MOSFET RF power amplifier design

[Buddhafollower]

I did more experiments this morning.
I added an additional diode in series with the first and increased the value of the resistor. Now the resistor is much larger, say 8.2kOhm and the idle current is much smaller, 280mA. Also in the 10MHz range the waveform appears blurred a bit for some reason... When i set the signal generator to 5Vpp, the sine wave is blurred in 10MHz. Also at 20Vpp in the same range the sine wave is blurred. Only at 10Vpp the sine wave appears normal or less blurred than before. From that point on there is a blurring of the waveform and because of this i don't think the amplifier can go any further.
Previously the idle current was way too great and the waveform was very good way after 10MHz. Now At 1MHz the sine wave is a bit bent towards right, an almost imperceptible bend, the idle current is much more acceptable and so the heating of the heat sink/.

[Buddhafollower]

In case of the AB class, the diode is there to set the bases at 0.7V or so. This is so that i won't need a voltage divider network. Probably made up of two or rather four resistors to give me 0.7V at the bases. It is in a way ''automatic biasing''.

[Buddhafollower]

Yes, i forgot to mention that it doesn't need an artificial load of 50Ohm. The transistors can do without that when they operate. You need the load only if you want to adapt an impedance.

[spacecase0]

your max voltage from the emitter to your base is 5V, so if you are putting in 10V or 20V, and the transistor is still working, then your waveform is is likely distorted as you are running into the max. current out of your signal generator.
you are going to need multiple stages of amplification with bipolar transistors to go from your signal generator to your final output.
this multiple amplifiers is not needed with mosfets, and that is the big reason why I like them better.

and I sent you the first part of that book,
could you make sure it is readable and get back to me,
if it seems good then I will send more chapters.

[Buddhafollower]

Since i added the second diode in series with the first, at 10MHz and beyond the sine wave is blurry.
I have to experiment more with this design.
In the beginning when the idle current was immense, the amplifier could go up to any frequency, even beyond 20MHz without distortion of the sine wave.
Now there is this issue i have to resolve.

[Buddhafollower]

You were right when you said that the device works without the diode. It does, but there are two problems: 1) is that the sine wave is a little bent to the right between 1-3MHz and this disappears when i lower the input voltage to about 2Vpp (from the signal generator) so i will have 0.707Vrms at the input. 2) the more i increase the idle current, at present is at 150mA, the further it goes up in frequency and the better the waveform seems to get.

[spacecase0]

good to here that you are figuring it out.

[Buddhafollower]

There are some bipolar transistors 2SC... people call them ''pills'' because they are rather small and white. My question is if you want HF then why don't you use IRFP250? or IRFP150? Or simply high power mosfets from IXYS? Why do people need to pay good money for these white transistors to get their high power out, when you can very simply and very well get HF and high power out of mosfets such as i have mentioned earlier? people truly have complicated minds.
I have two such transistors from IXYS called IXFB52N90P
https://www.littelfuse.com/~/media/e...asheet.pdf.pdf

I truly wonder sometimes what can you get out of these mosfets.
The only big problem this type of mosfet has is the input capacitance. I don't know if they would work in HF

[Buddhafollower]

This is a good one from IXYS:
https://www.tme.eu/Document/44812aae...Q%2960N20T.pdf
And another one that seems to be good, both for ZVS HF Oscillators and Power Amplifiers for HF:
https://www.tme.eu/Document/2ba08f3c...IXFH40N50Q.pdf

[spacecase0]

there are a few reasons why people will use bipolar and not mosfets,
likely the largest reason is that high power mosfets are relatively new on the market.
and most newer amplifiers I look at are mosfet types

[Buddhafollower]

This IXFB52N90P has an input capacitance of 19nF. How will it behave in HF or if it will reach so far, how much drive power will it require compared to a mosfet that has, say, 5nF input capacitance and almost the same characteristics as this one?
I bought these two because they are quite good as a ZVS Oscillator but of course, naturally i am afraid nowadays to build such a thing and to supply power to the Tesla transformers. Even a rather small such oscillator made with two IRFP250 when connected to the Tesla transformer, produced waves on the screen of the monitor. This is too much for me. One single experiment of this kind and it's enough. One can easily burn electronic devices around with this thing.

[spacecase0]

19nF at 20MHz is equivalent to 0.419 ohms
here is an online calculator to let you easily figure it out on your own.
https://www.66pacific.com/calculator...alculator.aspx
even at 2MHz it is 4.2 ohms, so usable there and lower frequency,
so my guess, just looking at that number, is that mosfet is meant for audio frequencies

what is your voltage in ?
1V, 2V ? (this is for your 19nF at 20MHz, I am going to round off the 0.419 to 0.5)
at 1V your power in will be about 2W
at 2V your power in will be about 8W

same thing for 5nF,
20MHz 1.6 ohms (I am going to round off to 1.5)
so, 1V is 0.66 W
2V is 2.66 W

[Buddhafollower]

19nF at 20MHz is equivalent to 0.419 ohms
here is an online calculator to let you easily figure it out on your own.
https://www.66pacific.com/calculator...alculator.aspx
even at 2MHz it is 4.2 ohms, so usable there and lower frequency,
so my guess, just looking at that number, is that mosfet is meant for audio frequencies

what is your voltage in ?
1V, 2V ? (this is for your 19nF at 20MHz, I am going to round off the 0.419 to 0.5)
at 1V your power in will be about 2W
at 2V your power in will be about 8W

same thing for 5nF,
20MHz 1.6 ohms (I am going to round off to 1.5)
so, 1V is 0.66 W
2V is 2.66 W

So here you used Ohm's Law for Power i see.
I thought that the smaller the input capacitance, the better and the higher this thing can go somehow.

[Buddhafollower]

The calculation i do not understand. If the frequency is 1MHz and the output capacitance is 780pF such as it is for IRFP250, what power will i get?

[spacecase0]

The calculation i do not understand. If the frequency is 1MHz and the output capacitance is 780pF such as it is for IRFP250, what power will i get?

yes, the smaller the capacitance the better
1MHz at 780pF is 204 ohms