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MOSFET input and output impendance matching
Posted: Fri Jul 16, 2021 9:01 am
by ronald001
I am wondering for some time now, how you can determine or calculate the in/output impendance match.
Usually a transformer is used at the input, and 2 pieces of 25 Ohm coax + 50 Ohm coax is used at the output.
This raises the following questions:
- I understand the transformer part at the input, and the 2 pieces of 25 Ohm coax at the output. But what is the purpose of the 50 Ohm coax?
- With what formula can be determined what the lenght should be of the 25 Ohm coax?
- (1) With the following data that can be found in the datasheet, how can you determine the match?
- (2) Is the formula for the above Z=R+jX / R=Z-jX?
Re: MOSFET input and output impendance matching
Posted: Sat Jul 17, 2021 12:47 am
by sinus trouble
To put it as basic as possible?
The S-parameters are to plot actual impedance of the device at a specific frequency on the smith chart (Circle thing) The end goal is to transform that to match a 50 Ohm (Center line) impedance.
The centre line of the Smith Chart is the resistive impedance! Following the lines downwards will lead to capacitive reactance! Following the lines upwards will be inductive reactance!
Either way you do it would most likely work? But inductance and capacitance will need to be somewhat proportional.
There is no 'Formula' that i know of? You would need to know the specification of the coax or components you are using?
I would highly recommend using Sim Smith to give you an idea of whats going on in a tuned circuit!
Good Luck!
Re: MOSFET input and output impendance matching
Posted: Sat Jul 17, 2021 12:39 pm
by Bton-FM
The 50 ohm coax is a balun combining the two sides of the dual MOSFET (or LDMOS). It also provides an impedance transformation.
+1 to what Sinus said. SimSmith is a really great tool to help calculate impedance matching networks. It’s quite intuitive to use and it makes it easy to visualise how you get from one impedance to another.
Re: MOSFET input and output impendance matching
Posted: Sun Jul 18, 2021 1:23 am
by sinus trouble
Bton is correct!
The LDMOS is basically two N channel FETs which need to be operating out of phase of each other! Hence the way that the two 25 ohm coax transmission lines are configured!
The 50 ohm coax balun (plus a few capacitors) forms the output match!
Re: MOSFET input and output impendance matching
Posted: Mon Jul 19, 2021 8:33 am
by ronald001
Thanks Sinus and Bton!
- So i know what a Smith chart is, but didn't know that the 50 Ohm coax was part of the balun.
- I also know that LDMOS is basically a 2 channel FET, and that you need to combine them with the 25 ohm coax. But they need to be combined out of phase? Since the 2 two pieces of 25 Ohm coax have the same length... the signal should be IN phase?
- I will have a look at SimSmith, thank you for the suggestion.
In the above screenshot, the output impendance is known as Zsource, and it contains 2 value's ( Z and J )
Since coax has a specific R value at a giving lenght and frequency - how does this value relates / how do you transform this value to Z and J?
In other words, do you need to recalculate Z and J to one R value to determine the lengt of the coaxes?
Re: MOSFET input and output impendance matching
Posted: Mon Jul 19, 2021 8:34 am
by ronald001
ronald001 wrote: ↑Mon Jul 19, 2021 8:33 am
Thanks Sinus and Bton!
- So i know what a Smith chart is, but didn't know that the 50 Ohm coax was part of the balun.
- I also know that LDMOS is basically a 2 channel FET, and that you need to combine them with the 25 ohm coax. But they need to be combined out of phase? Since the 2 two pieces of 25 Ohm coax have the same length... the signal should be IN phase?
- I will have a look at SimSmith, thank you for the suggestion.
In the above screenshot, the output impendance is known as Zsource, and it contains 2 value's ( Z and J )
Since coax has a specific Z value at a giving lenght and frequency - how does this value relates / how do you transform this value to Z and J?
In other words, do you need to recalculate Z and J to one Z value to determine the lengt of the coaxes?
Re: MOSFET input and output impendance matching
Posted: Mon Jul 19, 2021 8:34 am
by ronald001
ronald001 wrote: ↑Mon Jul 19, 2021 8:34 am
ronald001 wrote: ↑Mon Jul 19, 2021 8:33 am
Thanks Sinus and Bton!
- So i know what a Smith chart is, but didn't know that the 50 Ohm coax was part of the balun.
- I also know that LDMOS is basically a 2 channel FET, and that you need to combine them with the 25 ohm coax. But they need to be combined out of phase? Since the 2 two pieces of 25 Ohm coax have the same length... the signal should be IN phase?
- I will have a look at SimSmith, thank you for the suggestion.
In the above screenshot, the output impendance is known as Zsource, and it contains 2 value's ( Z and J )
Since coax has a specific Z value at a giving lenght and frequency - how does this value relates / how do you transform this value to Z and J?
In other words, do you need to recalculate Z and J to one Z value to determine the lengt of the coaxes?
Re: MOSFET input and output impendance matching
Posted: Mon Jul 19, 2021 11:51 pm
by sinus trouble
ronald001 wrote: ↑Mon Jul 19, 2021 8:34 am But they need to be combined out of phase? Since the 2 two pieces of 25 Ohm coax have the same length... the signal should be IN phase?
My Mistake, I think you are correct!
After looking at a schematic, The input transformer is centre tapped causing the dual FET to conduct with a 180 degrees phase shift from eachother.