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19th July 2019, 13:26 #1
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RF Mosfet Class E PA for Dummies or absolute idiots
I have about 100W RF PA with MOSFET about class E, and I would like to understand how such a thing is proposed.
I looked into books like this or this
frankly, more I understand from this
Thing simpler and for beginnings i will ignore everything before G MOSFET see in picture.
Frequency of PA is 13,56MHz and all power (sin) go to 50Ohm load.
RF Choke L1?
If I understand correctly
L1 must form a resonant circuit with the capacitor in MOSFET Coos.
Her I have first problem.
IXFH12N50F have Coss 290pF , but this is for Vds 25V And I have more. How do I find a Coss for 50V?
In any case, L1 is 10 turn on any white coated ferrite OD 24mm and I measure 10uH on 100kHz
And this have resonant frequency about 3MHz?
What can't I see where the problem is?
Impednce Matching networks.
This convert low output impedance of MOSFET Zin to other Zou I thing 50Ohm.
I understand that, but I don't know how to calculate impedance of MOSFET amplifier?
Or if possible how I measure this impedance without expensive instruments like network analyzer?
That's enough for a start.

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19th July 2019, 20:16 #2
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Re: RF Mosfet Class E PA for Dummies or absolute idiots
But that amplifier is not ClassE.. It seems ClassA
In order to understand ClassE Power Amplifiers, you have to read Grebennikov's "High Efficiency Power Amplifiers" textbook.There are more but it's good for beginner.

20th July 2019, 07:56 #3
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Re: RF Mosfet Class E PA for Dummies or absolute idiots
How do you judge class A?
Whistleblower that anything before G is just hinted and in reality I would expect the first stage of the amplifier with another transistor.
in any case, just a fool would make a RF amplifier of 100W 13.56MHz (most often induction heating) in class A, if it has a 100W output power, so at a typical class A performance of 35% would have to 185W to heat.
These amplifiers are class Cup.
Also, I do not understand much, but here is a typical case of amplifier in class E.
in my opinion, it is the same as in the black and white picture above. The first look is missing the capacitor Cv, but it is the internal parasitic capacity of the Coos MOSFET transistor.
Plus it's extra Impedance matching network for impedance adaptation of LPF impedance and mainly load impedance.
Without this element, some of the power would change to standing wave.

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20th July 2019, 08:41 #4
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20th July 2019, 13:50 #5
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Re: RF Mosfet Class E PA for Dummies or absolute idiots
Dear carpenter
Cv is not Internal Parasitic Capacitance of MOS, it's essential of ClassE Amplifiers.

20th July 2019, 14:51 #6
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Re: RF Mosfet Class E PA for Dummies or absolute idiots
I'm not saying it's necessarily, but it can be.
Cv is the sum of all capacities between D S of switching MOSFET (capacity in series with L1).
L1 + CV is designed as resonant LC circuit with resonst frequency = frequency of PA.
And this circuits can be designed with or without external Cv.
In this specific case. IXFH12N50F have Coos 290pF on Vds=25V and this increase with Voltage Vds. Maybe on 50V 330pF (estimate maybe more)
for f=13,56MHz and C 330pF is resonant with L=420nH.
After all, in the referenced document is in Table 1 calculated value Cv and L1 for Class E PA with IRF510 and for band 15M is external Cv missing (C is only IRF510 internal Coos 120pF) and L1 is 500nH 10T T506 core.

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20th July 2019, 15:08 #7
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Re: RF Mosfet Class E PA for Dummies or absolute idiots
Dear carpenter,
DS Capacitance is not a fixed capacitor, instead it's voltage depended variable capacitance and therefore using this parasitic element will cause some troubles.
The Switch Mode Power Amplifier Guru Mr. Grebennikov and Nathan Sokal ( founder of ClassE) have both mentioned this phenomena and therefore there is a "controllable" fixed capacitors in their textbooks and papers.

20th July 2019, 15:53 #8
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Re: RF Mosfet Class E PA for Dummies or absolute idiots
In this specific case. IXFH12N50F have Coos 290pF on Vds=25V and this increase with Voltage Vds.
   Updated   
See this IXYS application note for 13.56 MHz class E PA. http://ixys.com/Documents/AppNotes/CO1.pdf
It's using series resonant tank.
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