half of the LDMOS to do the calculation, we also use half the power. So for a 300W push pull amplifier operating with 50v, the equation is 50*50 / 2 * (300/2) = 8.33 ohms output impedance for one half of the dual LDMOS. Now since we are operating it in balanced push pull, the output impedance will double = 2 * 8.33 = 16.66. Its the same as connecting two resistors in series, the resistance doubles. This is not going to be exact but its close enough.

Now back to SimSmith, enter in the impedance on the left hand side (see my example), and then use the scroller on the mouse to change the length and impedance of the coax up and down while looking at the SWR, you're aiming for an SWR of 1.5 or less. Lengths are also in feet. The coax impedance can be any impedance, however for obvious reasons you'll need to stick to known values eg 25/35/50 etc...

300w.PNG

pish pull circuit.PNG

[Bton-FM]

How do you calculate the input impedance on a device?

[thewisepranker]

Nice one NRG, I was also going to suggest SimSmith. I think the best bit about it is that you can use the Smith chart, though. I really like the Smith chart because you can see very quickly where you are and the paths required to get to where you want to be, and ultimately the correct element to apply. For anyone unfamiliar with the Smith chart, it can appear quite daunting at first but with a small amount of practice it's very useful.

[thewisepranker]

How do you calculate the input impedance on a device?

You don't calculate it, you either assume it or you measure it.
You either assume and interpolate the data from an always incomplete and very granular datasheet, or you get a few samples and measure it for yourself if you've got the luxury of the equipment required to do it.
Some of the better datasheets like those from Philips have hyperlinks in them to more comprehensive data sets of the S parameters, but lots of the links are broken now.

[ronald001]

I use SimSmith do all the calculations for me, rather than messing around with complex mathematical equations involving reactive impedance.

You can get SimSimth here https://www.w0qe.com/SimSmith.html

It's very useful for simulating matching networks involving lumped elements and coax of any impedance etc... and its also free software.

Calculating the coax lengths and impedance for push pull amplifiers is very easy with SimSmith. You first need to draw out the circuit which I've already done - see the attachment.

Next you need to know the output impedance of the dual LDMOS device you're using (as seen from drain to drain - balanced configuration) - you can refer to the datasheet for this, or if its not there you can calculate it. To calculate it, take one half of the dual LDMOS device and use the formula which Albert has posted on the forums previously. Output impedance = VCC * VCC / 2 * Power out. Also since we are using one half of the LDMOS to do the calculation, we also use half the power. So for a 300W push pull amplifier operating with 50v, the equation is 50*50 / 2 * (300/2) = 8.33 ohms output impedance for one half of the dual LDMOS. Now since we are operating it in balanced push pull, the output impedance will double = 2 * 8.33 = 16.66. Its the same as connecting two resistors in series, the resistance doubles. This is not going to be exact but its close enough.

Now back to SimSmith, enter in the impedance on the left hand side (see my example), and then use the scroller on the mouse to change the length and impedance of the coax up and down while looking at the SWR, you're aiming for an SWR of 1.5 or less. Lengths are also in feet. The coax impedance can be any impedance, however for obvious reasons you'll need to stick to known values eg 25/35/50 etc...

300w.PNG
pish pull circuit.PNG

Very detailled explanation Mr.NRG - thanks!
Will try to follow the steps above and see if i can calculate it aswell with RFSIM

[Bton-FM]

How do you calculate the input impedance on a device?

You don't calculate it, you either assume it or you measure it.
You either assume and interpolate the data from an always incomplete and very granular datasheet, or you get a few samples and measure it for yourself if you've got the luxury of the equipment required to do it.
Some of the better datasheets like those from Philips have hyperlinks in them to more comprehensive data sets of the S parameters, but lots of the links are broken now.

What data can I use to Interpolate the input impedance and how?