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MOSFET RF power amplifier design

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  • #61
    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.

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    • #62
      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.
      Last edited by Buddhafollower; 07-21-2021, 09:26 AM.

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      • #63
        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.
        Last edited by spacecase0; 07-21-2021, 10:41 PM.

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        • #64
          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.

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          • #65
            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?
            Attached Files

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