EMRFD Message Archive 598

Message Date From Subject
598 2007-04-05 06:06:00 Ed Almos Input Capacitance of Large MOSFETs
I am considering the use of two IRF740B devices in the final of an HF
power amplifier. Normally this would not be a problem but examination
of the datasheet shows device input capacitance as 1400 pF. This is
not good, especially on the bands above 14MHz.

Suggestions concerning how I should deal with this problem would be
appreciated.

Edward Almos HA6SST
599 2007-04-05 06:14:07 DSNman@comcast.ne... Re: Input Capacitance of Large MOSFETs
Attachments :
    Ed,

    That capacitance would suggest that the devices may not good for a very high frequency.

    --
    Loren Moline
    WA7SKT

    -------------- Original message ----------------------
    From: "Ed Almos" <edwardalmos@gmail.com>
    > I am considering the use of two IRF740B devices in the final of an HF
    > power amplifier. Normally this would not be a problem but examination
    > of the datasheet shows device input capacitance as 1400 pF. This is
    > not good, especially on the bands above 14MHz.
    >
    > Suggestions concerning how I should deal with this problem would be
    > appreciated.
    >
    > Edward Almos HA6SST
    >
    >
    601 2007-04-05 07:05:06 Shawn Upton Re: Input Capacitance of Large MOSFETs
    Be careful. The input capacitance is not linear--it
    changes as the FET goes from off to on hard. Might
    not be an issue for a linear amp though.

    If this is for a linear amp, look further into the
    datasheet; there should be a plot showing gate charge
    versus Vgs. It will have a linear ramp for the FET
    being off and then Ids ramping up, then a flat spot as
    Vds ramps down, and then a different curve for the FET
    on hard. For grounded source, use the flat spot to
    calculate input capacitance.

    For a class C or similar, think of having to move that
    total gate charge onto and off of the gate for each
    cycle. You could translate that into a capacitance
    for required gate drive; I'm not sure if that's what
    the "capacitance" that the datasheet refers to or not.

    Shawn kb1ckt

    604 2007-04-05 08:16:21 Graham Haddock Re: Input Capacitance of Large MOSFETs
    Ed:

    I would use a real RF power transistor rather than a low frequency
    power switch in an RF Power amplifier, if you are planning to
    go above 10 MHz or so. This is a good example of the difference
    between a switching transistor and an RF Power transistor.

    For single band PA operation, you can series resonate the gate
    capacitance of a switch and make it work, but very difficult to deal
    with in a multi band amplifier.

    I would recommend that you look at the Mitsubishi "RD Series" of RF
    power transistors. They have 16 watt RF output devices
    for under USD $5.00. (RD16HHF1)

    They are very rugged and easy to design with.

    http://www.rfparts.com/mitstran.html

    607 2007-04-05 11:53:47 Allison Parent Re: Input Capacitance of Large MOSFETs
    612 2007-04-05 16:36:00 arv evans Re: Input Capacitance of Large MOSFETs
    Ed - HA6SST

    This URL may provide some ideas about how to do it:

           http://www.arrl.org/qex/2006/09/qx9ohsawa.pdf

    Arv - K7HKL

    Ed Almos wrote:

    I am considering the use of two IRF740B devices in the final of an HF
    power amplifier. Normally this would not be a problem but examination
    of the datasheet shows device input capacitance as 1400 pF. This is
    not good, especially on the bands above 14MHz.

    Suggestions concerning how I should deal with this problem would be
    appreciated.

    Edward Almos HA6SST

    614 2007-04-06 00:33:50 Ed Almos Thanks Arv - K7HKL
    The link you supplied was most useful.

    It appears that the best way to deal with the large gate capacitance
    is to use a very low impedance input. I have not figured out the
    reason for the large capacitors across the input but the power MOSFETs
    are not expensive, if a few die for the cause during experiments then
    this is not a problem.

    Edward Almos HA6SST