MOSFET RF power amplifier design

[Buddhafollower]

There has got to be a logical way out of this. There are two capacitances, an input and an output one. I suppose the input capacitance has no influence over the power gain of the mosfet since there is hardly any current that goes from the gate to the source. The output capacitance is important but have i a way to find out the current? I don't know. There has to be a way to find out this current from drain to source.

[Buddhafollower]

Many times before in the past i have seen that the datasheets for different bipolar junction transistors and mosfets lack different things. For example some soviet bjt's lack the beta parameter. Many bjt datasheets lack the basic Ic to Vce graph with which you can design class A amplifiers. It is as if they have been written by mentally handicapped individuals. Things are going from bad to worse and will definitely get worse before they get any better so i'am not at all surprised that this is the situation.

[spacecase0]

sorry, I had misread, I did not see you had said "output", I had calculated input...
input capacitance determines how much power in you need to drive it,
output capacitance is another thing entirely
ever shut off a mosfet sharply and seen the voltage give a ringing effect ?
that output capacitance is critical there
read this
https://www.ti.com/lit/ml/slua618a/slua618a.pdf
it is pretty good if you need to learn all of that.
if you need more info, look up snubber circuits,
I personally don't like snubber circuits,
just seems like there are better ways to deal with the issue.

as far as incomplete data specifications,
I have never seen one with all the specifications for any part.
some get close, and some are really bad.
even if you had one totally correct and complete,
batch to batch, things change,
you have to either have forgiving designs or test what you actually get when you buy them.

edit:
input capacitance sets how fast you can turn on a mosfet with a given input current.
so it does matter if it it to large.
and some of the parts you mentioned would take a substantial amount of power to work at the frequencies you mentioned,
and I have never seen a specification for watts in to the gate,
but you have to wonder if you are putting 5W into a gate, can it deal with that ?

output capacitance determines how the mosfet interacts with your load.

[Buddhafollower]

www.modernhamguy.com

I wish this man will do a series of tutorials about the Class AB amplifiers with bjt and mosfets. You know, the type of power amplifiers we all like to have around.

[Buddhafollower]

Please take a look at this schematic diagram. It is said that the capacitor is there so that the transformer is at signal ground even though it may not be at DC ground. What does this mean and how can I compute that capacitor?

[Buddhafollower]

In this image below there is a power stage with two NPN transistors.
There is a resistor in series with a capacitor that goes from each collector to each base. I believe this to be negative feedback. Based on what can I calculate the values of the components?

[spacecase0]

start on page 55 of the book I am sending you.
that will let you calculate the resistor value,
why is the capacitor is there ?
I am not totally sure, it is going to make sure the feedback is smoothed out
maybe a stability thing ?
value is likely needs to be large enough,
I would likely choose 10 times the value when you convert the resistor you chose to impedance
so if the resistor is 10 ohms, choose a cap. that is 1 ohm reactance at your lowest frequency. (or something like that)
that math would be true in each schematic you posted

[spacecase0]

go look at the last page I sent you today, it shows the capacitor in that spot.
if they don't tell you how to caculate it, then my calculations are likely correct

[Buddhafollower]

In this image below there is a power stage with two NPN transistors.
There is a resistor in series with a capacitor that goes from each collector to each base. I believe this to be negative feedback. Based on what can I calculate the values of the components?

[spacecase0]

In this image below there is a power stage with two NPN transistors.
There is a resistor in series with a capacitor that goes from each collector to each base. I believe this to be negative feedback. Based on what can I calculate the values of the components?

I think that my answer in my post (#67) in this thread applies here as well
or at least for the capacitor value
I could write it out better if it does not make sense.

[Buddhafollower]

This negative feedback i imagine has the advantage of increasing the bandwidth of the stage i guess. But how can i calculate the value of the resistor? I mean based on what parameters? Because as you said the capacitor will be one that has an reactance of 1Ohm at the lowest frequency which in my case is 1MHz and so the capacitor will have a value of 150nF. 150nF capacitor has a reactance of 1.1 Ohm at 1MHz which for me is close enough. And so in this case what will the value of the resistor be? Ten times the impedance of the capacitor at the lowest frequency (1MHz)?
Sorry for writing in big letters, it is quite hard for me to focus my eyesight when the letters are 12.

[Buddhafollower]

http://www.zen22142.zen.co.uk/Design/lazarnfb.htm

Here it says that for a certain amplifier, the feedback equation is this one:

Rfb = ((Vcc / 2 ) - Vbe) x B1 / I1

I took my 15.5V and divided it by 2 so i got 7.75Vdc from which i subtracted 0.7V so i was left with 7.05V. Then the beta of the transistor 2SC3264 is 140 and this value i divided by the current for one transistor which is about 80mA. So overall i got a resistance of 12Ohm.
And so if what you said is correct, the i will have a resistance of 12Ohm or 10Ohm in series of 150nF capacitor.
Is this right?

[spacecase0]

at least as font,
there is a default here,
if your text is larger I will likely not notice at all.
based on "based on what parameters",
I played with default font and it seems to be 14 here,

so,
seems as if you have figured it out...
my only question is if you got the idea that if you are using a transformer,
and a center tapped one,
did you think about how the individual transistor is a pair, and needs half the impedance as the entire system
anyway,
I could go through all the math on it,
but I already have a full time job that pays me almost nothing, (but I have lots of stability here)
I am so worn out every day,
and can't even afford hot water to clean up at night,
so,
I can answer your question for sure,
but it will be hard on me, might take me a week to figure it out.
so,
my recommendation is to do your best math and then go try it.
trying things is going to tell you answers.

[Buddhafollower]

I am so sorry you have to go through all this... I too have some problems with money.... I just don t have enough even to buy the necessary components i need for my projects. Not being able to do science as i would like really affects me so much so that i wish i were dead. Hopefully all this cheap game won t last too long from now on. This is a really sick world.

I am not sure i understand your question.
I cannot afford to pay you for your help nor anyone else. I barely afford to make a living. All these questions of mine simply because i mainly want to realize my dream of building a power amplifier with which to be able to power up a Tesla transformer so i can send electricity wirelessly. Just through the Aether, no ground wire. So far my amplifiers were unable to achieve this desideratum. The system needs the ground connection.... I don t know why.
Poor idiot....

Like i said, i do not understand your question well enough. It seems like something is missing.
So each transistor has assigned one half of the power transformer.

[Buddhafollower]

He he he I tried it by my calculations. The thing does not work.