Bton-FM wrote: ↑Thu Apr 23, 2020 3:56 pm Also it’s impedance when talking about AC.

ronald001 wrote: ↑Thu Apr 23, 2020 8:41 pm

Bton-FM wrote: ↑Thu Apr 23, 2020 3:56 pm Also it’s impedance when talking about AC.

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

Bton-FM wrote: ↑Sat Apr 25, 2020 7:01 pm How do you calculate the input impedance on a device?

nrgkits.nz wrote: ↑Sat Apr 25, 2020 1:57 pm 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

thewisepranker wrote: ↑Sun Apr 26, 2020 2:30 am

Bton-FM wrote: ↑Sat Apr 25, 2020 7:01 pm 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.